rocksdb/db/db_impl.cc

//  Copyright (c) 2013, Facebook, Inc.  All rights reserved.
//  This source code is licensed under the BSD-style license found in the
//  LICENSE file in the root directory of this source tree. An additional grant
//  of patent rights can be found in the PATENTS file in the same directory.
//
// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file. See the AUTHORS file for names of contributors.

#include "db/db_impl.h"

#include <algorithm>
#include <climits>
#include <cstdio>
#include <set>
#include <stdexcept>
#include <stdint.h>
#include <string>
#include <unordered_set>
#include <vector>

#include "db/builder.h"
#include "db/dbformat.h"
#include "db/db_iter.h"
#include "db/filename.h"
#include "db/log_reader.h"
#include "db/log_writer.h"
#include "db/memtable.h"
#include "db/memtablelist.h"
#include "db/merge_helper.h"
#include "db/prefix_filter_iterator.h"
#include "db/table_cache.h"
#include "db/table_stats_collector.h"
#include "db/transaction_log_impl.h"
#include "db/version_set.h"
#include "db/write_batch_internal.h"
#include "port/port.h"
#include "rocksdb/compaction_filter.h"
#include "rocksdb/db.h"
#include "rocksdb/env.h"
#include "rocksdb/merge_operator.h"
#include "rocksdb/statistics.h"
#include "rocksdb/status.h"
#include "rocksdb/table.h"
#include "port/port.h"
#include "table/block.h"
#include "table/merger.h"
#include "table/two_level_iterator.h"
#include "util/auto_roll_logger.h"
#include "util/build_version.h"
#include "util/coding.h"
#include "util/logging.h"
#include "util/mutexlock.h"
#include "util/perf_context_imp.h"
#include "util/stop_watch.h"

namespace rocksdb {

void dumpLeveldbBuildVersion(Logger * log);

// Information kept for every waiting writer
struct DBImpl::Writer {
  Status status;
  WriteBatch* batch;
  bool sync;
  bool disableWAL;
  bool done;
  port::CondVar cv;

  explicit Writer(port::Mutex* mu) : cv(mu) { }
};

struct DBImpl::CompactionState {
  Compaction* const compaction;

  // If there were two snapshots with seq numbers s1 and
  // s2 and s1 < s2, and if we find two instances of a key k1 then lies
  // entirely within s1 and s2, then the earlier version of k1 can be safely
  // deleted because that version is not visible in any snapshot.
  std::vector<SequenceNumber> existing_snapshots;

  // Files produced by compaction
  struct Output {
    uint64_t number;
    uint64_t file_size;
    InternalKey smallest, largest;
    SequenceNumber smallest_seqno, largest_seqno;
  };
  std::vector<Output> outputs;
  std::list<uint64_t> allocated_file_numbers;

  // State kept for output being generated
  unique_ptr<WritableFile> outfile;
  unique_ptr<TableBuilder> builder;

  uint64_t total_bytes;

  Output* current_output() { return &outputs[outputs.size()-1]; }

  explicit CompactionState(Compaction* c)
      : compaction(c),
        total_bytes(0) {
  }

  // Create a client visible context of this compaction
  CompactionFilter::Context GetFilterContext() {
    CompactionFilter::Context context;
    context.is_full_compaction = compaction->IsFullCompaction();
    return context;
  }
};

// Fix user-supplied options to be reasonable
template <class T, class V>
static void ClipToRange(T* ptr, V minvalue, V maxvalue) {
  if (static_cast<V>(*ptr) > maxvalue) *ptr = maxvalue;
  if (static_cast<V>(*ptr) < minvalue) *ptr = minvalue;
}
Options SanitizeOptions(const std::string& dbname,
                        const InternalKeyComparator* icmp,
                        const InternalFilterPolicy* ipolicy,
                        const Options& src) {
  Options result = src;
  result.comparator = icmp;
  result.filter_policy = (src.filter_policy != nullptr) ? ipolicy : nullptr;
  ClipToRange(&result.max_open_files,            20,     1000000);
  ClipToRange(&result.write_buffer_size,         ((size_t)64)<<10,
                                                 ((size_t)64)<<30);
  ClipToRange(&result.block_size,                1<<10,  4<<20);

  // if user sets arena_block_size, we trust user to use this value. Otherwise,
  // calculate a proper value from writer_buffer_size;
  if (result.arena_block_size <= 0) {
    result.arena_block_size = result.write_buffer_size / 10;
  }

  result.min_write_buffer_number_to_merge = std::min(
    result.min_write_buffer_number_to_merge, result.max_write_buffer_number-1);
  if (result.info_log == nullptr) {
    Status s = CreateLoggerFromOptions(dbname, result.db_log_dir, src.env,
                                       result, &result.info_log);
    if (!s.ok()) {
      // No place suitable for logging
      result.info_log = nullptr;
    }
  }
  if (result.block_cache == nullptr && !result.no_block_cache) {
    result.block_cache = NewLRUCache(8 << 20);
  }
  result.compression_per_level = src.compression_per_level;
  if (result.block_size_deviation < 0 || result.block_size_deviation > 100) {
    result.block_size_deviation = 0;
  }
  if (result.max_mem_compaction_level >= result.num_levels) {
    result.max_mem_compaction_level = result.num_levels - 1;
  }
  if (result.soft_rate_limit > result.hard_rate_limit) {
    result.soft_rate_limit = result.hard_rate_limit;
  }
  if (result.compaction_filter) {
    Log(result.info_log, "Compaction filter specified, ignore factory");
  }
  if (result.prefix_extractor) {
    // If a prefix extractor has been supplied and a PrefixHashRepFactory is
    // being used, make sure that the latter uses the former as its transform
    // function.
    auto factory = dynamic_cast<PrefixHashRepFactory*>(
      result.memtable_factory.get());
    if (factory &&
        factory->GetTransform() != result.prefix_extractor) {
      Log(result.info_log, "A prefix hash representation factory was supplied "
          "whose prefix extractor does not match options.prefix_extractor. "
          "Falling back to skip list representation factory");
      result.memtable_factory = std::make_shared<SkipListFactory>();
    } else if (factory) {
      Log(result.info_log, "Prefix hash memtable rep is in use.");
    }
  }

  if (result.wal_dir.empty()) {
    // Use dbname as default
    result.wal_dir = dbname;
  }

  // -- Sanitize the table stats collector
  // All user defined stats collectors will be wrapped by
  // UserKeyTableStatsCollector since for them they only have the knowledge of
  // the user keys; internal keys are invisible to them.
  auto& collectors = result.table_stats_collectors;
  for (size_t i = 0; i < result.table_stats_collectors.size(); ++i) {
    assert(collectors[i]);
    collectors[i] =
      std::make_shared<UserKeyTableStatsCollector>(collectors[i]);
  }

  // Add collector to collect internal key statistics
  collectors.push_back(
      std::make_shared<InternalKeyStatsCollector>()
  );

  if (!result.flush_block_policy_factory) {
    result.SetUpDefaultFlushBlockPolicyFactory();
  }

  return result;
}

CompressionType GetCompressionType(const Options& options, int level,
                                   const bool enable_compression) {
  if (!enable_compression) {
    // disable compression
    return kNoCompression;
  }
  // If the use has specified a different compression level for each level,
  // then pick the compresison for that level.
  if (!options.compression_per_level.empty()) {
    const int n = options.compression_per_level.size() - 1;
    // It is possible for level_ to be -1; in that case, we use level
    // 0's compression.  This occurs mostly in backwards compatibility
    // situations when the builder doesn't know what level the file
    // belongs to.  Likewise, if level_ is beyond the end of the
    // specified compression levels, use the last value.
    return options.compression_per_level[std::max(0, std::min(level, n))];
  } else {
    return options.compression;
  }
}

DBImpl::DBImpl(const Options& options, const std::string& dbname)
    : env_(options.env),
      dbname_(dbname),
      internal_comparator_(options.comparator),
      options_(SanitizeOptions(
          dbname, &internal_comparator_, &internal_filter_policy_, options)),
      internal_filter_policy_(options.filter_policy),
      owns_info_log_(options_.info_log != options.info_log),
      db_lock_(nullptr),
      mutex_(options.use_adaptive_mutex),
      shutting_down_(nullptr),
      bg_cv_(&mutex_),
      mem_rep_factory_(options_.memtable_factory),
      mem_(new MemTable(internal_comparator_, mem_rep_factory_,
        NumberLevels(), options_)),
      logfile_number_(0),
      tmp_batch_(),
      bg_compaction_scheduled_(0),
      bg_flush_scheduled_(0),
      bg_logstats_scheduled_(false),
      manual_compaction_(nullptr),
      logger_(nullptr),
      disable_delete_obsolete_files_(false),
      delete_obsolete_files_last_run_(0),
      purge_wal_files_last_run_(0),
      last_stats_dump_time_microsec_(0),
      default_interval_to_delete_obsolete_WAL_(600),
      stall_level0_slowdown_(0),
      stall_memtable_compaction_(0),
      stall_level0_num_files_(0),
      stall_level0_slowdown_count_(0),
      stall_memtable_compaction_count_(0),
      stall_level0_num_files_count_(0),
      started_at_(options.env->NowMicros()),
      flush_on_destroy_(false),
      stats_(options.num_levels),
      delayed_writes_(0),
      storage_options_(options),
      bg_work_gate_closed_(false),
      refitting_level_(false) {

  mem_->Ref();

  env_->GetAbsolutePath(dbname, &db_absolute_path_);

  stall_leveln_slowdown_.resize(options.num_levels);
  stall_leveln_slowdown_count_.resize(options.num_levels);
  for (int i = 0; i < options.num_levels; ++i) {
    stall_leveln_slowdown_[i] = 0;
    stall_leveln_slowdown_count_[i] = 0;
  }

  // Reserve ten files or so for other uses and give the rest to TableCache.
  const int table_cache_size = options_.max_open_files - 10;
  table_cache_.reset(new TableCache(dbname_, &options_,
                                    storage_options_, table_cache_size));

  versions_.reset(new VersionSet(dbname_, &options_, storage_options_,
                                 table_cache_.get(), &internal_comparator_));

  dumpLeveldbBuildVersion(options_.info_log.get());
  options_.Dump(options_.info_log.get());

  char name[100];
  Status st = env_->GetHostName(name, 100L);
  if (st.ok()) {
    host_name_ = name;
  } else {
    Log(options_.info_log, "Can't get hostname, use localhost as host name.");
    host_name_ = "localhost";
  }
  last_log_ts = 0;

  LogFlush(options_.info_log);
}

DBImpl::~DBImpl() {
  // Wait for background work to finish
  if (flush_on_destroy_ && mem_->GetFirstSequenceNumber() != 0) {
    FlushMemTable(FlushOptions());
  }
  mutex_.Lock();
  shutting_down_.Release_Store(this);  // Any non-nullptr value is ok
  while (bg_compaction_scheduled_ ||
         bg_flush_scheduled_ ||
         bg_logstats_scheduled_) {
    bg_cv_.Wait();
  }
  mutex_.Unlock();

  if (db_lock_ != nullptr) {
    env_->UnlockFile(db_lock_);
  }

  if (mem_ != nullptr) mem_->Unref();
  imm_.UnrefAll();
  LogFlush(options_.info_log);
}

// Do not flush and close database elegantly. Simulate a crash.
void DBImpl::TEST_Destroy_DBImpl() {
  // ensure that no new memtable flushes can occur
  flush_on_destroy_ = false;

  // wait till all background compactions are done.
  mutex_.Lock();
  while (bg_compaction_scheduled_ ||
         bg_flush_scheduled_ ||
         bg_logstats_scheduled_) {
    bg_cv_.Wait();
  }

  // Prevent new compactions from occuring.
  bg_work_gate_closed_ = true;
  const int LargeNumber = 10000000;
  bg_compaction_scheduled_ += LargeNumber;

  mutex_.Unlock();
  LogFlush(options_.info_log);

  // force release the lock file.
  if (db_lock_ != nullptr) {
    env_->UnlockFile(db_lock_);
  }

  log_.reset();
  versions_.reset();
  table_cache_.reset();
}

uint64_t DBImpl::TEST_Current_Manifest_FileNo() {
  return versions_->ManifestFileNumber();
}

Status DBImpl::NewDB() {
  VersionEdit new_db(NumberLevels());
  new_db.SetComparatorName(user_comparator()->Name());
  new_db.SetLogNumber(0);
  new_db.SetNextFile(2);
  new_db.SetLastSequence(0);

  const std::string manifest = DescriptorFileName(dbname_, 1);
  unique_ptr<WritableFile> file;
  Status s = env_->NewWritableFile(manifest, &file, storage_options_);
  if (!s.ok()) {
    return s;
  }
  file->SetPreallocationBlockSize(options_.manifest_preallocation_size);
  {
    log::Writer log(std::move(file));
    std::string record;
    new_db.EncodeTo(&record);
    s = log.AddRecord(record);
  }
  if (s.ok()) {
    // Make "CURRENT" file that points to the new manifest file.
    s = SetCurrentFile(env_, dbname_, 1);
  } else {
    env_->DeleteFile(manifest);
  }
  return s;
}

void DBImpl::MaybeIgnoreError(Status* s) const {
  if (s->ok() || options_.paranoid_checks) {
    // No change needed
  } else {
    Log(options_.info_log, "Ignoring error %s", s->ToString().c_str());
    *s = Status::OK();
  }
}

const Status DBImpl::CreateArchivalDirectory() {
  if (options_.WAL_ttl_seconds > 0 || options_.WAL_size_limit_MB > 0) {
    std::string archivalPath = ArchivalDirectory(options_.wal_dir);
    return env_->CreateDirIfMissing(archivalPath);
  }
  return Status::OK();
}

void DBImpl::PrintStatistics() {
  auto dbstats = options_.statistics;
  if (dbstats) {
    Log(options_.info_log,
        "STATISTCS:\n %s",
        dbstats->ToString().c_str());
  }
}

void DBImpl::MaybeDumpStats() {
  if (options_.stats_dump_period_sec == 0) return;

  const uint64_t now_micros = env_->NowMicros();

  if (last_stats_dump_time_microsec_ +
      options_.stats_dump_period_sec * 1000000
      <= now_micros) {
    // Multiple threads could race in here simultaneously.
    // However, the last one will update last_stats_dump_time_microsec_
    // atomically. We could see more than one dump during one dump
    // period in rare cases.
    last_stats_dump_time_microsec_ = now_micros;
    std::string stats;
    GetProperty("rocksdb.stats", &stats);
    Log(options_.info_log, "%s", stats.c_str());
    PrintStatistics();
  }
}

// Returns the list of live files in 'sst_live' and the list
// of all files in the filesystem in 'all_files'.
void DBImpl::FindObsoleteFiles(DeletionState& deletion_state, bool force) {
  mutex_.AssertHeld();

  // if deletion is disabled, do nothing
  if (disable_delete_obsolete_files_) {
    return;
  }

  // store the current filenum, lognum, etc
  deletion_state.manifest_file_number = versions_->ManifestFileNumber();
  deletion_state.log_number = versions_->LogNumber();
  deletion_state.prev_log_number = versions_->PrevLogNumber();

  // This method is costly when the number of files is large.
  // Do not allow it to trigger more often than once in
  // delete_obsolete_files_period_micros.
  if (!force && options_.delete_obsolete_files_period_micros != 0) {
    const uint64_t now_micros = env_->NowMicros();
    if (delete_obsolete_files_last_run_ +
        options_.delete_obsolete_files_period_micros > now_micros) {
      return;
    }
    delete_obsolete_files_last_run_ = now_micros;
  }

