rocksdb/HISTORY.md

# Rocksdb Change Log
## Unreleased
### Public API Change
* Now DB::Close() will return Aborted() error when there is unreleased snapshot. Users can retry after all snapshots are released.

### New Features
* Add an option `snap_refresh_nanos` (default to 0.1s) to periodically refresh the snapshot list in compaction jobs. Assign to 0 to disable the feature.
* Add an option `unordered_write` which trades snapshot guarantees with higher write throughput. When used with WRITE_PREPARED transactions with two_write_queues=true, it offers higher throughput with however no compromise on guarantees.
* Allow DBImplSecondary to remove memtables with obsolete data after replaying MANIFEST and WAL.
* Add an option `failed_move_fall_back_to_copy` (default is true) for external SST ingestion. When `move_files` is true and hard link fails, ingestion falls back to copy if `failed_move_fall_back_to_copy` is true. Otherwise, ingestion reports an error.

### Performance Improvements
* Reduce binary search when iterator reseek into the same data block.
* DBIter::Next() can skip user key checking if previous entry's seqnum is 0.
* Merging iterator to avoid child iterator reseek for some cases
* Reduce iterator key comparision for upper/lower bound check.
* Log Writer will flush after finishing the whole record, rather than a fragment.

### General Improvements
* Added new status code kColumnFamilyDropped to distinguish between Column Family Dropped and DB Shutdown in progress.

### Bug Fixes


## 6.2.0 (4/30/2019)
### New Features
* Add an option `strict_bytes_per_sync` that causes a file-writing thread to block rather than exceed the limit on bytes pending writeback specified by `bytes_per_sync` or `wal_bytes_per_sync`.
* Improve range scan performance by avoiding per-key upper bound check in BlockBasedTableIterator.
* Introduce Periodic Compaction for Level style compaction. Files are re-compacted periodically and put in the same level.
* Block-based table index now contains exact highest key in the file, rather than an upper bound. This may improve Get() and iterator Seek() performance in some situations, especially when direct IO is enabled and block cache is disabled. A setting BlockBasedTableOptions::index_shortening is introduced to control this behavior. Set it to kShortenSeparatorsAndSuccessor to get the old behavior.
* When reading from option file/string/map, customized envs can be filled according to object registry.
* Improve range scan performance when using explicit user readahead by not creating new table readers for every iterator.

### Public API Change
* Change the behavior of OptimizeForPointLookup(): move away from hash-based block-based-table index, and use whole key memtable filtering.
* Change the behavior of OptimizeForSmallDb(): use a 16MB block cache, put index and filter blocks into it, and cost the memtable size to it. DBOptions.OptimizeForSmallDb() and ColumnFamilyOptions.OptimizeForSmallDb() start to take an optional cache object.
* Added BottommostLevelCompaction::kForceOptimized to avoid double compacting newly compacted files in the bottommost level compaction of manual compaction. Note this option may prohibit the manual compaction to produce a single file in the bottommost level.

### Bug Fixes
* Adjust WriteBufferManager's dummy entry size to block cache from 1MB to 256KB.
* Fix a race condition between WritePrepared::Get and ::Put with duplicate keys.
* Fix crash when memtable prefix bloom is enabled and read/write a key out of domain of prefix extractor.
* Close a WAL file before another thread deletes it.
* Fix an assertion failure `IsFlushPending() == true` caused by one bg thread releasing the db mutex in ~ColumnFamilyData and another thread clearing `flush_requested_` flag.

## 6.1.1 (4/9/2019)
### New Features
* When reading from option file/string/map, customized comparators and/or merge operators can be filled according to object registry.

### Public API Change

### Bug Fixes
* Fix a bug in 2PC where a sequence of txn prepare, memtable flush, and crash could result in losing the prepared transaction.
* Fix a bug in Encryption Env which could cause encrypted files to be read beyond file boundaries.

## 6.1.0 (3/27/2019)
### New Features
* Introduce two more stats levels, kExceptHistogramOrTimers and kExceptTimers.
* Added a feature to perform data-block sampling for compressibility, and report stats to user.
* Add support for trace filtering.
* Add DBOptions.avoid_unnecessary_blocking_io. If true, we avoid file deletion when destorying ColumnFamilyHandle and Iterator. Instead, a job is scheduled to delete the files in background.

### Public API Change
* Remove bundled fbson library.
* statistics.stats_level_ becomes atomic. It is preferred to use statistics.set_stats_level() and statistics.get_stats_level() to access it.
* Introduce a new IOError subcode, PathNotFound, to indicate trying to open a nonexistent file or directory for read.
* Add initial support for multiple db instances sharing the same data in single-writer, multi-reader mode.
* Removed some "using std::xxx" from public headers.

### Bug Fixes
* Fix JEMALLOC_CXX_THROW macro missing from older Jemalloc versions, causing build failures on some platforms.
* Fix SstFileReader not able to open file ingested with write_glbal_seqno=true.

## 6.0.0 (2/19/2019)
### New Features
* Enabled checkpoint on readonly db (DBImplReadOnly).
* Make DB ignore dropped column families while committing results of atomic flush.
* RocksDB may choose to preopen some files even if options.max_open_files != -1. This may make DB open slightly longer.
* For users of dictionary compression with ZSTD v0.7.0+, we now reuse the same digested dictionary when compressing each of an SST file's data blocks for faster compression speeds.
* For all users of dictionary compression who set `cache_index_and_filter_blocks == true`, we now store dictionary data used for decompression in the block cache for better control over memory usage. For users of ZSTD v1.1.4+ who compile with -DZSTD_STATIC_LINKING_ONLY, this includes a digested dictionary, which is used to increase decompression speed.
* Add support for block checksums verification for external SST files before ingestion.
* Introduce stats history which periodically saves Statistics snapshots and added `GetStatsHistory` API to retrieve these snapshots.
* Add a place holder in manifest which indicate a record from future that can be safely ignored.
* Add support for trace sampling.
* Enable properties block checksum verification for block-based tables.
* For all users of dictionary compression, we now generate a separate dictionary for compressing each bottom-level SST file. Previously we reused a single dictionary for a whole compaction to bottom level. The new approach achieves better compression ratios; however, it uses more memory and CPU for buffering/sampling data blocks and training dictionaries.
* Add whole key bloom filter support in memtable.
* Files written by `SstFileWriter` will now use dictionary compression if it is configured in the file writer's `CompressionOptions`.

### Public API Change
* Disallow CompactionFilter::IgnoreSnapshots() = false, because it is not very useful and the behavior is confusing. The filter will filter everything if there is no snapshot declared by the time the compaction starts. However, users can define a snapshot after the compaction starts and before it finishes and this new snapshot won't be repeatable, because after the compaction finishes, some keys may be dropped.
* CompactionPri = kMinOverlappingRatio also uses compensated file size, which boosts file with lots of tombstones to be compacted first.
* Transaction::GetForUpdate is extended with a do_validate parameter with default value of true. If false it skips validating the snapshot before doing the read. Similarly ::Merge, ::Put, ::Delete, and ::SingleDelete are extended with assume_tracked with default value of false. If true it indicates that call is assumed to be after a ::GetForUpdate.
* `TableProperties::num_entries` and `TableProperties::num_deletions` now also account for number of range tombstones.
* Remove geodb, spatial_db, document_db, json_document, date_tiered_db, and redis_lists.
* With "ldb ----try_load_options", when wal_dir specified by the option file doesn't exist, ignore it.
* Change time resolution in FileOperationInfo.
* Deleting Blob files also go through SStFileManager.
* Remove CuckooHash memtable.
* The counter stat `number.block.not_compressed` now also counts blocks not compressed due to poor compression ratio.
* Remove ttl option from `CompactionOptionsFIFO`. The option has been deprecated and ttl in `ColumnFamilyOptions` is used instead.
* Support SST file ingestion across multiple column families via DB::IngestExternalFiles. See the function's comment about atomicity.
* Remove Lua compaction filter.

### Bug Fixes
* Fix a deadlock caused by compaction and file ingestion waiting for each other in the event of write stalls.
* Fix a memory leak when files with range tombstones are read in mmap mode and block cache is enabled
* Fix handling of corrupt range tombstone blocks such that corruptions cannot cause deleted keys to reappear
* Lock free MultiGet
* Fix incorrect `NotFound` point lookup result when querying the endpoint of a file that has been extended by a range tombstone.
* Fix with pipelined write, write leaders's callback failure lead to the whole write group fail.

### Change Default Options
* Change options.compaction_pri's default to kMinOverlappingRatio

## 5.18.0 (11/30/2018)
### New Features
* Introduced `JemallocNodumpAllocator` memory allocator. When being use, block cache will be excluded from core dump.
* Introduced `PerfContextByLevel` as part of `PerfContext` which allows storing perf context at each level. Also replaced `__thread` with `thread_local` keyword for perf_context. Added per-level perf context for bloom filter and `Get` query.
* With level_compaction_dynamic_level_bytes = true, level multiplier may be adjusted automatically when Level 0 to 1 compaction is lagged behind.
* Introduced DB option `atomic_flush`. If true, RocksDB supports flushing multiple column families and atomically committing the result to MANIFEST. Useful when WAL is disabled.
* Added `num_deletions` and `num_merge_operands` members to `TableProperties`.
* Added "rocksdb.min-obsolete-sst-number-to-keep" DB property that reports the lower bound on SST file numbers that are being kept from deletion, even if the SSTs are obsolete.
* Add xxhash64 checksum support
* Introduced `MemoryAllocator`, which lets the user specify custom memory allocator for block based table.
* Improved `DeleteRange` to prevent read performance degradation. The feature is no longer marked as experimental.

### Public API Change
* `DBOptions::use_direct_reads` now affects reads issued by `BackupEngine` on the database's SSTs.
* `NO_ITERATORS` is divided into two counters `NO_ITERATOR_CREATED` and `NO_ITERATOR_DELETE`. Both of them are only increasing now, just as other counters.