  // Make a list of all of the live files; set is slow, should not
  // be used.
  deletion_state.sst_live.assign(pending_outputs_.begin(),
                                 pending_outputs_.end());
  versions_->AddLiveFiles(&deletion_state.sst_live);

  // set of all files in the directory
  env_->GetChildren(dbname_, &deletion_state.all_files); // Ignore errors

  //Add log files in wal_dir
  if (options_.wal_dir != dbname_) {
    std::vector<std::string> log_files;
    env_->GetChildren(options_.wal_dir, &log_files); // Ignore errors
    deletion_state.all_files.insert(
      deletion_state.all_files.end(),
      log_files.begin(),
      log_files.end()
    );
  }
}

// Diffs the files listed in filenames and those that do not
// belong to live files are posibly removed. Also, removes all the
// files in sst_delete_files and log_delete_files.
// It is not necessary to hold the mutex when invoking this method.
void DBImpl::PurgeObsoleteFiles(DeletionState& state) {
  // if deletion is disabled, do nothing
  if (disable_delete_obsolete_files_) {
    return;
  }

  uint64_t number;
  FileType type;
  std::vector<std::string> old_log_files;

  // Now, convert live list to an unordered set, WITHOUT mutex held;
  // set is slow.
  std::unordered_set<uint64_t> live_set(state.sst_live.begin(),
                                        state.sst_live.end());

  state.all_files.reserve(state.all_files.size() +
      state.sst_delete_files.size());
  for (auto file : state.sst_delete_files) {
    state.all_files.push_back(TableFileName("", file->number));
    delete file;
  }

  state.all_files.reserve(state.all_files.size() +
      state.log_delete_files.size());
  for (auto filenum : state.log_delete_files) {
    if (filenum > 0) {
      state.all_files.push_back(LogFileName("", filenum));
    }
  }

  for (size_t i = 0; i < state.all_files.size(); i++) {
    if (ParseFileName(state.all_files[i], &number, &type)) {
      bool keep = true;
      switch (type) {
        case kLogFile:
          keep = ((number >= state.log_number) ||
                  (number == state.prev_log_number));
          break;
        case kDescriptorFile:
          // Keep my manifest file, and any newer incarnations'
          // (in case there is a race that allows other incarnations)
          keep = (number >= state.manifest_file_number);
          break;
        case kTableFile:
          keep = (live_set.find(number) != live_set.end());
          break;
        case kTempFile:
          // Any temp files that are currently being written to must
          // be recorded in pending_outputs_, which is inserted into "live"
          keep = (live_set.find(number) != live_set.end());
          break;
        case kInfoLogFile:
          keep = true;
          if (number != 0) {
            old_log_files.push_back(state.all_files[i]);
          }
          break;
        case kCurrentFile:
        case kDBLockFile:
        case kIdentityFile:
        case kMetaDatabase:
          keep = true;
          break;
      }

      if (!keep) {
        if (type == kTableFile) {
          // evict from cache
          table_cache_->Evict(number);
        }
        Log(options_.info_log, "Delete type=%d #%llu", int(type), number);

        Status st;
        if (type == kLogFile && (options_.WAL_ttl_seconds > 0 ||
              options_.WAL_size_limit_MB > 0)) {
            st = env_->RenameFile(dbname_ + "/" + state.all_files[i],
                                  ArchivedLogFileName(options_.wal_dir,
                                                      number));
            if (!st.ok()) {
              Log(options_.info_log, "RenameFile logfile #%llu FAILED", number);
            }
        } else {
          st = env_->DeleteFile(dbname_ + "/" + state.all_files[i]);
          if (!st.ok()) {
            Log(options_.info_log, "Delete type=%d #%llu FAILED\n",
                int(type), number);
          }
        }
      }
    }
  }

  // Delete old info log files.
  size_t old_log_file_count = old_log_files.size();
  // NOTE: Currently we only support log purge when options_.db_log_dir is
  // located in `dbname` directory.
  if (old_log_file_count >= options_.keep_log_file_num &&
      options_.db_log_dir.empty()) {
    std::sort(old_log_files.begin(), old_log_files.end());
    size_t end = old_log_file_count - options_.keep_log_file_num;
    for (unsigned int i = 0; i <= end; i++) {
      std::string& to_delete = old_log_files.at(i);
      // Log(options_.info_log, "Delete type=%d %s\n",
      //     int(kInfoLogFile), to_delete.c_str());
      env_->DeleteFile(dbname_ + "/" + to_delete);
    }
  }
  PurgeObsoleteWALFiles();
  LogFlush(options_.info_log);
}

void DBImpl::DeleteObsoleteFiles() {
  mutex_.AssertHeld();
  DeletionState deletion_state;
  FindObsoleteFiles(deletion_state, true);
  PurgeObsoleteFiles(deletion_state);
}

// 1. Go through all archived files and
//    a. if ttl is enabled, delete outdated files
//    b. if archive size limit is enabled, delete empty files,
//        compute file number and size.
// 2. If size limit is enabled:
//    a. compute how many files should be deleted
//    b. get sorted non-empty archived logs
//    c. delete what should be deleted
void DBImpl::PurgeObsoleteWALFiles() {
  bool const ttl_enabled = options_.WAL_ttl_seconds > 0;
  bool const size_limit_enabled =  options_.WAL_size_limit_MB > 0;
  if (!ttl_enabled && !size_limit_enabled) {
    return;
  }

  int64_t current_time;
  Status s = env_->GetCurrentTime(&current_time);
  if (!s.ok()) {
    Log(options_.info_log, "Can't get current time: %s", s.ToString().c_str());
    assert(false);
    return;
  }
  uint64_t const now_seconds = static_cast<uint64_t>(current_time);
  uint64_t const time_to_check = (ttl_enabled && !size_limit_enabled) ?
    options_.WAL_ttl_seconds / 2 : default_interval_to_delete_obsolete_WAL_;

  if (purge_wal_files_last_run_ + time_to_check > now_seconds) {
    return;
  }

  purge_wal_files_last_run_ = now_seconds;

  std::string archival_dir = ArchivalDirectory(options_.wal_dir);
  std::vector<std::string> files;
  s = env_->GetChildren(archival_dir, &files);
  if (!s.ok()) {
    Log(options_.info_log, "Can't get archive files: %s", s.ToString().c_str());
    assert(false);
    return;
  }

  size_t log_files_num = 0;
  uint64_t log_file_size = 0;

  for (auto& f : files) {
    uint64_t number;
    FileType type;
    if (ParseFileName(f, &number, &type) && type == kLogFile) {
      std::string const file_path = archival_dir + "/" + f;
      if (ttl_enabled) {
        uint64_t file_m_time;
        Status const s = env_->GetFileModificationTime(file_path,
          &file_m_time);
        if (!s.ok()) {
          Log(options_.info_log, "Can't get file mod time: %s: %s",
              file_path.c_str(), s.ToString().c_str());
          continue;
        }
        if (now_seconds - file_m_time > options_.WAL_ttl_seconds) {
          Status const s = env_->DeleteFile(file_path);
          if (!s.ok()) {
            Log(options_.info_log, "Can't delete file: %s: %s",
                file_path.c_str(), s.ToString().c_str());
            continue;
          }
          continue;
        }
      }

      if (size_limit_enabled) {
        uint64_t file_size;
        Status const s = env_->GetFileSize(file_path, &file_size);
        if (!s.ok()) {
          Log(options_.info_log, "Can't get file size: %s: %s",
              file_path.c_str(), s.ToString().c_str());
          return;
        } else {
          if (file_size > 0) {
            log_file_size = std::max(log_file_size, file_size);
            ++log_files_num;
          } else {
            Status s = env_->DeleteFile(file_path);
            if (!s.ok()) {
              Log(options_.info_log, "Can't delete file: %s: %s",
                  file_path.c_str(), s.ToString().c_str());
              continue;
            }
          }
        }
      }
    }
  }

  if (0 == log_files_num || !size_limit_enabled) {
    return;
  }

  size_t const files_keep_num = options_.WAL_size_limit_MB *
    1024 * 1024 / log_file_size;
  if (log_files_num <= files_keep_num) {
    return;
  }

  size_t files_del_num = log_files_num - files_keep_num;
  VectorLogPtr archived_logs;
  AppendSortedWalsOfType(archival_dir, archived_logs, kArchivedLogFile);

  if (files_del_num > archived_logs.size()) {
    Log(options_.info_log, "Trying to delete more archived log files than "
        "exist. Deleting all");
    files_del_num = archived_logs.size();
  }

  for (size_t i = 0; i < files_del_num; ++i) {
    std::string const file_path = archived_logs[i]->PathName();
    Status const s = DeleteFile(file_path);
    if (!s.ok()) {
      Log(options_.info_log, "Can't delete file: %s: %s",
          file_path.c_str(), s.ToString().c_str());
      continue;
    }
  }
}

// If externalTable is set, then apply recovered transactions
// to that table. This is used for readonly mode.
Status DBImpl::Recover(VersionEdit* edit, MemTable* external_table,
    bool error_if_log_file_exist) {
  mutex_.AssertHeld();

  assert(db_lock_ == nullptr);
  if (!external_table) {
    // We call CreateDirIfMissing() as the directory may already exist (if we
    // are reopening a DB), when this happens we don't want creating the
    // directory to cause an error. However, we need to check if creating the
    // directory fails or else we may get an obscure message about the lock
    // file not existing. One real-world example of this occurring is if
    // env->CreateDirIfMissing() doesn't create intermediate directories, e.g.
    // when dbname_ is "dir/db" but when "dir" doesn't exist.
    Status s = env_->CreateDirIfMissing(dbname_);
    if (!s.ok()) {
      return s;
    }

    s = env_->LockFile(LockFileName(dbname_), &db_lock_);
    if (!s.ok()) {
      return s;
    }

    if (!env_->FileExists(CurrentFileName(dbname_))) {
      if (options_.create_if_missing) {
        // TODO: add merge_operator name check
        s = NewDB();
        if (!s.ok()) {
          return s;
        }
      } else {
        return Status::InvalidArgument(
            dbname_, "does not exist (create_if_missing is false)");
      }
    } else {
      if (options_.error_if_exists) {
        return Status::InvalidArgument(
            dbname_, "exists (error_if_exists is true)");
      }
    }
    // Check for the IDENTITY file and create it if not there
    if (!env_->FileExists(IdentityFileName(dbname_))) {
      s = SetIdentityFile(env_, dbname_);
      if (!s.ok()) {
        return s;
      }
    }
  }

  Status s = versions_->Recover();
  if (s.ok()) {
    SequenceNumber max_sequence(0);

    // Recover from all newer log files than the ones named in the
    // descriptor (new log files may have been added by the previous
    // incarnation without registering them in the descriptor).
    //
    // Note that PrevLogNumber() is no longer used, but we pay
    // attention to it in case we are recovering a database
    // produced by an older version of rocksdb.
    const uint64_t min_log = versions_->LogNumber();
    const uint64_t prev_log = versions_->PrevLogNumber();
    std::vector<std::string> filenames;
    s = env_->GetChildren(options_.wal_dir, &filenames);
    if (!s.ok()) {
      return s;
    }
    uint64_t number;
    FileType type;
    std::vector<uint64_t> logs;
    for (size_t i = 0; i < filenames.size(); i++) {
      if (ParseFileName(filenames[i], &number, &type)
          && type == kLogFile
          && ((number >= min_log) || (number == prev_log))) {
        logs.push_back(number);
      }
    }

    if (logs.size() > 0 && error_if_log_file_exist) {
      return Status::Corruption(""
          "The db was opened in readonly mode with error_if_log_file_exist"
          "flag but a log file already exists");
    }

    // Recover in the order in which the logs were generated
    std::sort(logs.begin(), logs.end());
    for (size_t i = 0; i < logs.size(); i++) {
      s = RecoverLogFile(logs[i], edit, &max_sequence, external_table);
      // The previous incarnation may not have written any MANIFEST
      // records after allocating this log number.  So we manually
      // update the file number allocation counter in VersionSet.
      versions_->MarkFileNumberUsed(logs[i]);
    }

    if (s.ok()) {
      if (versions_->LastSequence() < max_sequence) {
        versions_->SetLastSequence(max_sequence);
      }
      SetTickerCount(options_.statistics, SEQUENCE_NUMBER,
                     versions_->LastSequence());
    }
  }

  return s;
}

Status DBImpl::RecoverLogFile(uint64_t log_number,
                              VersionEdit* edit,
                              SequenceNumber* max_sequence,
                              MemTable* external_table) {
  struct LogReporter : public log::Reader::Reporter {
    Env* env;
    Logger* info_log;
    const char* fname;
    Status* status;  // nullptr if options_.paranoid_checks==false or
                     //            options_.skip_log_error_on_recovery==true
    virtual void Corruption(size_t bytes, const Status& s) {
      Log(info_log, "%s%s: dropping %d bytes; %s",
          (this->status == nullptr ? "(ignoring error) " : ""),
          fname, static_cast<int>(bytes), s.ToString().c_str());
      if (this->status != nullptr && this->status->ok()) *this->status = s;
    }
  };

  mutex_.AssertHeld();

  // Open the log file
  std::string fname = LogFileName(options_.wal_dir, log_number);
  unique_ptr<SequentialFile> file;
  Status status = env_->NewSequentialFile(fname, &file, storage_options_);
  if (!status.ok()) {
    MaybeIgnoreError(&status);
    return status;
  }

  // Create the log reader.
  LogReporter reporter;
  reporter.env = env_;
  reporter.info_log = options_.info_log.get();
  reporter.fname = fname.c_str();
  reporter.status = (options_.paranoid_checks &&
                     !options_.skip_log_error_on_recovery ? &status : nullptr);
  // We intentially make log::Reader do checksumming even if
  // paranoid_checks==false so that corruptions cause entire commits
  // to be skipped instead of propagating bad information (like overly
  // large sequence numbers).
  log::Reader reader(std::move(file), &reporter, true/*checksum*/,
                     0/*initial_offset*/);
  Log(options_.info_log, "Recovering log #%llu",
      (unsigned long long) log_number);

  // Read all the records and add to a memtable
  std::string scratch;
  Slice record;
  WriteBatch batch;
  MemTable* mem = nullptr;
  if (external_table) {
    mem = external_table;
  }
  while (reader.ReadRecord(&record, &scratch) && status.ok()) {
    if (record.size() < 12) {
      reporter.Corruption(
          record.size(), Status::Corruption("log record too small"));
      continue;
    }
    WriteBatchInternal::SetContents(&batch, record);

    if (mem == nullptr) {
      mem = new MemTable(internal_comparator_, mem_rep_factory_,
        NumberLevels(), options_);
      mem->Ref();
    }
    status = WriteBatchInternal::InsertInto(&batch, mem, &options_);
    MaybeIgnoreError(&status);
    if (!status.ok()) {
      break;
    }
    const SequenceNumber last_seq =
        WriteBatchInternal::Sequence(&batch) +
        WriteBatchInternal::Count(&batch) - 1;
    if (last_seq > *max_sequence) {
      *max_sequence = last_seq;
    }

    if (!external_table &&
        mem->ApproximateMemoryUsage() > options_.write_buffer_size) {
      status = WriteLevel0TableForRecovery(mem, edit);
      if (!status.ok()) {
        // Reflect errors immediately so that conditions like full
        // file-systems cause the DB::Open() to fail.
        break;
      }
      mem->Unref();
      mem = nullptr;
    }
  }

  if (status.ok() && mem != nullptr && !external_table) {
    status = WriteLevel0TableForRecovery(mem, edit);
    // Reflect errors immediately so that conditions like full
    // file-systems cause the DB::Open() to fail.
  }

  if (mem != nullptr && !external_table) mem->Unref();
  return status;
}

Status DBImpl::WriteLevel0TableForRecovery(MemTable* mem, VersionEdit* edit) {
  mutex_.AssertHeld();
  const uint64_t start_micros = env_->NowMicros();
  FileMetaData meta;
  meta.number = versions_->NewFileNumber();
  pending_outputs_.insert(meta.number);
  Iterator* iter = mem->NewIterator();
  const SequenceNumber newest_snapshot = snapshots_.GetNewest();
  const SequenceNumber earliest_seqno_in_memtable =
    mem->GetFirstSequenceNumber();
  Log(options_.info_log, "Level-0 table #%llu: started",
      (unsigned long long) meta.number);