### Bug Fixes
* Fix corner case where a write group leader blocked due to write stall blocks other writers in queue with WriteOptions::no_slowdown set.
* Fix in-memory range tombstone truncation to avoid erroneously covering newer keys at a lower level, and include range tombstones in compacted files whose largest key is the range tombstone's start key.
* Properly set the stop key for a truncated manual CompactRange
* Fix slow flush/compaction when DB contains many snapshots. The problem became noticeable to us in DBs with 100,000+ snapshots, though it will affect others at different thresholds.
* Fix the bug that WriteBatchWithIndex's SeekForPrev() doesn't see the entries with the same key.
* Fix the bug where user comparator was sometimes fed with InternalKey instead of the user key. The bug manifests when during GenerateBottommostFiles.
* Fix a bug in WritePrepared txns where if the number of old snapshots goes beyond the snapshot cache size (128 default) the rest will not be checked when evicting a commit entry from the commit cache.
* Fixed Get correctness bug in the presence of range tombstones where merge operands covered by a range tombstone always result in NotFound.
* Start populating `NO_FILE_CLOSES` ticker statistic, which was always zero previously.
* The default value of NewBloomFilterPolicy()'s argument use_block_based_builder is changed to false. Note that this new default may cause large temp memory usage when building very large SST files.

## 5.17.0 (10/05/2018)
### Public API Change
* `OnTableFileCreated` will now be called for empty files generated during compaction. In that case, `TableFileCreationInfo::file_path` will be "(nil)" and `TableFileCreationInfo::file_size` will be zero.
* Add `FlushOptions::allow_write_stall`, which controls whether Flush calls start working immediately, even if it causes user writes to stall, or will wait until flush can be performed without causing write stall (similar to `CompactRangeOptions::allow_write_stall`). Note that the default value is false, meaning we add delay to Flush calls until stalling can be avoided when possible. This is behavior change compared to previous RocksDB versions, where Flush calls didn't check if they might cause stall or not.
* Application using PessimisticTransactionDB is expected to rollback/commit recovered transactions before starting new ones. This assumption is used to skip concurrency control during recovery.
* Expose column family id to `OnCompactionCompleted`.

### New Features
* TransactionOptions::skip_concurrency_control allows pessimistic transactions to skip the overhead of concurrency control. Could be used for optimizing certain transactions or during recovery.

### Bug Fixes
* Avoid creating empty SSTs and subsequently deleting them in certain cases during compaction.
* Sync CURRENT file contents during checkpoint.

## 5.16.3 (10/1/2018)
### Bug Fixes
* Fix crash caused when `CompactFiles` run with `CompactionOptions::compression == CompressionType::kDisableCompressionOption`. Now that setting causes the compression type to be chosen according to the column family-wide compression options.

## 5.16.2 (9/21/2018)
### Bug Fixes
* Fix bug in partition filters with format_version=4.

## 5.16.1 (9/17/2018)
### Bug Fixes
* Remove trace_analyzer_tool from rocksdb_lib target in TARGETS file.
* Fix RocksDB Java build and tests.
* Remove sync point in Block destructor.

## 5.16.0 (8/21/2018)
### Public API Change
* The merge operands are passed to `MergeOperator::ShouldMerge` in the reversed order relative to how they were merged (passed to FullMerge or FullMergeV2) for performance reasons
* GetAllKeyVersions() to take an extra argument of `max_num_ikeys`.
* Using ZSTD dictionary trainer (i.e., setting `CompressionOptions::zstd_max_train_bytes` to a nonzero value) now requires ZSTD version 1.1.3 or later.

### New Features
* Changes the format of index blocks by delta encoding the index values, which are the block handles. This saves the encoding of BlockHandle::offset of the non-head index entries in each restart interval. The feature is backward compatible but not forward compatible. It is disabled by default unless format_version 4 or above is used.
* Add a new tool: trace_analyzer. Trace_analyzer analyzes the trace file generated by using trace_replay API. It can convert the binary format trace file to a human readable txt file, output the statistics of the analyzed query types such as access statistics and size statistics, combining the dumped whole key space file to analyze, support query correlation analyzing, and etc. Current supported query types are: Get, Put, Delete, SingleDelete, DeleteRange, Merge, Iterator (Seek, SeekForPrev only).
* Add hash index support to data blocks, which helps reducing the cpu utilization of point-lookup operations. This feature is backward compatible with the data block created without the hash index. It is disabled by default unless BlockBasedTableOptions::data_block_index_type is set to data_block_index_type = kDataBlockBinaryAndHash.

### Bug Fixes
* Fix a bug in misreporting the estimated partition index size in properties block.

## 5.15.0 (7/17/2018)
### Public API Change
* Remove managed iterator. ReadOptions.managed is not effective anymore.
* For bottommost_compression, a compatible CompressionOptions is added via `bottommost_compression_opts`. To keep backward compatible, a new boolean `enabled` is added to CompressionOptions. For compression_opts, it will be always used no matter what value of `enabled` is. For bottommost_compression_opts, it will only be used when user set `enabled=true`, otherwise, compression_opts will be used for bottommost_compression as default.
* With LRUCache, when high_pri_pool_ratio > 0, midpoint insertion strategy will be enabled to put low-pri items to the tail of low-pri list (the midpoint) when they first inserted into the cache. This is to make cache entries never get hit age out faster, improving cache efficiency when large background scan presents.
* For users of `Statistics` objects created via `CreateDBStatistics()`, the format of the string returned by its `ToString()` method has changed.
* The "rocksdb.num.entries" table property no longer counts range deletion tombstones as entries.