  Status s;
  {
    mutex_.Unlock();
    s = BuildTable(dbname_, env_, options_, storage_options_,
                   table_cache_.get(), iter, &meta,
                   user_comparator(), newest_snapshot,
                   earliest_seqno_in_memtable, true);
    LogFlush(options_.info_log);
    mutex_.Lock();
  }

  Log(options_.info_log, "Level-0 table #%llu: %lld bytes %s",
      (unsigned long long) meta.number,
      (unsigned long long) meta.file_size,
      s.ToString().c_str());
  delete iter;

  pending_outputs_.erase(meta.number);

  // Note that if file_size is zero, the file has been deleted and
  // should not be added to the manifest.
  int level = 0;
  if (s.ok() && meta.file_size > 0) {
    edit->AddFile(level, meta.number, meta.file_size,
                  meta.smallest, meta.largest,
                  meta.smallest_seqno, meta.largest_seqno);
  }

  CompactionStats stats;
  stats.micros = env_->NowMicros() - start_micros;
  stats.bytes_written = meta.file_size;
  stats.files_out_levelnp1 = 1;
  stats_[level].Add(stats);
  return s;
}


Status DBImpl::WriteLevel0Table(std::vector<MemTable*> &mems, VersionEdit* edit,
                                uint64_t* filenumber) {
  mutex_.AssertHeld();
  const uint64_t start_micros = env_->NowMicros();
  FileMetaData meta;
  meta.number = versions_->NewFileNumber();
  *filenumber = meta.number;
  pending_outputs_.insert(meta.number);

  std::vector<Iterator*> list;
  for (MemTable* m : mems) {
    Log(options_.info_log,
        "Flushing memtable with log file: %llu\n",
        m->GetLogNumber());
    list.push_back(m->NewIterator());
  }
  Iterator* iter = NewMergingIterator(&internal_comparator_, &list[0],
                                      list.size());
  const SequenceNumber newest_snapshot = snapshots_.GetNewest();
  const SequenceNumber earliest_seqno_in_memtable =
    mems[0]->GetFirstSequenceNumber();
  Log(options_.info_log, "Level-0 flush table #%llu: started", meta.number);

  Version* base = versions_->current();
  base->Ref();          // it is likely that we do not need this reference
  Status s;
  {
    mutex_.Unlock();
    // We skip compression if universal compression is used and the size
    // threshold is set for compression.
    bool enable_compression = (options_.compaction_style
        != kCompactionStyleUniversal ||
        options_.compaction_options_universal.compression_size_percent < 0);
    s = BuildTable(dbname_, env_, options_, storage_options_,
                   table_cache_.get(), iter, &meta,
                   user_comparator(), newest_snapshot,
                   earliest_seqno_in_memtable, enable_compression);
    LogFlush(options_.info_log);
    mutex_.Lock();
  }
  base->Unref();

  Log(options_.info_log, "Level-0 flush table #%llu: %lld bytes %s",
      (unsigned long long) meta.number,
      (unsigned long long) meta.file_size,
      s.ToString().c_str());
  delete iter;

  // re-acquire the most current version
  base = versions_->current();

  // There could be multiple threads writing to its own level-0 file.
  // The pending_outputs cannot be cleared here, otherwise this newly
  // created file might not be considered as a live-file by another
  // compaction thread that is concurrently deleting obselete files.
  // The pending_outputs can be cleared only after the new version is
  // committed so that other threads can recognize this file as a
  // valid one.
  // pending_outputs_.erase(meta.number);

  // Note that if file_size is zero, the file has been deleted and
  // should not be added to the manifest.
  int level = 0;
  if (s.ok() && meta.file_size > 0) {
    const Slice min_user_key = meta.smallest.user_key();
    const Slice max_user_key = meta.largest.user_key();
    // if we have more than 1 background thread, then we cannot
    // insert files directly into higher levels because some other
    // threads could be concurrently producing compacted files for
    // that key range.
    if (base != nullptr && options_.max_background_compactions <= 1 &&
        options_.compaction_style == kCompactionStyleLevel) {
      level = base->PickLevelForMemTableOutput(min_user_key, max_user_key);
    }
    edit->AddFile(level, meta.number, meta.file_size,
                  meta.smallest, meta.largest,
                  meta.smallest_seqno, meta.largest_seqno);
  }

  CompactionStats stats;
  stats.micros = env_->NowMicros() - start_micros;
  stats.bytes_written = meta.file_size;
  stats_[level].Add(stats);
  return s;
}

Status DBImpl::FlushMemTableToOutputFile(bool* madeProgress,
                                         DeletionState& deletion_state) {
  mutex_.AssertHeld();
  assert(imm_.size() != 0);

  if (!imm_.IsFlushPending(options_.min_write_buffer_number_to_merge)) {
    Log(options_.info_log, "FlushMemTableToOutputFile already in progress");
    Status s = Status::IOError("FlushMemTableToOutputFile already in progress");
    return s;
  }

  // Save the contents of the earliest memtable as a new Table
  uint64_t file_number;
  std::vector<MemTable*> mems;
  imm_.PickMemtablesToFlush(&mems);
  if (mems.empty()) {
    Log(options_.info_log, "Nothing in memstore to flush");
    Status s = Status::IOError("Nothing in memstore to flush");
    return s;
  }

  // record the logfile_number_ before we release the mutex
  // entries mems are (implicitly) sorted in ascending order by their created
  // time. We will use the first memtable's `edit` to keep the meta info for
  // this flush.
  MemTable* m = mems[0];
  VersionEdit* edit = m->GetEdits();
  edit->SetPrevLogNumber(0);
  // SetLogNumber(log_num) indicates logs with number smaller than log_num
  // will no longer be picked up for recovery.
  edit->SetLogNumber(
      mems.back()->GetNextLogNumber()
  );

  std::vector<uint64_t> logs_to_delete;
  for (auto mem : mems) {
    logs_to_delete.push_back(mem->GetLogNumber());
  }

  // This will release and re-acquire the mutex.
  Status s = WriteLevel0Table(mems, edit, &file_number);

  if (s.ok() && shutting_down_.Acquire_Load()) {
    s = Status::IOError(
      "Database shutdown started during memtable compaction"
    );
  }

  // Replace immutable memtable with the generated Table
  s = imm_.InstallMemtableFlushResults(
    mems, versions_.get(), s, &mutex_, options_.info_log.get(),
    file_number, pending_outputs_);

  if (s.ok()) {
    if (madeProgress) {
      *madeProgress = 1;
    }

    MaybeScheduleLogDBDeployStats();

    if (!disable_delete_obsolete_files_) {
      // add to deletion state
      deletion_state.log_delete_files.insert(
          deletion_state.log_delete_files.end(),
          logs_to_delete.begin(),
          logs_to_delete.end());
    }
  }
  return s;
}

void DBImpl::CompactRange(const Slice* begin, const Slice* end,
                          bool reduce_level, int target_level) {
  int max_level_with_files = 1;
  {
    MutexLock l(&mutex_);
    Version* base = versions_->current();
    for (int level = 1; level < NumberLevels(); level++) {
      if (base->OverlapInLevel(level, begin, end)) {
        max_level_with_files = level;
      }
    }
  }
  TEST_FlushMemTable(); // TODO(sanjay): Skip if memtable does not overlap
  for (int level = 0; level < max_level_with_files; level++) {
    TEST_CompactRange(level, begin, end);
  }

  if (reduce_level) {
    ReFitLevel(max_level_with_files, target_level);
  }
  LogFlush(options_.info_log);
}

// return the same level if it cannot be moved
int DBImpl::FindMinimumEmptyLevelFitting(int level) {
  mutex_.AssertHeld();
  int minimum_level = level;
  for (int i = level - 1; i > 0; --i) {
    // stop if level i is not empty
    if (versions_->NumLevelFiles(i) > 0) break;

    // stop if level i is too small (cannot fit the level files)
    if (versions_->MaxBytesForLevel(i) < versions_->NumLevelBytes(level)) break;

    minimum_level = i;
  }
  return minimum_level;
}

void DBImpl::ReFitLevel(int level, int target_level) {
  assert(level < NumberLevels());

  MutexLock l(&mutex_);

  // only allow one thread refitting
  if (refitting_level_) {
    Log(options_.info_log, "ReFitLevel: another thread is refitting");
    return;
  }
  refitting_level_ = true;

  // wait for all background threads to stop
  bg_work_gate_closed_ = true;
  while (bg_compaction_scheduled_ > 0 || bg_flush_scheduled_) {
    Log(options_.info_log,
        "RefitLevel: waiting for background threads to stop: %d %d",
        bg_compaction_scheduled_, bg_flush_scheduled_);
    bg_cv_.Wait();
  }

  // move to a smaller level
  int to_level = target_level;
  if (target_level < 0) {
    to_level = FindMinimumEmptyLevelFitting(level);
  }

  assert(to_level <= level);

  if (to_level < level) {
    Log(options_.info_log, "Before refitting:\n%s",
        versions_->current()->DebugString().data());

    VersionEdit edit(NumberLevels());
    for (const auto& f : versions_->current()->files_[level]) {
      edit.DeleteFile(level, f->number);
      edit.AddFile(to_level, f->number, f->file_size, f->smallest, f->largest,
                   f->smallest_seqno, f->largest_seqno);
    }
    Log(options_.info_log, "Apply version edit:\n%s",
        edit.DebugString().data());

    auto status = versions_->LogAndApply(&edit, &mutex_);

    Log(options_.info_log, "LogAndApply: %s\n", status.ToString().data());

    if (status.ok()) {
      Log(options_.info_log, "After refitting:\n%s",
          versions_->current()->DebugString().data());
    }
  }

  refitting_level_ = false;
  bg_work_gate_closed_ = false;
}

int DBImpl::NumberLevels() {
  return options_.num_levels;
}

int DBImpl::MaxMemCompactionLevel() {
  return options_.max_mem_compaction_level;
}

int DBImpl::Level0StopWriteTrigger() {
  return options_.level0_stop_writes_trigger;
}

Status DBImpl::Flush(const FlushOptions& options) {
  Status status = FlushMemTable(options);
  return status;
}

SequenceNumber DBImpl::GetLatestSequenceNumber() const {
  return versions_->LastSequence();
}

Status DBImpl::GetUpdatesSince(SequenceNumber seq,
                               unique_ptr<TransactionLogIterator>* iter) {

  RecordTick(options_.statistics, GET_UPDATES_SINCE_CALLS);
  if (seq > versions_->LastSequence()) {
    return Status::IOError("Requested sequence not yet written in the db");
  }
  //  Get all sorted Wal Files.
  //  Do binary search and open files and find the seq number.

  std::unique_ptr<VectorLogPtr> wal_files(new VectorLogPtr);
  Status s = GetSortedWalFiles(*wal_files);
  if (!s.ok()) {
    return s;
  }

  s = RetainProbableWalFiles(*wal_files, seq);
  if (!s.ok()) {
    return s;
  }
  iter->reset(
    new TransactionLogIteratorImpl(options_.wal_dir,
                                   &options_,
                                   storage_options_,
                                   seq,
                                   std::move(wal_files),
                                   this));
  return (*iter)->status();
}

Status DBImpl::RetainProbableWalFiles(VectorLogPtr& all_logs,
                                      const SequenceNumber target) {
  long start = 0; // signed to avoid overflow when target is < first file.
  long end = static_cast<long>(all_logs.size()) - 1;
  // Binary Search. avoid opening all files.
  while (end >= start) {
    long mid = start + (end - start) / 2;  // Avoid overflow.
    SequenceNumber current_seq_num = all_logs.at(mid)->StartSequence();
    if (current_seq_num == target) {
      end = mid;
      break;
    } else if (current_seq_num < target) {
      start = mid + 1;
    } else {
      end = mid - 1;
    }
  }
  size_t start_index = std::max(0l, end); // end could be -ve.
  // The last wal file is always included
  all_logs.erase(all_logs.begin(), all_logs.begin() + start_index);
  return Status::OK();
}

bool DBImpl::CheckWalFileExistsAndEmpty(const WalFileType type,
                                        const uint64_t number) {
  const std::string fname = (type == kAliveLogFile) ?
    LogFileName(options_.wal_dir, number) :
    ArchivedLogFileName(options_.wal_dir, number);
  uint64_t file_size;
  Status s = env_->GetFileSize(fname, &file_size);
  return (s.ok() && (file_size == 0));
}

Status DBImpl::ReadFirstRecord(const WalFileType type, const uint64_t number,
                               WriteBatch* const result) {

  if (type == kAliveLogFile) {
    std::string fname = LogFileName(options_.wal_dir, number);
    Status status = ReadFirstLine(fname, result);
    if (!status.ok()) {
      //  check if the file got moved to archive.
      std::string archived_file =
        ArchivedLogFileName(options_.wal_dir, number);
      Status s = ReadFirstLine(archived_file, result);
      if (!s.ok()) {
        return Status::IOError("Log File has been deleted: " + archived_file);
      }
    }
    return Status::OK();
  } else if (type == kArchivedLogFile) {
    std::string fname = ArchivedLogFileName(options_.wal_dir, number);
    Status status = ReadFirstLine(fname, result);
    return status;
  }
  return Status::NotSupported("File Type Not Known: " + std::to_string(type));
}

Status DBImpl::ReadFirstLine(const std::string& fname,
                             WriteBatch* const batch) {
  struct LogReporter : public log::Reader::Reporter {
    Env* env;
    Logger* info_log;
    const char* fname;
    Status* status;  // nullptr if options_.paranoid_checks==false
    virtual void Corruption(size_t bytes, const Status& s) {
      Log(info_log, "%s%s: dropping %d bytes; %s",
          (this->status == nullptr ? "(ignoring error) " : ""),
          fname, static_cast<int>(bytes), s.ToString().c_str());
      if (this->status != nullptr && this->status->ok()) *this->status = s;
    }
  };

  unique_ptr<SequentialFile> file;
  Status status = env_->NewSequentialFile(fname, &file, storage_options_);

  if (!status.ok()) {
    return status;
  }


  LogReporter reporter;
  reporter.env = env_;
  reporter.info_log = options_.info_log.get();
  reporter.fname = fname.c_str();
  reporter.status = (options_.paranoid_checks ? &status : nullptr);
  log::Reader reader(std::move(file), &reporter, true/*checksum*/,
                     0/*initial_offset*/);
  std::string scratch;
  Slice record;

  if (reader.ReadRecord(&record, &scratch) && status.ok()) {
    if (record.size() < 12) {
      reporter.Corruption(
          record.size(), Status::Corruption("log record too small"));
      return Status::IOError("Corruption noted");
      //  TODO read record's till the first no corrupt entry?
    }
    WriteBatchInternal::SetContents(batch, record);
    return Status::OK();
  }
  return Status::IOError("Error reading from file " + fname);
}

struct CompareLogByPointer {
  bool operator() (const unique_ptr<LogFile>& a,
                   const unique_ptr<LogFile>& b) {
    LogFileImpl* a_impl = dynamic_cast<LogFileImpl*>(a.get());
    LogFileImpl* b_impl = dynamic_cast<LogFileImpl*>(b.get());
    return *a_impl < *b_impl;
  }
};