### New Features
* Changes the format of index blocks by storing the key in their raw form rather than converting them to InternalKey. This saves 8 bytes per index key. The feature is backward compatible but not forward compatible. It is disabled by default unless format_version 3 or above is used.
* Avoid memcpy when reading mmap files with OpenReadOnly and max_open_files==-1.
* Support dynamically changing `ColumnFamilyOptions::ttl` via `SetOptions()`.
* Add a new table property, "rocksdb.num.range-deletions", which counts the number of range deletion tombstones in the table.
* Improve the performance of iterators doing long range scans by using readahead, when using direct IO.
* pin_top_level_index_and_filter (default true) in BlockBasedTableOptions can be used in combination with cache_index_and_filter_blocks to prefetch and pin the top-level index of partitioned index and filter blocks in cache. It has no impact when cache_index_and_filter_blocks is false.
* Write properties meta-block at the end of block-based table to save read-ahead IO.

### Bug Fixes
* Fix deadlock with enable_pipelined_write=true and max_successive_merges > 0
* Check conflict at output level in CompactFiles.
* Fix corruption in non-iterator reads when mmap is used for file reads
* Fix bug with prefix search in partition filters where a shared prefix would be ignored from the later partitions. The bug could report an eixstent key as missing. The bug could be triggered if prefix_extractor is set and partition filters is enabled.
* Change default value of `bytes_max_delete_chunk` to 0 in NewSstFileManager() as it doesn't work well with checkpoints.
* Fix a bug caused by not copying the block trailer with compressed SST file, direct IO, prefetcher and no compressed block cache.
* Fix write can stuck indefinitely if enable_pipelined_write=true. The issue exists since pipelined write was introduced in 5.5.0.

## 5.14.0 (5/16/2018)
### Public API Change
* Add a BlockBasedTableOption to align uncompressed data blocks on the smaller of block size or page size boundary, to reduce flash reads by avoiding reads spanning 4K pages.
* The background thread naming convention changed (on supporting platforms) to "rocksdb:<thread pool priority><thread number>", e.g., "rocksdb:low0".
* Add a new ticker stat rocksdb.number.multiget.keys.found to count number of keys successfully read in MultiGet calls
* Touch-up to write-related counters in PerfContext. New counters added: write_scheduling_flushes_compactions_time, write_thread_wait_nanos. Counters whose behavior was fixed or modified: write_memtable_time, write_pre_and_post_process_time, write_delay_time.
* Posix Env's NewRandomRWFile() will fail if the file doesn't exist.
* Now, `DBOptions::use_direct_io_for_flush_and_compaction` only applies to background writes, and `DBOptions::use_direct_reads` applies to both user reads and background reads. This conforms with Linux's `open(2)` manpage, which advises against simultaneously reading a file in buffered and direct modes, due to possibly undefined behavior and degraded performance.
* Iterator::Valid() always returns false if !status().ok(). So, now when doing a Seek() followed by some Next()s, there's no need to check status() after every operation.
* Iterator::Seek()/SeekForPrev()/SeekToFirst()/SeekToLast() always resets status().
* Introduced `CompressionOptions::kDefaultCompressionLevel`, which is a generic way to tell RocksDB to use the compression library's default level. It is now the default value for `CompressionOptions::level`. Previously the level defaulted to -1, which gave poor compression ratios in ZSTD.

### New Features
* Introduce TTL for level compaction so that all files older than ttl go through the compaction process to get rid of old data.
* TransactionDBOptions::write_policy can be configured to enable WritePrepared 2PC transactions. Read more about them in the wiki.
* Add DB properties "rocksdb.block-cache-capacity", "rocksdb.block-cache-usage", "rocksdb.block-cache-pinned-usage" to show block cache usage.
* Add `Env::LowerThreadPoolCPUPriority(Priority)` method, which lowers the CPU priority of background (esp. compaction) threads to minimize interference with foreground tasks.
* Fsync parent directory after deleting a file in delete scheduler.
* In level-based compaction, if bottom-pri thread pool was setup via `Env::SetBackgroundThreads()`, compactions to the bottom level will be delegated to that thread pool.
* `prefix_extractor` has been moved from ImmutableCFOptions to MutableCFOptions, meaning it can be dynamically changed without a DB restart.

### Bug Fixes
* Fsync after writing global seq number to the ingestion file in ExternalSstFileIngestionJob.
* Fix WAL corruption caused by race condition between user write thread and FlushWAL when two_write_queue is not set.
* Fix `BackupableDBOptions::max_valid_backups_to_open` to not delete backup files when refcount cannot be accurately determined.
* Fix memory leak when pin_l0_filter_and_index_blocks_in_cache is used with partitioned filters
* Disable rollback of merge operands in WritePrepared transactions to work around an issue in MyRocks. It can be enabled back by setting TransactionDBOptions::rollback_merge_operands to true.
* Fix wrong results by ReverseBytewiseComparator::FindShortSuccessor()

### Java API Changes
* Add `BlockBasedTableConfig.setBlockCache` to allow sharing a block cache across DB instances.
* Added SstFileManager to the Java API to allow managing SST files across DB instances.

## 5.13.0 (3/20/2018)
### Public API Change
* RocksDBOptionsParser::Parse()'s `ignore_unknown_options` argument will only be effective if the option file shows it is generated using a higher version of RocksDB than the current version.
* Remove CompactionEventListener.

### New Features
* SstFileManager now can cancel compactions if they will result in max space errors. SstFileManager users can also use SetCompactionBufferSize to specify how much space must be leftover during a compaction for auxiliary file functions such as logging and flushing.
* Avoid unnecessarily flushing in `CompactRange()` when the range specified by the user does not overlap unflushed memtables.
* If `ColumnFamilyOptions::max_subcompactions` is set greater than one, we now parallelize large manual level-based compactions.
* Add "rocksdb.live-sst-files-size" DB property to return total bytes of all SST files belong to the latest LSM tree.
* NewSstFileManager to add an argument bytes_max_delete_chunk with default 64MB. With this argument, a file larger than 64MB will be ftruncated multiple times based on this size.

### Bug Fixes
* Fix a leak in prepared_section_completed_ where the zeroed entries would not removed from the map.
* Fix WAL corruption caused by race condition between user write thread and backup/checkpoint thread.

## 5.12.0 (2/14/2018)
### Public API Change
* Iterator::SeekForPrev is now a pure virtual method. This is to prevent user who implement the Iterator interface fail to implement SeekForPrev by mistake.
* Add `include_end` option to make the range end exclusive when `include_end == false` in `DeleteFilesInRange()`.
* Add `CompactRangeOptions::allow_write_stall`, which makes `CompactRange` start working immediately, even if it causes user writes to stall. The default value is false, meaning we add delay to `CompactRange` calls until stalling can be avoided when possible. Note this delay is not present in previous RocksDB versions.
* Creating checkpoint with empty directory now returns `Status::InvalidArgument`; previously, it returned `Status::IOError`.
* Adds a BlockBasedTableOption to turn off index block compression.
* Close() method now returns a status when closing a db.

### New Features
* Improve the performance of iterators doing long range scans by using readahead.
* Add new function `DeleteFilesInRanges()` to delete files in multiple ranges at once for better performance.
* FreeBSD build support for RocksDB and RocksJava.
* Improved performance of long range scans with readahead.
* Updated to and now continuously tested in Visual Studio 2017.

### Bug Fixes
* Fix `DisableFileDeletions()` followed by `GetSortedWalFiles()` to not return obsolete WAL files that `PurgeObsoleteFiles()` is going to delete.
* Fix Handle error return from WriteBuffer() during WAL file close and DB close.
* Fix advance reservation of arena block addresses.
* Fix handling of empty string as checkpoint directory.

## 5.11.0 (01/08/2018)
### Public API Change
* Add `autoTune` and `getBytesPerSecond()` to RocksJava RateLimiter

### New Features
* Add a new histogram stat called rocksdb.db.flush.micros for memtable flush.
* Add "--use_txn" option to use transactional API in db_stress.
* Disable onboard cache for compaction output in Windows platform.
* Improve the performance of iterators doing long range scans by using readahead.

### Bug Fixes
* Fix a stack-use-after-scope bug in ForwardIterator.
* Fix builds on platforms including Linux, Windows, and PowerPC.
* Fix buffer overrun in backup engine for DBs with huge number of files.
* Fix a mislabel bug for bottom-pri compaction threads.
* Fix DB::Flush() keep waiting after flush finish under certain condition.

## 5.10.0 (12/11/2017)
### Public API Change
* When running `make` with environment variable `USE_SSE` set and `PORTABLE` unset, will use all machine features available locally. Previously this combination only compiled SSE-related features.

### New Features
* Provide lifetime hints when writing files on Linux. This reduces hardware write-amp on storage devices supporting multiple streams.
* Add a DB stat, `NUMBER_ITER_SKIP`, which returns how many internal keys were skipped during iterations (e.g., due to being tombstones or duplicate versions of a key).
* Add PerfContext counters, `key_lock_wait_count` and `key_lock_wait_time`, which measure the number of times transactions wait on key locks and total amount of time waiting.

### Bug Fixes
* Fix IOError on WAL write doesn't propagate to write group follower
* Make iterator invalid on merge error.
* Fix performance issue in `IngestExternalFile()` affecting databases with large number of SST files.
* Fix possible corruption to LSM structure when `DeleteFilesInRange()` deletes a subset of files spanned by a `DeleteRange()` marker.