Status DBImpl::AppendSortedWalsOfType(const std::string& path,
    VectorLogPtr& log_files, WalFileType log_type) {
  std::vector<std::string> all_files;
  const Status status = env_->GetChildren(path, &all_files);
  if (!status.ok()) {
    return status;
  }
  log_files.reserve(log_files.size() + all_files.size());
  VectorLogPtr::iterator pos_start;
  if (!log_files.empty()) {
    pos_start = log_files.end() - 1;
  } else {
    pos_start = log_files.begin();
  }
  for (const auto& f : all_files) {
    uint64_t number;
    FileType type;
    if (ParseFileName(f, &number, &type) && type == kLogFile){

      WriteBatch batch;
      Status s = ReadFirstRecord(log_type, number, &batch);
      if (!s.ok()) {
        if (CheckWalFileExistsAndEmpty(log_type, number)) {
          continue;
        }
        return s;
      }

      uint64_t size_bytes;
      s = env_->GetFileSize(LogFileName(path, number), &size_bytes);
      if (!s.ok()) {
        return s;
      }

      log_files.push_back(std::move(unique_ptr<LogFile>(new LogFileImpl(
        number, log_type, WriteBatchInternal::Sequence(&batch), size_bytes))));
    }
  }
  CompareLogByPointer compare_log_files;
  std::sort(pos_start, log_files.end(), compare_log_files);
  return status;
}

void DBImpl::TEST_CompactRange(int level, const Slice* begin,const Slice* end) {
  assert(level >= 0);

  InternalKey begin_storage, end_storage;

  ManualCompaction manual;
  manual.level = level;
  manual.done = false;
  manual.in_progress = false;
  // For universal compaction, we enforce every manual compaction to compact
  // all files.
  if (begin == nullptr ||
      options_.compaction_style == kCompactionStyleUniversal) {
    manual.begin = nullptr;
  } else {
    begin_storage = InternalKey(*begin, kMaxSequenceNumber, kValueTypeForSeek);
    manual.begin = &begin_storage;
  }
  if (end == nullptr ||
      options_.compaction_style == kCompactionStyleUniversal) {
    manual.end = nullptr;
  } else {
    end_storage = InternalKey(*end, 0, static_cast<ValueType>(0));
    manual.end = &end_storage;
  }

  MutexLock l(&mutex_);

  // When a manual compaction arrives, temporarily throttle down
  // the number of background compaction threads to 1. This is
  // needed to ensure that this manual compaction can compact
  // any range of keys/files. We artificialy increase
  // bg_compaction_scheduled_ by a large number, this causes
  // the system to have a single background thread. Now,
  // this manual compaction can progress without stomping
  // on any other concurrent compactions.
  const int LargeNumber = 10000000;
  const int newvalue = options_.max_background_compactions-1;
  bg_compaction_scheduled_ += LargeNumber;
  while (bg_compaction_scheduled_ > LargeNumber) {
    Log(options_.info_log, "Manual compaction request waiting for background threads to fall below 1");
    bg_cv_.Wait();
  }
  Log(options_.info_log, "Manual compaction starting");

  while (!manual.done) {
    while (manual_compaction_ != nullptr) {
      bg_cv_.Wait();
    }
    manual_compaction_ = &manual;
    if (bg_compaction_scheduled_ == LargeNumber) {
      bg_compaction_scheduled_ = newvalue;
    }
    MaybeScheduleFlushOrCompaction();
    while (manual_compaction_ == &manual) {
      bg_cv_.Wait();
    }
  }
  assert(!manual.in_progress);

  // wait till there are no background threads scheduled
  bg_compaction_scheduled_ += LargeNumber;
  while (bg_compaction_scheduled_ > LargeNumber + newvalue) {
    Log(options_.info_log, "Manual compaction resetting background threads");
    bg_cv_.Wait();
  }
  bg_compaction_scheduled_ = 0;
}

Status DBImpl::FlushMemTable(const FlushOptions& options) {
  // nullptr batch means just wait for earlier writes to be done
  Status s = Write(WriteOptions(), nullptr);
  if (s.ok() && options.wait) {
    // Wait until the compaction completes
    s = WaitForFlushMemTable();
  }
  return s;
}

Status DBImpl::WaitForFlushMemTable() {
  Status s;
  // Wait until the compaction completes
  MutexLock l(&mutex_);
  while (imm_.size() > 0 && bg_error_.ok()) {
    bg_cv_.Wait();
  }
  if (imm_.size() != 0) {
    s = bg_error_;
  }
  return s;
}

Status DBImpl::TEST_FlushMemTable() {
  return FlushMemTable(FlushOptions());
}

Status DBImpl::TEST_WaitForFlushMemTable() {
  return WaitForFlushMemTable();
}

Status DBImpl::TEST_WaitForCompact() {
  // Wait until the compaction completes

  // TODO: a bug here. This function actually does not necessarily
  // wait for compact. It actually waits for scheduled compaction
  // OR flush to finish.

  MutexLock l(&mutex_);
  while ((bg_compaction_scheduled_ || bg_flush_scheduled_) &&
         bg_error_.ok()) {
    bg_cv_.Wait();
  }
  return bg_error_;
}

void DBImpl::MaybeScheduleFlushOrCompaction() {
  mutex_.AssertHeld();
  if (bg_work_gate_closed_) {
    // gate closed for backgrond work
  } else if (shutting_down_.Acquire_Load()) {
    // DB is being deleted; no more background compactions
  } else {
    bool is_flush_pending =
      imm_.IsFlushPending(options_.min_write_buffer_number_to_merge);
    if (is_flush_pending &&
        (bg_flush_scheduled_ < options_.max_background_flushes)) {
      // memtable flush needed
      bg_flush_scheduled_++;
      env_->Schedule(&DBImpl::BGWorkFlush, this, Env::Priority::HIGH);
    }

    if ((manual_compaction_ ||
         versions_->NeedsCompaction() ||
         (is_flush_pending && (options_.max_background_flushes <= 0))) &&
        bg_compaction_scheduled_ < options_.max_background_compactions) {
      // compaction needed, or memtable flush needed but HIGH pool not enabled.
      bg_compaction_scheduled_++;
      env_->Schedule(&DBImpl::BGWorkCompaction, this, Env::Priority::LOW);
    }
  }
}

void DBImpl::BGWorkFlush(void* db) {
  reinterpret_cast<DBImpl*>(db)->BackgroundCallFlush();
}

void DBImpl::BGWorkCompaction(void* db) {
  reinterpret_cast<DBImpl*>(db)->BackgroundCallCompaction();
}

Status DBImpl::BackgroundFlush(bool* madeProgress,
                               DeletionState& deletion_state) {
  Status stat;
  while (stat.ok() &&
         imm_.IsFlushPending(options_.min_write_buffer_number_to_merge)) {
    Log(options_.info_log,
        "BackgroundCallFlush doing FlushMemTableToOutputFile, flush slots available %d",
        options_.max_background_flushes - bg_flush_scheduled_);
    stat = FlushMemTableToOutputFile(madeProgress, deletion_state);
  }
  return stat;
}

void DBImpl::BackgroundCallFlush() {
  bool madeProgress = false;
  DeletionState deletion_state;
  assert(bg_flush_scheduled_);
  MutexLock l(&mutex_);

  Status s;
  if (!shutting_down_.Acquire_Load()) {
    s = BackgroundFlush(&madeProgress, deletion_state);
    if (!s.ok()) {
      // Wait a little bit before retrying background compaction in
      // case this is an environmental problem and we do not want to
      // chew up resources for failed compactions for the duration of
      // the problem.
      bg_cv_.SignalAll();  // In case a waiter can proceed despite the error
      Log(options_.info_log, "Waiting after background flush error: %s",
          s.ToString().c_str());
      mutex_.Unlock();
      LogFlush(options_.info_log);
      env_->SleepForMicroseconds(1000000);
      mutex_.Lock();
    }
  }

  // If !s.ok(), this means that Flush failed. In that case, we want
  // to delete all obsolete files and we force FindObsoleteFiles()
  FindObsoleteFiles(deletion_state, !s.ok());
  // delete unnecessary files if any, this is done outside the mutex
  if (deletion_state.HaveSomethingToDelete()) {
    mutex_.Unlock();
    PurgeObsoleteFiles(deletion_state);
    mutex_.Lock();
  }

  bg_flush_scheduled_--;
  if (madeProgress) {
    MaybeScheduleFlushOrCompaction();
  }
  bg_cv_.SignalAll();
}


void DBImpl::TEST_PurgeObsoleteteWAL() {
  PurgeObsoleteWALFiles();
}

void DBImpl::BackgroundCallCompaction() {
  bool madeProgress = false;
  DeletionState deletion_state;

  MaybeDumpStats();

  MutexLock l(&mutex_);
  // Log(options_.info_log, "XXX BG Thread %llx process new work item", pthread_self());
  assert(bg_compaction_scheduled_);
  Status s;
  if (!shutting_down_.Acquire_Load()) {
    s = BackgroundCompaction(&madeProgress, deletion_state);
    if (!s.ok()) {
      // Wait a little bit before retrying background compaction in
      // case this is an environmental problem and we do not want to
      // chew up resources for failed compactions for the duration of
      // the problem.
      bg_cv_.SignalAll();  // In case a waiter can proceed despite the error
      Log(options_.info_log, "Waiting after background compaction error: %s",
          s.ToString().c_str());
      mutex_.Unlock();
      LogFlush(options_.info_log);
      env_->SleepForMicroseconds(1000000);
      mutex_.Lock();
    }
  }

  // If !s.ok(), this means that Compaction failed. In that case, we want
  // to delete all obsolete files we might have created and we force
  // FindObsoleteFiles(). This is because deletion_state does not catch
  // all created files if compaction failed.
  FindObsoleteFiles(deletion_state, !s.ok());
  // delete unnecessary files if any, this is done outside the mutex
  if (deletion_state.HaveSomethingToDelete()) {
    mutex_.Unlock();
    PurgeObsoleteFiles(deletion_state);
    mutex_.Lock();
  }

  bg_compaction_scheduled_--;

  MaybeScheduleLogDBDeployStats();

  // Previous compaction may have produced too many files in a level,
  // So reschedule another compaction if we made progress in the
  // last compaction.
  if (madeProgress) {
    MaybeScheduleFlushOrCompaction();
  }
  bg_cv_.SignalAll();

}

Status DBImpl::BackgroundCompaction(bool* madeProgress,
  DeletionState& deletion_state) {
  *madeProgress = false;
  mutex_.AssertHeld();

  // TODO: remove memtable flush from formal compaction
  while (imm_.IsFlushPending(options_.min_write_buffer_number_to_merge)) {
    Log(options_.info_log,
        "BackgroundCompaction doing FlushMemTableToOutputFile, compaction slots "
        "available %d",
        options_.max_background_compactions - bg_compaction_scheduled_);
    Status stat = FlushMemTableToOutputFile(madeProgress, deletion_state);
    if (!stat.ok()) {
      return stat;
    }
  }

  unique_ptr<Compaction> c;
  bool is_manual = (manual_compaction_ != nullptr) &&
                   (manual_compaction_->in_progress == false);
  InternalKey manual_end;
  if (is_manual) {
    ManualCompaction* m = manual_compaction_;
    assert(!m->in_progress);
    m->in_progress = true; // another thread cannot pick up the same work
    c.reset(versions_->CompactRange(m->level, m->begin, m->end));
    if (c) {
      manual_end = c->input(0, c->num_input_files(0) - 1)->largest;
    } else {
      m->done = true;
    }
    Log(options_.info_log,
        "Manual compaction at level-%d from %s .. %s; will stop at %s\n",
        m->level,
        (m->begin ? m->begin->DebugString().c_str() : "(begin)"),
        (m->end ? m->end->DebugString().c_str() : "(end)"),
        (m->done ? "(end)" : manual_end.DebugString().c_str()));
  } else if (!options_.disable_auto_compactions) {
    c.reset(versions_->PickCompaction());
  }

  Status status;
  if (!c) {
    // Nothing to do
    Log(options_.info_log, "Compaction nothing to do");
  } else if (!is_manual && c->IsTrivialMove()) {
    // Move file to next level
    assert(c->num_input_files(0) == 1);
    FileMetaData* f = c->input(0, 0);
    c->edit()->DeleteFile(c->level(), f->number);
    c->edit()->AddFile(c->level() + 1, f->number, f->file_size,
                       f->smallest, f->largest,
                       f->smallest_seqno, f->largest_seqno);
    status = versions_->LogAndApply(c->edit(), &mutex_);
    VersionSet::LevelSummaryStorage tmp;
    Log(options_.info_log, "Moved #%lld to level-%d %lld bytes %s: %s\n",
        static_cast<unsigned long long>(f->number),
        c->level() + 1,
        static_cast<unsigned long long>(f->file_size),
        status.ToString().c_str(),
        versions_->LevelSummary(&tmp));
    versions_->ReleaseCompactionFiles(c.get(), status);
    *madeProgress = true;
  } else {
    MaybeScheduleFlushOrCompaction(); // do more compaction work in parallel.
    CompactionState* compact = new CompactionState(c.get());
    status = DoCompactionWork(compact, deletion_state);
    CleanupCompaction(compact, status);
    versions_->ReleaseCompactionFiles(c.get(), status);
    c->ReleaseInputs();
    versions_->GetObsoleteFiles(&deletion_state.sst_delete_files);
    *madeProgress = true;
  }
  c.reset();

  if (status.ok()) {
    // Done
  } else if (shutting_down_.Acquire_Load()) {
    // Ignore compaction errors found during shutting down
  } else {
    Log(options_.info_log,
        "Compaction error: %s", status.ToString().c_str());
    if (options_.paranoid_checks && bg_error_.ok()) {
      bg_error_ = status;
    }
  }

  if (is_manual) {
    ManualCompaction* m = manual_compaction_;
    if (!status.ok()) {
      m->done = true;
    }
    // For universal compaction:
    //   Because universal compaction always happens at level 0, so one
    //   compaction will pick up all overlapped files. No files will be
    //   filtered out due to size limit and left for a successive compaction.
    //   So we can safely conclude the current compaction.
    //
    //   Also note that, if we don't stop here, then the current compaction
    //   writes a new file back to level 0, which will be used in successive
    //   compaction. Hence the manual compaction will never finish.
    if (options_.compaction_style == kCompactionStyleUniversal) {
      m->done = true;
    }
    if (!m->done) {
      // We only compacted part of the requested range.  Update *m
      // to the range that is left to be compacted.
      m->tmp_storage = manual_end;
      m->begin = &m->tmp_storage;
    }
    m->in_progress = false; // not being processed anymore
    manual_compaction_ = nullptr;
  }
  return status;
}

void DBImpl::CleanupCompaction(CompactionState* compact, Status status) {
  mutex_.AssertHeld();
  if (compact->builder != nullptr) {
    // May happen if we get a shutdown call in the middle of compaction
    compact->builder->Abandon();
    compact->builder.reset();
  } else {
    assert(compact->outfile == nullptr);
  }
  for (size_t i = 0; i < compact->outputs.size(); i++) {
    const CompactionState::Output& out = compact->outputs[i];
    pending_outputs_.erase(out.number);

    // If this file was inserted into the table cache then remove
    // them here because this compaction was not committed.
    if (!status.ok()) {
      table_cache_->Evict(out.number);
    }
  }
  delete compact;
}