## 5.9.0 (11/1/2017)
### Public API Change
* `BackupableDBOptions::max_valid_backups_to_open == 0` now means no backups will be opened during BackupEngine initialization. Previously this condition disabled limiting backups opened.
* `DBOptions::preserve_deletes` is a new option that allows one to specify that DB should not drop tombstones for regular deletes if they have sequence number larger than what was set by the new API call `DB::SetPreserveDeletesSequenceNumber(SequenceNumber seqnum)`. Disabled by default.
* API call `DB::SetPreserveDeletesSequenceNumber(SequenceNumber seqnum)` was added, users who wish to preserve deletes are expected to periodically call this function to advance the cutoff seqnum (all deletes made before this seqnum can be dropped by DB). It's user responsibility to figure out how to advance the seqnum in the way so the tombstones are kept for the desired period of time, yet are eventually processed in time and don't eat up too much space.
* `ReadOptions::iter_start_seqnum` was added;
if set to something > 0 user will see 2 changes in iterators behavior 1) only keys written with sequence larger than this parameter would be returned and 2) the `Slice` returned by iter->key() now points to the memory that keep User-oriented representation of the internal key, rather than user key. New struct `FullKey` was added to represent internal keys, along with a new helper function `ParseFullKey(const Slice& internal_key, FullKey* result);`.
* Deprecate trash_dir param in NewSstFileManager, right now we will rename deleted files to <name>.trash instead of moving them to trash directory
* Allow setting a custom trash/DB size ratio limit in the SstFileManager, after which files that are to be scheduled for deletion are deleted immediately, regardless of any delete ratelimit.
* Return an error on write if write_options.sync = true and write_options.disableWAL = true to warn user of inconsistent options. Previously we will not write to WAL and not respecting the sync options in this case.

### New Features
* CRC32C is now using the 3-way pipelined SSE algorithm `crc32c_3way` on supported platforms to improve performance. The system will choose to use this algorithm on supported platforms automatically whenever possible. If PCLMULQDQ is not supported it will fall back to the old Fast_CRC32 algorithm.
* `DBOptions::writable_file_max_buffer_size` can now be changed dynamically.
* `DBOptions::bytes_per_sync`, `DBOptions::compaction_readahead_size`, and `DBOptions::wal_bytes_per_sync` can now be changed dynamically, `DBOptions::wal_bytes_per_sync` will flush all memtables and switch to a new WAL file.
* Support dynamic adjustment of rate limit according to demand for background I/O. It can be enabled by passing `true` to the `auto_tuned` parameter in `NewGenericRateLimiter()`. The value passed as `rate_bytes_per_sec` will still be respected as an upper-bound.
* Support dynamically changing `ColumnFamilyOptions::compaction_options_fifo`.
* Introduce `EventListener::OnStallConditionsChanged()` callback. Users can implement it to be notified when user writes are stalled, stopped, or resumed.
* Add a new db property "rocksdb.estimate-oldest-key-time" to return oldest data timestamp. The property is available only for FIFO compaction with compaction_options_fifo.allow_compaction = false.
* Upon snapshot release, recompact bottommost files containing deleted/overwritten keys that previously could not be dropped due to the snapshot. This alleviates space-amp caused by long-held snapshots.
* Support lower bound on iterators specified via `ReadOptions::iterate_lower_bound`.
* Support for differential snapshots (via iterator emitting the sequence of key-values representing the difference between DB state at two different sequence numbers). Supports preserving and emitting puts and regular deletes, doesn't support SingleDeletes, MergeOperator, Blobs and Range Deletes.

### Bug Fixes
* Fix a potential data inconsistency issue during point-in-time recovery. `DB:Open()` will abort if column family inconsistency is found during PIT recovery.
* Fix possible metadata corruption in databases using `DeleteRange()`.

## 5.8.0 (08/30/2017)
### Public API Change
* Users of `Statistics::getHistogramString()` will see fewer histogram buckets and different bucket endpoints.
* `Slice::compare` and BytewiseComparator `Compare` no longer accept `Slice`s containing nullptr.
* `Transaction::Get` and `Transaction::GetForUpdate` variants with `PinnableSlice` added.

### New Features
* Add Iterator::Refresh(), which allows users to update the iterator state so that they can avoid some initialization costs of recreating iterators.
* Replace dynamic_cast<> (except unit test) so people can choose to build with RTTI off. With make, release mode is by default built with -fno-rtti and debug mode is built without it. Users can override it by setting USE_RTTI=0 or 1.
* Universal compactions including the bottom level can be executed in a dedicated thread pool. This alleviates head-of-line blocking in the compaction queue, which cause write stalling, particularly in multi-instance use cases. Users can enable this feature via `Env::SetBackgroundThreads(N, Env::Priority::BOTTOM)`, where `N > 0`.
* Allow merge operator to be called even with a single merge operand during compactions, by appropriately overriding `MergeOperator::AllowSingleOperand`.
* Add `DB::VerifyChecksum()`, which verifies the checksums in all SST files in a running DB.
* Block-based table support for disabling checksums by setting `BlockBasedTableOptions::checksum = kNoChecksum`.