// Allocate the file numbers for the output file. We allocate as
// many output file numbers as there are files in level+1.
// Insert them into pending_outputs so that they do not get deleted.
void DBImpl::AllocateCompactionOutputFileNumbers(CompactionState* compact) {
  mutex_.AssertHeld();
  assert(compact != nullptr);
  assert(compact->builder == nullptr);
  int filesNeeded = compact->compaction->num_input_files(1);
  for (int i = 0; i < filesNeeded; i++) {
    uint64_t file_number = versions_->NewFileNumber();
    pending_outputs_.insert(file_number);
    compact->allocated_file_numbers.push_back(file_number);
  }
}

// Frees up unused file number.
void DBImpl::ReleaseCompactionUnusedFileNumbers(CompactionState* compact) {
  mutex_.AssertHeld();
  for (const auto file_number : compact->allocated_file_numbers) {
    pending_outputs_.erase(file_number);
    // Log(options_.info_log, "XXX releasing unused file num %d", file_number);
  }
}

Status DBImpl::OpenCompactionOutputFile(CompactionState* compact) {
  assert(compact != nullptr);
  assert(compact->builder == nullptr);
  uint64_t file_number;
  // If we have not yet exhausted the pre-allocated file numbers,
  // then use the one from the front. Otherwise, we have to acquire
  // the heavyweight lock and allocate a new file number.
  if (!compact->allocated_file_numbers.empty()) {
    file_number = compact->allocated_file_numbers.front();
    compact->allocated_file_numbers.pop_front();
  } else {
    mutex_.Lock();
    file_number = versions_->NewFileNumber();
    pending_outputs_.insert(file_number);
    mutex_.Unlock();
  }
  CompactionState::Output out;
  out.number = file_number;
  out.smallest.Clear();
  out.largest.Clear();
  out.smallest_seqno = out.largest_seqno = 0;
  compact->outputs.push_back(out);

  // Make the output file
  std::string fname = TableFileName(dbname_, file_number);
  Status s = env_->NewWritableFile(fname, &compact->outfile, storage_options_);

  if (s.ok()) {
    // Over-estimate slightly so we don't end up just barely crossing
    // the threshold.
    compact->outfile->SetPreallocationBlockSize(
      1.1 * versions_->MaxFileSizeForLevel(compact->compaction->output_level()));

    CompressionType compression_type = GetCompressionType(
        options_, compact->compaction->output_level(),
        compact->compaction->enable_compression());

    compact->builder.reset(
        GetTableBuilder(options_, compact->outfile.get(), compression_type));
  }
  LogFlush(options_.info_log);
  return s;
}

Status DBImpl::FinishCompactionOutputFile(CompactionState* compact,
                                          Iterator* input) {
  assert(compact != nullptr);
  assert(compact->outfile);
  assert(compact->builder != nullptr);

  const uint64_t output_number = compact->current_output()->number;
  assert(output_number != 0);

  // Check for iterator errors
  Status s = input->status();
  const uint64_t current_entries = compact->builder->NumEntries();
  if (s.ok()) {
    s = compact->builder->Finish();
  } else {
    compact->builder->Abandon();
  }
  const uint64_t current_bytes = compact->builder->FileSize();
  compact->current_output()->file_size = current_bytes;
  compact->total_bytes += current_bytes;
  compact->builder.reset();

  // Finish and check for file errors
  if (s.ok() && !options_.disableDataSync) {
    if (options_.use_fsync) {
      StopWatch sw(env_, options_.statistics, COMPACTION_OUTFILE_SYNC_MICROS);
      s = compact->outfile->Fsync();
    } else {
      StopWatch sw(env_, options_.statistics, COMPACTION_OUTFILE_SYNC_MICROS);
      s = compact->outfile->Sync();
    }
  }
  if (s.ok()) {
    s = compact->outfile->Close();
  }
  compact->outfile.reset();

  if (s.ok() && current_entries > 0) {
    // Verify that the table is usable
    Iterator* iter = table_cache_->NewIterator(ReadOptions(),
                                               storage_options_,
                                               output_number,
                                               current_bytes);
    s = iter->status();
    delete iter;
    if (s.ok()) {
      Log(options_.info_log,
          "Generated table #%llu: %lld keys, %lld bytes",
          (unsigned long long) output_number,
          (unsigned long long) current_entries,
          (unsigned long long) current_bytes);
    }
  }
  return s;
}


Status DBImpl::InstallCompactionResults(CompactionState* compact) {
  mutex_.AssertHeld();

  // paranoia: verify that the files that we started with
  // still exist in the current version and in the same original level.
  // This ensures that a concurrent compaction did not erroneously
  // pick the same files to compact.
  if (!versions_->VerifyCompactionFileConsistency(compact->compaction)) {
    Log(options_.info_log,  "Compaction %d@%d + %d@%d files aborted",
      compact->compaction->num_input_files(0),
      compact->compaction->level(),
      compact->compaction->num_input_files(1),
      compact->compaction->level() + 1);
    return Status::IOError("Compaction input files inconsistent");
  }

  Log(options_.info_log,  "Compacted %d@%d + %d@%d files => %lld bytes",
      compact->compaction->num_input_files(0),
      compact->compaction->level(),
      compact->compaction->num_input_files(1),
      compact->compaction->level() + 1,
      static_cast<long long>(compact->total_bytes));

  // Add compaction outputs
  compact->compaction->AddInputDeletions(compact->compaction->edit());
  const int level = compact->compaction->level();
  for (size_t i = 0; i < compact->outputs.size(); i++) {
    const CompactionState::Output& out = compact->outputs[i];
    compact->compaction->edit()->AddFile(
        (options_.compaction_style == kCompactionStyleUniversal) ?
          level : level + 1,
        out.number, out.file_size, out.smallest, out.largest,
        out.smallest_seqno, out.largest_seqno);
  }
  return versions_->LogAndApply(compact->compaction->edit(), &mutex_);
}

//
// Given a sequence number, return the sequence number of the
// earliest snapshot that this sequence number is visible in.
// The snapshots themselves are arranged in ascending order of
// sequence numbers.
// Employ a sequential search because the total number of
// snapshots are typically small.
inline SequenceNumber DBImpl::findEarliestVisibleSnapshot(
  SequenceNumber in, std::vector<SequenceNumber>& snapshots,
  SequenceNumber* prev_snapshot) {
  SequenceNumber prev __attribute__((unused)) = 0;
  for (const auto cur : snapshots) {
    assert(prev <= cur);
    if (cur >= in) {
      *prev_snapshot = prev;
      return cur;
    }
    prev = cur; // assignment
    assert(prev);
  }
  Log(options_.info_log,
      "Looking for seqid %llu but maxseqid is %llu", in,
      snapshots[snapshots.size()-1]);
  assert(0);
  return 0;
}

Status DBImpl::DoCompactionWork(CompactionState* compact,
                                DeletionState& deletion_state) {
  assert(compact);
  int64_t imm_micros = 0;  // Micros spent doing imm_ compactions
  Log(options_.info_log,
      "Compacting %d@%d + %d@%d files, score %.2f slots available %d",
      compact->compaction->num_input_files(0),
      compact->compaction->level(),
      compact->compaction->num_input_files(1),
      compact->compaction->level() + 1,
      compact->compaction->score(),
      options_.max_background_compactions - bg_compaction_scheduled_);
  char scratch[256];
  compact->compaction->Summary(scratch, sizeof(scratch));
  Log(options_.info_log, "Compaction start summary: %s\n", scratch);

  assert(versions_->NumLevelFiles(compact->compaction->level()) > 0);
  assert(compact->builder == nullptr);
  assert(!compact->outfile);

  SequenceNumber visible_at_tip = 0;
  SequenceNumber earliest_snapshot;
  SequenceNumber latest_snapshot = 0;
  snapshots_.getAll(compact->existing_snapshots);
  if (compact->existing_snapshots.size() == 0) {
    // optimize for fast path if there are no snapshots
    visible_at_tip = versions_->LastSequence();
    earliest_snapshot = visible_at_tip;
  } else {
    latest_snapshot = compact->existing_snapshots.back();
    // Add the current seqno as the 'latest' virtual
    // snapshot to the end of this list.
    compact->existing_snapshots.push_back(versions_->LastSequence());
    earliest_snapshot = compact->existing_snapshots[0];
  }

  // Is this compaction producing files at the bottommost level?
  bool bottommost_level = compact->compaction->BottomMostLevel();

  // Allocate the output file numbers before we release the lock
  AllocateCompactionOutputFileNumbers(compact);

  // Release mutex while we're actually doing the compaction work
  mutex_.Unlock();

  const uint64_t start_micros = env_->NowMicros();
  unique_ptr<Iterator> input(versions_->MakeInputIterator(compact->compaction));
  input->SeekToFirst();
  Status status;
  ParsedInternalKey ikey;
  std::string current_user_key;
  bool has_current_user_key = false;
  SequenceNumber last_sequence_for_key __attribute__((unused)) =
    kMaxSequenceNumber;
  SequenceNumber visible_in_snapshot = kMaxSequenceNumber;
  std::string compaction_filter_value;
  std::vector<char> delete_key; // for compaction filter
  MergeHelper merge(user_comparator(), options_.merge_operator.get(),
                    options_.info_log.get(),
                    false /* internal key corruption is expected */);
  auto compaction_filter = options_.compaction_filter;
  std::unique_ptr<CompactionFilter> compaction_filter_from_factory = nullptr;
  if (!compaction_filter) {
    auto context = compact->GetFilterContext();
    compaction_filter_from_factory =
      options_.compaction_filter_factory->CreateCompactionFilter(context);
    compaction_filter = compaction_filter_from_factory.get();
  }

  for (; input->Valid() && !shutting_down_.Acquire_Load(); ) {
    // Prioritize immutable compaction work
    // TODO: remove memtable flush from normal compaction work
    if (imm_.imm_flush_needed.NoBarrier_Load() != nullptr) {
      const uint64_t imm_start = env_->NowMicros();
      LogFlush(options_.info_log);
      mutex_.Lock();
      if (imm_.IsFlushPending(options_.min_write_buffer_number_to_merge)) {
        FlushMemTableToOutputFile(nullptr, deletion_state);
        bg_cv_.SignalAll();  // Wakeup MakeRoomForWrite() if necessary
      }
      mutex_.Unlock();
      imm_micros += (env_->NowMicros() - imm_start);
    }

    Slice key = input->key();
    Slice value = input->value();

    if (compact->compaction->ShouldStopBefore(key) &&
        compact->builder != nullptr) {
      status = FinishCompactionOutputFile(compact, input.get());
      if (!status.ok()) {
        break;
      }
    }

    // Handle key/value, add to state, etc.
    bool drop = false;
    bool current_entry_is_merging = false;
    if (!ParseInternalKey(key, &ikey)) {
      // Do not hide error keys
      // TODO: error key stays in db forever? Figure out the intention/rationale
      // v10 error v8 : we cannot hide v8 even though it's pretty obvious.
      current_user_key.clear();
      has_current_user_key = false;
      last_sequence_for_key = kMaxSequenceNumber;
      visible_in_snapshot = kMaxSequenceNumber;
    } else {
      if (!has_current_user_key ||
          user_comparator()->Compare(ikey.user_key,
                                     Slice(current_user_key)) != 0) {
        // First occurrence of this user key
        current_user_key.assign(ikey.user_key.data(), ikey.user_key.size());
        has_current_user_key = true;
        last_sequence_for_key = kMaxSequenceNumber;
        visible_in_snapshot = kMaxSequenceNumber;

        // apply the compaction filter to the first occurrence of the user key
        if (compaction_filter &&
            ikey.type == kTypeValue &&
            (visible_at_tip || ikey.sequence > latest_snapshot)) {
          // If the user has specified a compaction filter and the sequence
          // number is greater than any external snapshot, then invoke the
          // filter.
          // If the return value of the compaction filter is true, replace
          // the entry with a delete marker.
          bool value_changed = false;
          compaction_filter_value.clear();
          bool to_delete =
            compaction_filter->Filter(compact->compaction->level(),
                                               ikey.user_key, value,
                                               &compaction_filter_value,
                                               &value_changed);
          if (to_delete) {
            // make a copy of the original key
            delete_key.assign(key.data(), key.data() + key.size());
            // convert it to a delete
            UpdateInternalKey(&delete_key[0], delete_key.size(),
                              ikey.sequence, kTypeDeletion);
            // anchor the key again
            key = Slice(&delete_key[0], delete_key.size());
            // needed because ikey is backed by key
            ParseInternalKey(key, &ikey);
            // no value associated with delete
            value.clear();
            RecordTick(options_.statistics, COMPACTION_KEY_DROP_USER);
          } else if (value_changed) {
            value = compaction_filter_value;
          }
        }

      }

      // If there are no snapshots, then this kv affect visibility at tip.
      // Otherwise, search though all existing snapshots to find
      // the earlist snapshot that is affected by this kv.
      SequenceNumber prev_snapshot = 0; // 0 means no previous snapshot
      SequenceNumber visible = visible_at_tip ?
        visible_at_tip :
        findEarliestVisibleSnapshot(ikey.sequence,
                                    compact->existing_snapshots,
                                    &prev_snapshot);

      if (visible_in_snapshot == visible) {
        // If the earliest snapshot is which this key is visible in
        // is the same as the visibily of a previous instance of the
        // same key, then this kv is not visible in any snapshot.
        // Hidden by an newer entry for same user key
        // TODO: why not > ?
        assert(last_sequence_for_key >= ikey.sequence);
        drop = true;    // (A)
        RecordTick(options_.statistics, COMPACTION_KEY_DROP_NEWER_ENTRY);
      } else if (ikey.type == kTypeDeletion &&
                 ikey.sequence <= earliest_snapshot &&
                 compact->compaction->IsBaseLevelForKey(ikey.user_key)) {
        // For this user key:
        // (1) there is no data in higher levels
        // (2) data in lower levels will have larger sequence numbers
        // (3) data in layers that are being compacted here and have
        //     smaller sequence numbers will be dropped in the next
        //     few iterations of this loop (by rule (A) above).
        // Therefore this deletion marker is obsolete and can be dropped.
        drop = true;
        RecordTick(options_.statistics, COMPACTION_KEY_DROP_OBSOLETE);
      } else if (ikey.type == kTypeMerge) {
        // We know the merge type entry is not hidden, otherwise we would
        // have hit (A)
        // We encapsulate the merge related state machine in a different
        // object to minimize change to the existing flow. Turn out this
        // logic could also be nicely re-used for memtable flush purge
        // optimization in BuildTable.
        merge.MergeUntil(input.get(), prev_snapshot, bottommost_level,
                         options_.statistics);
        current_entry_is_merging = true;
        if (merge.IsSuccess()) {
          // Successfully found Put/Delete/(end-of-key-range) while merging
          // Get the merge result
          key = merge.key();
          ParseInternalKey(key, &ikey);
          value = merge.value();
        } else {
          // Did not find a Put/Delete/(end-of-key-range) while merging
          // We now have some stack of merge operands to write out.
          // NOTE: key,value, and ikey are now referring to old entries.
          //       These will be correctly set below.
          assert(!merge.keys().empty());
          assert(merge.keys().size() == merge.values().size());

          // Hack to make sure last_sequence_for_key is correct
          ParseInternalKey(merge.keys().front(), &ikey);
        }
      }

      last_sequence_for_key = ikey.sequence;
      visible_in_snapshot = visible;
    }
#if 0
    Log(options_.info_log,
        "  Compact: %s, seq %d, type: %d %d, drop: %d, is_base: %d, "
        "%d smallest_snapshot: %d level: %d bottommost %d",
        ikey.user_key.ToString().c_str(),
        (int)ikey.sequence, ikey.type, kTypeValue, drop,
        compact->compaction->IsBaseLevelForKey(ikey.user_key),
        (int)last_sequence_for_key, (int)earliest_snapshot,
        compact->compaction->level(), bottommost_level);
#endif

    if (!drop) {
      // We may write a single key (e.g.: for Put/Delete or successful merge).
      // Or we may instead have to write a sequence/list of keys.
      // We have to write a sequence iff we have an unsuccessful merge
      bool has_merge_list = current_entry_is_merging && !merge.IsSuccess();
      const std::deque<std::string>* keys = nullptr;
      const std::deque<std::string>* values = nullptr;
      std::deque<std::string>::const_reverse_iterator key_iter;
      std::deque<std::string>::const_reverse_iterator value_iter;
      if (has_merge_list) {
        keys = &merge.keys();
        values = &merge.values();
        key_iter = keys->rbegin();    // The back (*rbegin()) is the first key
        value_iter = values->rbegin();

        key = Slice(*key_iter);
        value = Slice(*value_iter);
      }

      // If we have a list of keys to write, traverse the list.
      // If we have a single key to write, simply write that key.
      while (true) {
        // Invariant: key,value,ikey will always be the next entry to write
        char* kptr = (char*)key.data();
        std::string kstr;

        // Zeroing out the sequence number leads to better compression.
        // If this is the bottommost level (no files in lower levels)
        // and the earliest snapshot is larger than this seqno
        // then we can squash the seqno to zero.
        if (options_.compaction_style == kCompactionStyleLevel &&
            bottommost_level && ikey.sequence < earliest_snapshot &&
            ikey.type != kTypeMerge) {
          assert(ikey.type != kTypeDeletion);
          // make a copy because updating in place would cause problems
          // with the priority queue that is managing the input key iterator
          kstr.assign(key.data(), key.size());
          kptr = (char *)kstr.c_str();
          UpdateInternalKey(kptr, key.size(), (uint64_t)0, ikey.type);
        }

        Slice newkey(kptr, key.size());
        assert((key.clear(), 1)); // we do not need 'key' anymore

        // Open output file if necessary
        if (compact->builder == nullptr) {
          status = OpenCompactionOutputFile(compact);
          if (!status.ok()) {
            break;
          }
        }

        SequenceNumber seqno = GetInternalKeySeqno(newkey);
        if (compact->builder->NumEntries() == 0) {
          compact->current_output()->smallest.DecodeFrom(newkey);
          compact->current_output()->smallest_seqno = seqno;
        } else {
          compact->current_output()->smallest_seqno =
            std::min(compact->current_output()->smallest_seqno, seqno);
        }
        compact->current_output()->largest.DecodeFrom(newkey);
        compact->builder->Add(newkey, value);
        compact->current_output()->largest_seqno =
          std::max(compact->current_output()->largest_seqno, seqno);

        // Close output file if it is big enough
        if (compact->builder->FileSize() >=
            compact->compaction->MaxOutputFileSize()) {
          status = FinishCompactionOutputFile(compact, input.get());
          if (!status.ok()) {
            break;
          }
        }

        // If we have a list of entries, move to next element
        // If we only had one entry, then break the loop.
        if (has_merge_list) {
          ++key_iter;
          ++value_iter;

          // If at end of list
          if (key_iter == keys->rend() || value_iter == values->rend()) {
            // Sanity Check: if one ends, then both end
            assert(key_iter == keys->rend() && value_iter == values->rend());
            break;
          }

          // Otherwise not at end of list. Update key, value, and ikey.
          key = Slice(*key_iter);
          value = Slice(*value_iter);
          ParseInternalKey(key, &ikey);

        } else{
          // Only had one item to begin with (Put/Delete)
          break;
        }
      }
    }

    // MergeUntil has moved input to the next entry
    if (!current_entry_is_merging) {
      input->Next();
    }
  }

  if (status.ok() && shutting_down_.Acquire_Load()) {
    status = Status::IOError("Database shutdown started during compaction");
  }
  if (status.ok() && compact->builder != nullptr) {
    status = FinishCompactionOutputFile(compact, input.get());
  }
  if (status.ok()) {
    status = input->status();
  }
  input.reset();

  CompactionStats stats;
  stats.micros = env_->NowMicros() - start_micros - imm_micros;
  if (options_.statistics) {
    options_.statistics->measureTime(COMPACTION_TIME, stats.micros);
  }
  stats.files_in_leveln = compact->compaction->num_input_files(0);
  stats.files_in_levelnp1 = compact->compaction->num_input_files(1);

  int num_output_files = compact->outputs.size();
  if (compact->builder != nullptr) {
    // An error occured so ignore the last output.
    assert(num_output_files > 0);
    --num_output_files;
  }
  stats.files_out_levelnp1 = num_output_files;

  for (int i = 0; i < compact->compaction->num_input_files(0); i++)
    stats.bytes_readn += compact->compaction->input(0, i)->file_size;

  for (int i = 0; i < compact->compaction->num_input_files(1); i++)
    stats.bytes_readnp1 += compact->compaction->input(1, i)->file_size;

  for (int i = 0; i < num_output_files; i++) {
    stats.bytes_written += compact->outputs[i].file_size;
  }

  LogFlush(options_.info_log);
  mutex_.Lock();
  stats_[compact->compaction->output_level()].Add(stats);

  // if there were any unused file number (mostly in case of
  // compaction error), free up the entry from pending_putputs
  ReleaseCompactionUnusedFileNumbers(compact);

  if (status.ok()) {
    status = InstallCompactionResults(compact);
  }
  VersionSet::LevelSummaryStorage tmp;
  Log(options_.info_log,
      "compacted to: %s, %.1f MB/sec, level %d, files in(%d, %d) out(%d) "
      "MB in(%.1f, %.1f) out(%.1f), read-write-amplify(%.1f) "
      "write-amplify(%.1f) %s\n",
      versions_->LevelSummary(&tmp),
      (stats.bytes_readn + stats.bytes_readnp1 + stats.bytes_written) /
          (double) stats.micros,
      compact->compaction->output_level(),
      stats.files_in_leveln, stats.files_in_levelnp1, stats.files_out_levelnp1,
      stats.bytes_readn / 1048576.0,
      stats.bytes_readnp1 / 1048576.0,
      stats.bytes_written / 1048576.0,
      (stats.bytes_written + stats.bytes_readnp1 + stats.bytes_readn) /
          (double) stats.bytes_readn,
      stats.bytes_written / (double) stats.bytes_readn,
      status.ToString().c_str());

  return status;
}

namespace {
struct IterState {
  port::Mutex* mu;
  Version* version;
  std::vector<MemTable*> mem; // includes both mem_ and imm_
};

static void CleanupIteratorState(void* arg1, void* arg2) {
  IterState* state = reinterpret_cast<IterState*>(arg1);
  state->mu->Lock();
  for (unsigned int i = 0; i < state->mem.size(); i++) {
    state->mem[i]->Unref();
  }
  state->version->Unref();
  state->mu->Unlock();
  delete state;
}
}  // namespace

Iterator* DBImpl::NewInternalIterator(const ReadOptions& options,
                                      SequenceNumber* latest_snapshot) {
  IterState* cleanup = new IterState;
  mutex_.Lock();
  *latest_snapshot = versions_->LastSequence();

  // Collect together all needed child iterators for mem
  std::vector<Iterator*> list;
  mem_->Ref();
  list.push_back(mem_->NewIterator(options));

  cleanup->mem.push_back(mem_);

  // Collect together all needed child iterators for imm_
  std::vector<MemTable*> immutables;
  imm_.GetMemTables(&immutables);
  for (unsigned int i = 0; i < immutables.size(); i++) {
    MemTable* m = immutables[i];
    m->Ref();
    list.push_back(m->NewIterator(options));
    cleanup->mem.push_back(m);
  }

  // Collect iterators for files in L0 - Ln
  versions_->current()->AddIterators(options, storage_options_, &list);
  Iterator* internal_iter =
      NewMergingIterator(&internal_comparator_, &list[0], list.size());
  versions_->current()->Ref();

  cleanup->mu = &mutex_;
  cleanup->version = versions_->current();
  internal_iter->RegisterCleanup(CleanupIteratorState, cleanup, nullptr);

  mutex_.Unlock();
  LogFlush(options_.info_log);
  return internal_iter;
}

Iterator* DBImpl::TEST_NewInternalIterator() {
  SequenceNumber ignored;
  return NewInternalIterator(ReadOptions(), &ignored);
}

int64_t DBImpl::TEST_MaxNextLevelOverlappingBytes() {
  MutexLock l(&mutex_);
  return versions_->MaxNextLevelOverlappingBytes();
}

Status DBImpl::Get(const ReadOptions& options,
                   const Slice& key,
                   std::string* value) {
  return GetImpl(options, key, value);
}

Status DBImpl::GetImpl(const ReadOptions& options,
                       const Slice& key,
                       std::string* value,
                       bool* value_found) {
  Status s;

  StopWatch sw(env_, options_.statistics, DB_GET);
  SequenceNumber snapshot;
  mutex_.Lock();
  if (options.snapshot != nullptr) {
    snapshot = reinterpret_cast<const SnapshotImpl*>(options.snapshot)->number_;
  } else {
    snapshot = versions_->LastSequence();
  }

  MemTable* mem = mem_;
  MemTableList imm = imm_;
  Version* current = versions_->current();
  mem->Ref();
  imm.RefAll();
  current->Ref();

  // Unlock while reading from files and memtables
  mutex_.Unlock();
  bool have_stat_update = false;
  Version::GetStats stats;


  // Prepare to store a list of merge operations if merge occurs.
  std::deque<std::string> merge_operands;

  // First look in the memtable, then in the immutable memtable (if any).
  // s is both in/out. When in, s could either be OK or MergeInProgress.
  // merge_operands will contain the sequence of merges in the latter case.
  LookupKey lkey(key, snapshot);
  if (mem->Get(lkey, value, &s, &merge_operands, options_)) {
    // Done
  } else if (imm.Get(lkey, value, &s, &merge_operands, options_)) {
    // Done
  } else {
    current->Get(options, lkey, value, &s, &merge_operands, &stats,
                 options_, value_found);
    have_stat_update = true;
  }
  mutex_.Lock();

  if (!options_.disable_seek_compaction &&
      have_stat_update && current->UpdateStats(stats)) {
    MaybeScheduleFlushOrCompaction();
  }
  mem->Unref();
  imm.UnrefAll();
  current->Unref();
  mutex_.Unlock();

  LogFlush(options_.info_log);
  // Note, tickers are atomic now - no lock protection needed any more.
  RecordTick(options_.statistics, NUMBER_KEYS_READ);
  RecordTick(options_.statistics, BYTES_READ, value->size());
  return s;
}

std::vector<Status> DBImpl::MultiGet(const ReadOptions& options,
                                     const std::vector<Slice>& keys,
                                     std::vector<std::string>* values) {

  StopWatch sw(env_, options_.statistics, DB_MULTIGET);
  SequenceNumber snapshot;
  mutex_.Lock();
  if (options.snapshot != nullptr) {
    snapshot = reinterpret_cast<const SnapshotImpl*>(options.snapshot)->number_;
  } else {
    snapshot = versions_->LastSequence();
  }

  MemTable* mem = mem_;
  MemTableList imm = imm_;
  Version* current = versions_->current();
  mem->Ref();
  imm.RefAll();
  current->Ref();

  // Unlock while reading from files and memtables

  mutex_.Unlock();
  bool have_stat_update = false;
  Version::GetStats stats;

  // Prepare to store a list of merge operations if merge occurs.
  std::deque<std::string> merge_operands;

  // Note: this always resizes the values array
  int numKeys = keys.size();
  std::vector<Status> statList(numKeys);
  values->resize(numKeys);

  // Keep track of bytes that we read for statistics-recording later
  uint64_t bytesRead = 0;

  // For each of the given keys, apply the entire "get" process as follows:
  // First look in the memtable, then in the immutable memtable (if any).
  // s is both in/out. When in, s could either be OK or MergeInProgress.
  // merge_operands will contain the sequence of merges in the latter case.
  for (int i=0; i<numKeys; ++i) {
    merge_operands.clear();
    Status& s = statList[i];
    std::string* value = &(*values)[i];

    LookupKey lkey(keys[i], snapshot);
    if (mem->Get(lkey, value, &s, &merge_operands, options_)) {
      // Done
    } else if (imm.Get(lkey, value, &s, &merge_operands, options_)) {
      // Done
    } else {
      current->Get(options, lkey, value, &s, &merge_operands, &stats, options_);
      have_stat_update = true;
    }

    if (s.ok()) {
      bytesRead += value->size();
    }
  }

  // Post processing (decrement reference counts and record statistics)
  mutex_.Lock();
  if (!options_.disable_seek_compaction &&
      have_stat_update && current->UpdateStats(stats)) {
    MaybeScheduleFlushOrCompaction();
  }
  mem->Unref();
  imm.UnrefAll();
  current->Unref();
  mutex_.Unlock();

  LogFlush(options_.info_log);
  RecordTick(options_.statistics, NUMBER_MULTIGET_CALLS);
  RecordTick(options_.statistics, NUMBER_MULTIGET_KEYS_READ, numKeys);
  RecordTick(options_.statistics, NUMBER_MULTIGET_BYTES_READ, bytesRead);

  return statList;
}

bool DBImpl::KeyMayExist(const ReadOptions& options,
                         const Slice& key,
                         std::string* value,
                         bool* value_found) {
  if (value_found != nullptr) {
    *value_found = true; // falsify later if key-may-exist but can't fetch value
  }
  ReadOptions roptions = options;
  roptions.read_tier = kBlockCacheTier; // read from block cache only
  return GetImpl(roptions, key, value, value_found).ok();
}

Iterator* DBImpl::NewIterator(const ReadOptions& options) {
  SequenceNumber latest_snapshot;
  Iterator* iter = NewInternalIterator(options, &latest_snapshot);
  iter = NewDBIterator(
             &dbname_, env_, options_, user_comparator(), iter,
             (options.snapshot != nullptr
              ? reinterpret_cast<const SnapshotImpl*>(options.snapshot)->number_
              : latest_snapshot));
  if (options.prefix) {
    // use extra wrapper to exclude any keys from the results which
    // don't begin with the prefix
    iter = new PrefixFilterIterator(iter, *options.prefix,
                                    options_.prefix_extractor);
  }
  return iter;
}

const Snapshot* DBImpl::GetSnapshot() {
  MutexLock l(&mutex_);
  return snapshots_.New(versions_->LastSequence());
}

void DBImpl::ReleaseSnapshot(const Snapshot* s) {
  MutexLock l(&mutex_);
  snapshots_.Delete(reinterpret_cast<const SnapshotImpl*>(s));
}

// Convenience methods
Status DBImpl::Put(const WriteOptions& o, const Slice& key, const Slice& val) {
  return DB::Put(o, key, val);
}

Status DBImpl::Merge(const WriteOptions& o, const Slice& key,
                     const Slice& val) {
  if (!options_.merge_operator) {
    return Status::NotSupported("Provide a merge_operator when opening DB");
  } else {
    return DB::Merge(o, key, val);
  }
}

Status DBImpl::Delete(const WriteOptions& options, const Slice& key) {
  return DB::Delete(options, key);
}

Status DBImpl::Write(const WriteOptions& options, WriteBatch* my_batch) {
  Writer w(&mutex_);
  w.batch = my_batch;
  w.sync = options.sync;
  w.disableWAL = options.disableWAL;
  w.done = false;