### Bug Fixes
* Fix wrong latencies in `rocksdb.db.get.micros`, `rocksdb.db.write.micros`, and `rocksdb.sst.read.micros`.
* Fix incorrect dropping of deletions during intra-L0 compaction.
* Fix transient reappearance of keys covered by range deletions when memtable prefix bloom filter is enabled.
* Fix potentially wrong file smallest key when range deletions separated by snapshot are written together.

## 5.7.0 (07/13/2017)
### Public API Change
* DB property "rocksdb.sstables" now prints keys in hex form.

### New Features
* Measure estimated number of reads per file. The information can be accessed through DB::GetColumnFamilyMetaData or "rocksdb.sstables" DB property.
* RateLimiter support for throttling background reads, or throttling the sum of background reads and writes. This can give more predictable I/O usage when compaction reads more data than it writes, e.g., due to lots of deletions.
* [Experimental] FIFO compaction with TTL support. It can be enabled by setting CompactionOptionsFIFO.ttl > 0.
* Introduce `EventListener::OnBackgroundError()` callback. Users can implement it to be notified of errors causing the DB to enter read-only mode, and optionally override them.
* Partitioned Index/Filters exiting the experimental mode. To enable partitioned indexes set index_type to kTwoLevelIndexSearch and to further enable partitioned filters set partition_filters to true. To configure the partition size set metadata_block_size.


### Bug Fixes
* Fix discarding empty compaction output files when `DeleteRange()` is used together with subcompactions.

## 5.6.0 (06/06/2017)
### Public API Change
* Scheduling flushes and compactions in the same thread pool is no longer supported by setting `max_background_flushes=0`. Instead, users can achieve this by configuring their high-pri thread pool to have zero threads.
* Replace `Options::max_background_flushes`, `Options::max_background_compactions`, and `Options::base_background_compactions` all with `Options::max_background_jobs`, which automatically decides how many threads to allocate towards flush/compaction.
* options.delayed_write_rate by default take the value of options.rate_limiter rate.
* Replace global variable `IOStatsContext iostats_context` with `IOStatsContext* get_iostats_context()`; replace global variable `PerfContext perf_context` with `PerfContext* get_perf_context()`.

### New Features
* Change ticker/histogram statistics implementations to use core-local storage. This improves aggregation speed compared to our previous thread-local approach, particularly for applications with many threads.
* Users can pass a cache object to write buffer manager, so that they can cap memory usage for memtable and block cache using one single limit.
* Flush will be triggered when 7/8 of the limit introduced by write_buffer_manager or db_write_buffer_size is triggered, so that the hard threshold is hard to hit.
* Introduce WriteOptions.low_pri. If it is true, low priority writes will be throttled if the compaction is behind.
* `DB::IngestExternalFile()` now supports ingesting files into a database containing range deletions.

### Bug Fixes
* Shouldn't ignore return value of fsync() in flush.

## 5.5.0 (05/17/2017)
### New Features
* FIFO compaction to support Intra L0 compaction too with CompactionOptionsFIFO.allow_compaction=true.
* DB::ResetStats() to reset internal stats.
* Statistics::Reset() to reset user stats.
* ldb add option --try_load_options, which will open DB with its own option file.
* Introduce WriteBatch::PopSavePoint to pop the most recent save point explicitly.
* Support dynamically change `max_open_files` option via SetDBOptions()
* Added DB::CreateColumnFamilie() and DB::DropColumnFamilies() to bulk create/drop column families.
* Add debugging function `GetAllKeyVersions` to see internal versions of a range of keys.
* Support file ingestion with universal compaction style
* Support file ingestion behind with option `allow_ingest_behind`
* New option enable_pipelined_write which may improve write throughput in case writing from multiple threads and WAL enabled.

### Bug Fixes
* Fix the bug that Direct I/O uses direct reads for non-SST file

## 5.4.0 (04/11/2017)
### Public API Change
* random_access_max_buffer_size no longer has any effect
* Removed Env::EnableReadAhead(), Env::ShouldForwardRawRequest()
* Support dynamically change `stats_dump_period_sec` option via SetDBOptions().
* Added ReadOptions::max_skippable_internal_keys to set a threshold to fail a request as incomplete when too many keys are being skipped when using iterators.
* DB::Get in place of std::string accepts PinnableSlice, which avoids the extra memcpy of value to std::string in most of cases.
    * PinnableSlice releases the pinned resources that contain the value when it is destructed or when ::Reset() is called on it.
    * The old API that accepts std::string, although discouraged, is still supported.
* Replace Options::use_direct_writes with Options::use_direct_io_for_flush_and_compaction. Read Direct IO wiki for details.
* Added CompactionEventListener and EventListener::OnFlushBegin interfaces.

### New Features
* Memtable flush can be avoided during checkpoint creation if total log file size is smaller than a threshold specified by the user.
* Introduce level-based L0->L0 compactions to reduce file count, so write delays are incurred less often.
* (Experimental) Partitioning filters which creates an index on the partitions. The feature can be enabled by setting partition_filters when using kFullFilter. Currently the feature also requires two-level indexing to be enabled. Number of partitions is the same as the number of partitions for indexes, which is controlled by metadata_block_size.