  StopWatch sw(env_, options_.statistics, DB_WRITE);
  MutexLock l(&mutex_);
  writers_.push_back(&w);
  while (!w.done && &w != writers_.front()) {
    w.cv.Wait();
  }
  if (w.done) {
    return w.status;
  }

  // May temporarily unlock and wait.
  Status status = MakeRoomForWrite(my_batch == nullptr);
  uint64_t last_sequence = versions_->LastSequence();
  Writer* last_writer = &w;
  if (status.ok() && my_batch != nullptr) {  // nullptr batch is for compactions
    // TODO: BuildBatchGroup physically concatenate/copy all write batches into
    // a new one. Mem copy is done with the lock held. Ideally, we only need
    // the lock to obtain the last_writer and the references to all batches.
    // Creation (copy) of the merged batch could have been done outside of the
    // lock protected region.
    WriteBatch* updates = BuildBatchGroup(&last_writer);

    // Add to log and apply to memtable.  We can release the lock
    // during this phase since &w is currently responsible for logging
    // and protects against concurrent loggers and concurrent writes
    // into mem_.
    {
      mutex_.Unlock();
      const SequenceNumber current_sequence = last_sequence + 1;
      WriteBatchInternal::SetSequence(updates, current_sequence);
      int my_batch_count = WriteBatchInternal::Count(updates);
      last_sequence += my_batch_count;
      // Record statistics
      RecordTick(options_.statistics, NUMBER_KEYS_WRITTEN, my_batch_count);
      RecordTick(options_.statistics,
                 BYTES_WRITTEN,
                 WriteBatchInternal::ByteSize(updates));
      if (options.disableWAL) {
        flush_on_destroy_ = true;
      }

      if (!options.disableWAL) {
        StopWatchNano timer(env_);
        StartPerfTimer(&timer);
        status = log_->AddRecord(WriteBatchInternal::Contents(updates));
        BumpPerfTime(&perf_context.wal_write_time, &timer);
        if (status.ok() && options.sync) {
          if (options_.use_fsync) {
            StopWatch(env_, options_.statistics, WAL_FILE_SYNC_MICROS);
            status = log_->file()->Fsync();
          } else {
            StopWatch(env_, options_.statistics, WAL_FILE_SYNC_MICROS);
            status = log_->file()->Sync();
          }
        }
      }
      if (status.ok()) {
        status = WriteBatchInternal::InsertInto(updates, mem_, &options_, this,
                                                options_.filter_deletes);
        if (!status.ok()) {
          // Panic for in-memory corruptions
          // Note that existing logic was not sound. Any partial failure writing
          // into the memtable would result in a state that some write ops might
          // have succeeded in memtable but Status reports error for all writes.
          throw std::runtime_error("In memory WriteBatch corruption!");
        }
        SetTickerCount(options_.statistics, SEQUENCE_NUMBER, last_sequence);
      }
      LogFlush(options_.info_log);
      mutex_.Lock();
      if (status.ok()) {
        versions_->SetLastSequence(last_sequence);
      }
    }
    if (updates == &tmp_batch_) tmp_batch_.Clear();
  }
  if (options_.paranoid_checks && !status.ok() && bg_error_.ok()) {
    bg_error_ = status; // stop compaction & fail any further writes
  }

  while (true) {
    Writer* ready = writers_.front();
    writers_.pop_front();
    if (ready != &w) {
      ready->status = status;
      ready->done = true;
      ready->cv.Signal();
    }
    if (ready == last_writer) break;
  }

  // Notify new head of write queue
  if (!writers_.empty()) {
    writers_.front()->cv.Signal();
  }
  return status;
}

// REQUIRES: Writer list must be non-empty
// REQUIRES: First writer must have a non-nullptr batch
WriteBatch* DBImpl::BuildBatchGroup(Writer** last_writer) {
  assert(!writers_.empty());
  Writer* first = writers_.front();
  WriteBatch* result = first->batch;
  assert(result != nullptr);

  size_t size = WriteBatchInternal::ByteSize(first->batch);

  // Allow the group to grow up to a maximum size, but if the
  // original write is small, limit the growth so we do not slow
  // down the small write too much.
  size_t max_size = 1 << 20;
  if (size <= (128<<10)) {
    max_size = size + (128<<10);
  }

  *last_writer = first;
  std::deque<Writer*>::iterator iter = writers_.begin();
  ++iter;  // Advance past "first"
  for (; iter != writers_.end(); ++iter) {
    Writer* w = *iter;
    if (w->sync && !first->sync) {
      // Do not include a sync write into a batch handled by a non-sync write.
      break;
    }

    if (!w->disableWAL && first->disableWAL) {
      // Do not include a write that needs WAL into a batch that has
      // WAL disabled.
      break;
    }

    if (w->batch != nullptr) {
      size += WriteBatchInternal::ByteSize(w->batch);
      if (size > max_size) {
        // Do not make batch too big
        break;
      }

      // Append to *reuslt
      if (result == first->batch) {
        // Switch to temporary batch instead of disturbing caller's batch
        result = &tmp_batch_;
        assert(WriteBatchInternal::Count(result) == 0);
        WriteBatchInternal::Append(result, first->batch);
      }
      WriteBatchInternal::Append(result, w->batch);
    }
    *last_writer = w;
  }
  return result;
}

// This function computes the amount of time in microseconds by which a write
// should be delayed based on the number of level-0 files according to the
// following formula:
// if n < bottom, return 0;
// if n >= top, return 1000;
// otherwise, let r = (n - bottom) /
//                    (top - bottom)
//  and return r^2 * 1000.
// The goal of this formula is to gradually increase the rate at which writes
// are slowed. We also tried linear delay (r * 1000), but it seemed to do
// slightly worse. There is no other particular reason for choosing quadratic.
uint64_t DBImpl::SlowdownAmount(int n, int top, int bottom) {
  uint64_t delay;
  if (n >= top) {
    delay = 1000;
  }
  else if (n < bottom) {
    delay = 0;
  }
  else {
    // If we are here, we know that:
    //   level0_start_slowdown <= n < level0_slowdown
    // since the previous two conditions are false.
    float how_much =
      (float) (n - bottom) /
              (top - bottom);
    delay = how_much * how_much * 1000;
  }
  assert(delay <= 1000);
  return delay;
}

// REQUIRES: mutex_ is held
// REQUIRES: this thread is currently at the front of the writer queue
Status DBImpl::MakeRoomForWrite(bool force) {
  mutex_.AssertHeld();
  assert(!writers_.empty());
  bool allow_delay = !force;
  bool allow_hard_rate_limit_delay = !force;
  bool allow_soft_rate_limit_delay = !force;
  uint64_t rate_limit_delay_millis = 0;
  Status s;
  double score;

  while (true) {
    if (!bg_error_.ok()) {
      // Yield previous error
      s = bg_error_;
      break;
    } else if (
        allow_delay &&
        versions_->NumLevelFiles(0) >=
          options_.level0_slowdown_writes_trigger) {
      // We are getting close to hitting a hard limit on the number of
      // L0 files.  Rather than delaying a single write by several
      // seconds when we hit the hard limit, start delaying each
      // individual write by 0-1ms to reduce latency variance.  Also,
      // this delay hands over some CPU to the compaction thread in
      // case it is sharing the same core as the writer.
      mutex_.Unlock();
      uint64_t delayed;
      {
        StopWatch sw(env_, options_.statistics, STALL_L0_SLOWDOWN_COUNT);
        env_->SleepForMicroseconds(
          SlowdownAmount(versions_->NumLevelFiles(0),
                         options_.level0_slowdown_writes_trigger,
                         options_.level0_stop_writes_trigger)
        );
        delayed = sw.ElapsedMicros();
      }
      RecordTick(options_.statistics, STALL_L0_SLOWDOWN_MICROS, delayed);
      stall_level0_slowdown_ += delayed;
      stall_level0_slowdown_count_++;
      allow_delay = false;  // Do not delay a single write more than once
      //Log(options_.info_log,
      //    "delaying write %llu usecs for level0_slowdown_writes_trigger\n",
      //     (long long unsigned int)delayed);
      mutex_.Lock();
      delayed_writes_++;
    } else if (!force &&
               (mem_->ApproximateMemoryUsage() <= options_.write_buffer_size)) {
      // There is room in current memtable
      if (allow_delay) {
        DelayLoggingAndReset();
      }
      break;
    } else if (imm_.size() == options_.max_write_buffer_number - 1) {
      // We have filled up the current memtable, but the previous
      // ones are still being compacted, so we wait.
      DelayLoggingAndReset();
      Log(options_.info_log, "wait for memtable compaction...\n");
      uint64_t stall;
      {
        StopWatch sw(env_, options_.statistics,
          STALL_MEMTABLE_COMPACTION_COUNT);
        bg_cv_.Wait();
        stall = sw.ElapsedMicros();
      }
      RecordTick(options_.statistics, STALL_MEMTABLE_COMPACTION_MICROS, stall);
      stall_memtable_compaction_ += stall;
      stall_memtable_compaction_count_++;
    } else if (versions_->NumLevelFiles(0) >=
               options_.level0_stop_writes_trigger) {
      // There are too many level-0 files.
      DelayLoggingAndReset();
      Log(options_.info_log, "wait for fewer level0 files...\n");
      uint64_t stall;
      {
        StopWatch sw(env_, options_.statistics, STALL_L0_NUM_FILES_COUNT);
        bg_cv_.Wait();
        stall = sw.ElapsedMicros();
      }
      RecordTick(options_.statistics, STALL_L0_NUM_FILES_MICROS, stall);
      stall_level0_num_files_ += stall;
      stall_level0_num_files_count_++;
    } else if (
        allow_hard_rate_limit_delay &&
        options_.hard_rate_limit > 1.0 &&
        (score = versions_->MaxCompactionScore()) > options_.hard_rate_limit) {
      // Delay a write when the compaction score for any level is too large.
      int max_level = versions_->MaxCompactionScoreLevel();
      mutex_.Unlock();
      uint64_t delayed;
      {
        StopWatch sw(env_, options_.statistics, HARD_RATE_LIMIT_DELAY_COUNT);
        env_->SleepForMicroseconds(1000);
        delayed = sw.ElapsedMicros();
      }
      stall_leveln_slowdown_[max_level] += delayed;
      stall_leveln_slowdown_count_[max_level]++;
      // Make sure the following value doesn't round to zero.
      uint64_t rate_limit = std::max((delayed / 1000), (uint64_t) 1);
      rate_limit_delay_millis += rate_limit;
      RecordTick(options_.statistics, RATE_LIMIT_DELAY_MILLIS, rate_limit);
      if (options_.rate_limit_delay_max_milliseconds > 0 &&
          rate_limit_delay_millis >=
          (unsigned)options_.rate_limit_delay_max_milliseconds) {
        allow_hard_rate_limit_delay = false;
      }
      // Log(options_.info_log,
      //    "delaying write %llu usecs for rate limits with max score %.2f\n",
      //    (long long unsigned int)delayed, score);
      mutex_.Lock();
    } else if (
        allow_soft_rate_limit_delay &&
        options_.soft_rate_limit > 0.0 &&
        (score = versions_->MaxCompactionScore()) > options_.soft_rate_limit) {
      // Delay a write when the compaction score for any level is too large.
      // TODO: add statistics
      mutex_.Unlock();
      {
        StopWatch sw(env_, options_.statistics, SOFT_RATE_LIMIT_DELAY_COUNT);
        env_->SleepForMicroseconds(SlowdownAmount(
          score,
          options_.soft_rate_limit,
          options_.hard_rate_limit)
        );
        rate_limit_delay_millis += sw.ElapsedMicros();
      }
      allow_soft_rate_limit_delay = false;
      mutex_.Lock();
    } else {
      // Attempt to switch to a new memtable and trigger compaction of old
      DelayLoggingAndReset();
      assert(versions_->PrevLogNumber() == 0);
      uint64_t new_log_number = versions_->NewFileNumber();
      unique_ptr<WritableFile> lfile;
      EnvOptions soptions(storage_options_);
      soptions.use_mmap_writes = false;
      s = env_->NewWritableFile(
            LogFileName(options_.wal_dir, new_log_number),
            &lfile,
            soptions
          );
      if (!s.ok()) {
        // Avoid chewing through file number space in a tight loop.
        versions_->ReuseFileNumber(new_log_number);
        break;
      }
      // Our final size should be less than write_buffer_size
      // (compression, etc) but err on the side of caution.
      lfile->SetPreallocationBlockSize(1.1 * options_.write_buffer_size);
      logfile_number_ = new_log_number;
      log_.reset(new log::Writer(std::move(lfile)));
      mem_->SetNextLogNumber(logfile_number_);
      imm_.Add(mem_);
      if (force) {
        imm_.FlushRequested();
      }
      mem_ = new MemTable(
          internal_comparator_, mem_rep_factory_, NumberLevels(), options_);
      mem_->Ref();
      Log(options_.info_log,
          "New memtable created with log file: #%llu\n",
          logfile_number_);
      mem_->SetLogNumber(logfile_number_);
      force = false;   // Do not force another compaction if have room
      MaybeScheduleFlushOrCompaction();
    }
  }
  return s;
}

bool DBImpl::GetProperty(const Slice& property, std::string* value) {
  value->clear();

  MutexLock l(&mutex_);
  Slice in = property;
  Slice prefix("rocksdb.");
  if (!in.starts_with(prefix)) return false;
  in.remove_prefix(prefix.size());

  if (in.starts_with("num-files-at-level")) {
    in.remove_prefix(strlen("num-files-at-level"));
    uint64_t level;
    bool ok = ConsumeDecimalNumber(&in, &level) && in.empty();
    if (!ok || (int)level >= NumberLevels()) {
      return false;
    } else {
      char buf[100];
      snprintf(buf, sizeof(buf), "%d",
               versions_->NumLevelFiles(static_cast<int>(level)));
      *value = buf;
      return true;
    }
  } else if (in == "levelstats") {
    char buf[1000];
    snprintf(buf, sizeof(buf),
             "Level Files Size(MB)\n"
             "--------------------\n");
    value->append(buf);

    for (int level = 0; level < NumberLevels(); level++) {
      snprintf(buf, sizeof(buf),
               "%3d %8d %8.0f\n",
               level,
               versions_->NumLevelFiles(level),
               versions_->NumLevelBytes(level) / 1048576.0);
      value->append(buf);
    }
    return true;

  } else if (in == "stats") {
    char buf[1000];
    uint64_t total_bytes_written = 0;
    uint64_t total_bytes_read = 0;
    uint64_t micros_up = env_->NowMicros() - started_at_;
    // Add "+1" to make sure seconds_up is > 0 and avoid NaN later
    double seconds_up = (micros_up + 1) / 1000000.0;
    uint64_t total_slowdown = 0;
    uint64_t total_slowdown_count = 0;
    uint64_t interval_bytes_written = 0;
    uint64_t interval_bytes_read = 0;
    uint64_t interval_bytes_new = 0;
    double   interval_seconds_up = 0;

    // Pardon the long line but I think it is easier to read this way.
    snprintf(buf, sizeof(buf),
             "                               Compactions\n"
             "Level  Files Size(MB) Score Time(sec)  Read(MB) Write(MB)    Rn(MB)  Rnp1(MB)  Wnew(MB) RW-Amplify Read(MB/s) Write(MB/s)      Rn     Rnp1     Wnp1     NewW    Count  Ln-stall Stall-cnt\n"
             "--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------\n"
             );
    value->append(buf);
    for (int level = 0; level < NumberLevels(); level++) {
      int files = versions_->NumLevelFiles(level);
      if (stats_[level].micros > 0 || files > 0) {
        int64_t bytes_read = stats_[level].bytes_readn +
                             stats_[level].bytes_readnp1;
        int64_t bytes_new = stats_[level].bytes_written -
                            stats_[level].bytes_readnp1;
        double amplify = (stats_[level].bytes_readn == 0)
            ? 0.0
            : (stats_[level].bytes_written +
               stats_[level].bytes_readnp1 +
               stats_[level].bytes_readn) /
                (double) stats_[level].bytes_readn;

        total_bytes_read += bytes_read;
        total_bytes_written += stats_[level].bytes_written;

        snprintf(
            buf, sizeof(buf),
            "%3d %8d %8.0f %5.1f %9.0f %9.0f %9.0f %9.0f %9.0f %9.0f %10.1f %9.1f %11.1f %8d %8d %8d %8d %8d %9.1f %9lu\n",
            level,
            files,
            versions_->NumLevelBytes(level) / 1048576.0,
            versions_->NumLevelBytes(level) /
                versions_->MaxBytesForLevel(level),
            stats_[level].micros / 1e6,
            bytes_read / 1048576.0,
            stats_[level].bytes_written / 1048576.0,
            stats_[level].bytes_readn / 1048576.0,
            stats_[level].bytes_readnp1 / 1048576.0,
            bytes_new / 1048576.0,
            amplify,
            // +1 to avoid division by 0
            (bytes_read / 1048576.0) / ((stats_[level].micros+1) / 1000000.0),
            (stats_[level].bytes_written / 1048576.0) /
                ((stats_[level].micros+1) / 1000000.0),
            stats_[level].files_in_leveln,
            stats_[level].files_in_levelnp1,
            stats_[level].files_out_levelnp1,
            stats_[level].files_out_levelnp1 - stats_[level].files_in_levelnp1,
            stats_[level].count,
            stall_leveln_slowdown_[level] / 1000000.0,
            (unsigned long) stall_leveln_slowdown_count_[level]);
        total_slowdown += stall_leveln_slowdown_[level];
        total_slowdown_count += stall_leveln_slowdown_count_[level];
        value->append(buf);
      }
    }

    interval_bytes_new = stats_[0].bytes_written - last_stats_.bytes_new_;
    interval_bytes_read = total_bytes_read - last_stats_.bytes_read_;
    interval_bytes_written = total_bytes_written - last_stats_.bytes_written_;
    interval_seconds_up = seconds_up - last_stats_.seconds_up_;

    snprintf(buf, sizeof(buf), "Uptime(secs): %.1f total, %.1f interval\n",
             seconds_up, interval_seconds_up);
    value->append(buf);

    snprintf(buf, sizeof(buf),
             "Compaction IO cumulative (GB): "
             "%.2f new, %.2f read, %.2f write, %.2f read+write\n",
             stats_[0].bytes_written / (1048576.0 * 1024),
             total_bytes_read / (1048576.0 * 1024),
             total_bytes_written / (1048576.0 * 1024),
             (total_bytes_read + total_bytes_written) / (1048576.0 * 1024));
    value->append(buf);

    snprintf(buf, sizeof(buf),
             "Compaction IO cumulative (MB/sec): "
             "%.1f new, %.1f read, %.1f write, %.1f read+write\n",
             stats_[0].bytes_written / 1048576.0 / seconds_up,
             total_bytes_read / 1048576.0 / seconds_up,
             total_bytes_written / 1048576.0 / seconds_up,
             (total_bytes_read + total_bytes_written) / 1048576.0 / seconds_up);
    value->append(buf);

    // +1 to avoid divide by 0 and NaN
    snprintf(buf, sizeof(buf),
             "Amplification cumulative: %.1f write, %.1f compaction\n",
             (double) total_bytes_written / (stats_[0].bytes_written+1),
             (double) (total_bytes_written + total_bytes_read)
                  / (stats_[0].bytes_written+1));
    value->append(buf);

    snprintf(buf, sizeof(buf),
             "Compaction IO interval (MB): "
             "%.2f new, %.2f read, %.2f write, %.2f read+write\n",
             interval_bytes_new / 1048576.0,
             interval_bytes_read/ 1048576.0,
             interval_bytes_written / 1048576.0,
             (interval_bytes_read + interval_bytes_written) / 1048576.0);
    value->append(buf);

    snprintf(buf, sizeof(buf),
             "Compaction IO interval (MB/sec): "
             "%.1f new, %.1f read, %.1f write, %.1f read+write\n",
             interval_bytes_new / 1048576.0 / interval_seconds_up,
             interval_bytes_read / 1048576.0 / interval_seconds_up,
             interval_bytes_written / 1048576.0 / interval_seconds_up,
             (interval_bytes_read + interval_bytes_written)
                 / 1048576.0 / interval_seconds_up);
    value->append(buf);

    // +1 to avoid divide by 0 and NaN
    snprintf(buf, sizeof(buf),
             "Amplification interval: %.1f write, %.1f compaction\n",
             (double) interval_bytes_written / (interval_bytes_new+1),
             (double) (interval_bytes_written + interval_bytes_read) /
                  (interval_bytes_new+1));
    value->append(buf);

    snprintf(buf, sizeof(buf),
            "Stalls(secs): %.3f level0_slowdown, %.3f level0_numfiles, "
            "%.3f memtable_compaction, %.3f leveln_slowdown\n",
            stall_level0_slowdown_ / 1000000.0,
            stall_level0_num_files_ / 1000000.0,
            stall_memtable_compaction_ / 1000000.0,
            total_slowdown / 1000000.0);
    value->append(buf);

    snprintf(buf, sizeof(buf),
            "Stalls(count): %lu level0_slowdown, %lu level0_numfiles, "
            "%lu memtable_compaction, %lu leveln_slowdown\n",
            (unsigned long) stall_level0_slowdown_count_,
            (unsigned long) stall_level0_num_files_count_,
            (unsigned long) stall_memtable_compaction_count_,
            (unsigned long) total_slowdown_count);
    value->append(buf);

    last_stats_.bytes_read_ = total_bytes_read;
    last_stats_.bytes_written_ = total_bytes_written;
    last_stats_.bytes_new_ = stats_[0].bytes_written;
    last_stats_.seconds_up_ = seconds_up;

    return true;
  } else if (in == "sstables") {
    *value = versions_->current()->DebugString();
    return true;
  } else if (in == "num-immutable-mem-table") {
    *value = std::to_string(imm_.size());
    return true;
  }

  return false;
}

void DBImpl::GetApproximateSizes(
    const Range* range, int n,
    uint64_t* sizes) {
  // TODO(opt): better implementation
  Version* v;
  {
    MutexLock l(&mutex_);
    versions_->current()->Ref();
    v = versions_->current();
  }

  for (int i = 0; i < n; i++) {
    // Convert user_key into a corresponding internal key.
    InternalKey k1(range[i].start, kMaxSequenceNumber, kValueTypeForSeek);
    InternalKey k2(range[i].limit, kMaxSequenceNumber, kValueTypeForSeek);
    uint64_t start = versions_->ApproximateOffsetOf(v, k1);
    uint64_t limit = versions_->ApproximateOffsetOf(v, k2);
    sizes[i] = (limit >= start ? limit - start : 0);
  }

  {
    MutexLock l(&mutex_);
    v->Unref();
  }
}

inline void DBImpl::DelayLoggingAndReset() {
  if (delayed_writes_ > 0) {
    Log(options_.info_log, "delayed %d write...\n", delayed_writes_ );
    delayed_writes_ = 0;
  }
}

Status DBImpl::DeleteFile(std::string name) {
  uint64_t number;
  FileType type;
  WalFileType log_type;
  if (!ParseFileName(name, &number, &type, &log_type) ||
      (type != kTableFile && type != kLogFile)) {
    Log(options_.info_log, "DeleteFile %s failed.\n", name.c_str());
    return Status::InvalidArgument("Invalid file name");
  }

  Status status;
  if (type == kLogFile) {
    // Only allow deleting archived log files
    if (log_type != kArchivedLogFile) {
      Log(options_.info_log, "DeleteFile %s failed.\n", name.c_str());
      return Status::NotSupported("Delete only supported for archived logs");
    }
    status = env_->DeleteFile(options_.wal_dir + "/" + name.c_str());
    if (!status.ok()) {
      Log(options_.info_log, "DeleteFile %s failed.\n", name.c_str());
    }
    return status;
  }

  int level;
  FileMetaData metadata;
  int maxlevel = NumberLevels();
  VersionEdit edit(maxlevel);
  DeletionState deletion_state;
  {
    MutexLock l(&mutex_);
    status = versions_->GetMetadataForFile(number, &level, &metadata);
    if (!status.ok()) {
      Log(options_.info_log, "DeleteFile %s failed. File not found\n",
                             name.c_str());
      return Status::InvalidArgument("File not found");
    }
    assert((level > 0) && (level < maxlevel));

    // If the file is being compacted no need to delete.
    if (metadata.being_compacted) {
      Log(options_.info_log,
          "DeleteFile %s Skipped. File about to be compacted\n", name.c_str());
      return Status::OK();
    }

    // Only the files in the last level can be deleted externally.
    // This is to make sure that any deletion tombstones are not
    // lost. Check that the level passed is the last level.
    for (int i = level + 1; i < maxlevel; i++) {
      if (versions_->NumLevelFiles(i) != 0) {
        Log(options_.info_log,
            "DeleteFile %s FAILED. File not in last level\n", name.c_str());
        return Status::InvalidArgument("File not in last level");
      }
    }
    edit.DeleteFile(level, number);
    status = versions_->LogAndApply(&edit, &mutex_);
    if (status.ok()) {
      versions_->GetObsoleteFiles(&deletion_state.sst_delete_files);
    }
    FindObsoleteFiles(deletion_state, false);
  } // lock released here
  LogFlush(options_.info_log);

  if (status.ok()) {
    // remove files outside the db-lock
    PurgeObsoleteFiles(deletion_state);
  }
  return status;
}

void DBImpl::GetLiveFilesMetaData(std::vector<LiveFileMetaData> *metadata) {
  MutexLock l(&mutex_);
  return versions_->GetLiveFilesMetaData(metadata);
}

// Default implementations of convenience methods that subclasses of DB
// can call if they wish
Status DB::Put(const WriteOptions& opt, const Slice& key, const Slice& value) {
  WriteBatch batch;
  batch.Put(key, value);
  return Write(opt, &batch);
}

Status DB::Delete(const WriteOptions& opt, const Slice& key) {
  WriteBatch batch;
  batch.Delete(key);
  return Write(opt, &batch);
}

Status DB::Merge(const WriteOptions& opt, const Slice& key,
                 const Slice& value) {
  WriteBatch batch;
  batch.Merge(key, value);
  return Write(opt, &batch);
}

DB::~DB() { }

Status DB::Open(const Options& options, const std::string& dbname, DB** dbptr) {
  *dbptr = nullptr;
  EnvOptions soptions;

  if (options.block_cache != nullptr && options.no_block_cache) {
    return Status::InvalidArgument(
        "no_block_cache is true while block_cache is not nullptr");
  }

  DBImpl* impl = new DBImpl(options, dbname);
  Status s = impl->env_->CreateDirIfMissing(impl->options_.wal_dir);
  if (!s.ok()) {
    delete impl;
    return s;
  }

  s = impl->CreateArchivalDirectory();
  if (!s.ok()) {
    delete impl;
    return s;
  }
  impl->mutex_.Lock();
  VersionEdit edit(impl->NumberLevels());
  s = impl->Recover(&edit); // Handles create_if_missing, error_if_exists
  if (s.ok()) {
    uint64_t new_log_number = impl->versions_->NewFileNumber();
    unique_ptr<WritableFile> lfile;
    soptions.use_mmap_writes = false;
    s = options.env->NewWritableFile(
      LogFileName(impl->options_.wal_dir, new_log_number),
      &lfile,
      soptions
    );
    if (s.ok()) {
      lfile->SetPreallocationBlockSize(1.1 * options.write_buffer_size);
      edit.SetLogNumber(new_log_number);
      impl->logfile_number_ = new_log_number;
      impl->log_.reset(new log::Writer(std::move(lfile)));
      s = impl->versions_->LogAndApply(&edit, &impl->mutex_);
    }
    if (s.ok()) {
      impl->mem_->SetLogNumber(impl->logfile_number_);
      impl->DeleteObsoleteFiles();
      impl->MaybeScheduleFlushOrCompaction();
      impl->MaybeScheduleLogDBDeployStats();
    }
  }
  impl->mutex_.Unlock();

  if (options.compaction_style == kCompactionStyleUniversal) {
    int num_files;
    for (int i = 1; i < impl->NumberLevels(); i++) {
      num_files = impl->versions_->NumLevelFiles(i);
      if (num_files > 0) {
        s = Status::InvalidArgument("Not all files are at level 0. Cannot "
          "open with universal compaction style.");
        break;
      }
    }
  }

  if (s.ok()) {
    *dbptr = impl;
  } else {
    delete impl;
  }
  return s;
}

Snapshot::~Snapshot() {
}

Status DestroyDB(const std::string& dbname, const Options& options) {
  const InternalKeyComparator comparator(options.comparator);
  const InternalFilterPolicy filter_policy(options.filter_policy);
  const Options& soptions(SanitizeOptions(
    dbname, &comparator, &filter_policy, options));
  Env* env = soptions.env;
  std::vector<std::string> filenames;
  std::vector<std::string> archiveFiles;

  std::string archivedir = ArchivalDirectory(dbname);
  // Ignore error in case directory does not exist
  env->GetChildren(dbname, &filenames);

  if (dbname != soptions.wal_dir) {
    std::vector<std::string> logfilenames;
    env->GetChildren(soptions.wal_dir, &logfilenames);
    filenames.insert(filenames.end(), logfilenames.begin(), logfilenames.end());
    archivedir = ArchivalDirectory(soptions.wal_dir);
  }

  if (filenames.empty()) {
    return Status::OK();
  }

  FileLock* lock;
  const std::string lockname = LockFileName(dbname);
  Status result = env->LockFile(lockname, &lock);
  if (result.ok()) {
    uint64_t number;
    FileType type;
    for (size_t i = 0; i < filenames.size(); i++) {
      if (ParseFileName(filenames[i], &number, &type) &&
          type != kDBLockFile) {  // Lock file will be deleted at end
        Status del;
        if (type == kMetaDatabase) {
          del = DestroyDB(dbname + "/" + filenames[i], options);
        } else if (type == kLogFile) {
          del = env->DeleteFile(soptions.wal_dir + "/" + filenames[i]);
        } else {
          del = env->DeleteFile(dbname + "/" + filenames[i]);
        }
        if (result.ok() && !del.ok()) {
          result = del;
        }
      }
    }

    env->GetChildren(archivedir, &archiveFiles);
    // Delete archival files.
    for (size_t i = 0; i < archiveFiles.size(); ++i) {
      if (ParseFileName(archiveFiles[i], &number, &type) &&
          type == kLogFile) {
        Status del = env->DeleteFile(archivedir + "/" + archiveFiles[i]);
        if (result.ok() && !del.ok()) {
          result = del;
        }
      }
    }
    // ignore case where no archival directory is present.
    env->DeleteDir(archivedir);

    env->UnlockFile(lock);  // Ignore error since state is already gone
    env->DeleteFile(lockname);
    env->DeleteDir(dbname);  // Ignore error in case dir contains other files
    env->DeleteDir(soptions.wal_dir);
  }
  return result;
}

//
// A global method that can dump out the build version
void dumpLeveldbBuildVersion(Logger * log) {
  Log(log, "Git sha %s", rocksdb_build_git_sha);
  Log(log, "Compile time %s %s",
      rocksdb_build_compile_time, rocksdb_build_compile_date);
}

}  // namespace rocksdb