## 5.3.0 (03/08/2017)
### Public API Change
* Remove disableDataSync option.
* Remove timeout_hint_us option from WriteOptions. The option has been deprecated and has no effect since 3.13.0.
* Remove option min_partial_merge_operands. Partial merge operands will always be merged in flush or compaction if there are more than one.
* Remove option verify_checksums_in_compaction. Compaction will always verify checksum.

### Bug Fixes
* Fix the bug that iterator may skip keys

## 5.2.0 (02/08/2017)
### Public API Change
* NewLRUCache() will determine number of shard bits automatically based on capacity, if the user doesn't pass one. This also impacts the default block cache when the user doesn't explict provide one.
* Change the default of delayed slowdown value to 16MB/s and further increase the L0 stop condition to 36 files.
* Options::use_direct_writes and Options::use_direct_reads are now ready to use.
* (Experimental) Two-level indexing that partition the index and creates a 2nd level index on the partitions. The feature can be enabled by setting kTwoLevelIndexSearch as IndexType and configuring index_per_partition.

### New Features
* Added new overloaded function GetApproximateSizes that allows to specify if memtable stats should be computed only without computing SST files' stats approximations.
* Added new function GetApproximateMemTableStats that approximates both number of records and size of memtables.
* Add Direct I/O mode for SST file I/O

### Bug Fixes
* RangeSync() should work if ROCKSDB_FALLOCATE_PRESENT is not set
* Fix wrong results in a data race case in Get()
* Some fixes related to 2PC.
* Fix bugs of data corruption in direct I/O

## 5.1.0 (01/13/2017)
* Support dynamically change `delete_obsolete_files_period_micros` option via SetDBOptions().
* Added EventListener::OnExternalFileIngested which will be called when IngestExternalFile() add a file successfully.
* BackupEngine::Open and BackupEngineReadOnly::Open now always return error statuses matching those of the backup Env.

### Bug Fixes
* Fix the bug that if 2PC is enabled, checkpoints may loss some recent transactions.
* When file copying is needed when creating checkpoints or bulk loading files, fsync the file after the file copying.

## 5.0.0 (11/17/2016)
### Public API Change
* Options::max_bytes_for_level_multiplier is now a double along with all getters and setters.
* Support dynamically change `delayed_write_rate` and `max_total_wal_size` options via SetDBOptions().
* Introduce DB::DeleteRange for optimized deletion of large ranges of contiguous keys.
* Support dynamically change `delayed_write_rate` option via SetDBOptions().
* Options::allow_concurrent_memtable_write and Options::enable_write_thread_adaptive_yield are now true by default.
* Remove Tickers::SEQUENCE_NUMBER to avoid confusion if statistics object is shared among RocksDB instance. Alternatively DB::GetLatestSequenceNumber() can be used to get the same value.
* Options.level0_stop_writes_trigger default value changes from 24 to 32.
* New compaction filter API: CompactionFilter::FilterV2(). Allows to drop ranges of keys.
* Removed flashcache support.
* DB::AddFile() is deprecated and is replaced with DB::IngestExternalFile(). DB::IngestExternalFile() remove all the restrictions that existed for DB::AddFile.

### New Features
* Add avoid_flush_during_shutdown option, which speeds up DB shutdown by not flushing unpersisted data (i.e. with disableWAL = true). Unpersisted data will be lost. The options is dynamically changeable via SetDBOptions().
* Add memtable_insert_with_hint_prefix_extractor option. The option is mean to reduce CPU usage for inserting keys into memtable, if keys can be group by prefix and insert for each prefix are sequential or almost sequential. See include/rocksdb/options.h for more details.
* Add LuaCompactionFilter in utilities.  This allows developers to write compaction filters in Lua.  To use this feature, LUA_PATH needs to be set to the root directory of Lua.
* No longer populate "LATEST_BACKUP" file in backup directory, which formerly contained the number of the latest backup. The latest backup can be determined by finding the highest numbered file in the "meta/" subdirectory.

## 4.13.0 (10/18/2016)
### Public API Change
* DB::GetOptions() reflect dynamic changed options (i.e. through DB::SetOptions()) and return copy of options instead of reference.
* Added Statistics::getAndResetTickerCount().

### New Features
* Add DB::SetDBOptions() to dynamic change base_background_compactions and max_background_compactions.
* Added Iterator::SeekForPrev(). This new API will seek to the last key that less than or equal to the target key.

## 4.12.0 (9/12/2016)
### Public API Change
* CancelAllBackgroundWork() flushes all memtables for databases containing writes that have bypassed the WAL (writes issued with WriteOptions::disableWAL=true) before shutting down background threads.
* Merge options source_compaction_factor, max_grandparent_overlap_bytes and expanded_compaction_factor into max_compaction_bytes.
* Remove ImmutableCFOptions.
* Add a compression type ZSTD, which can work with ZSTD 0.8.0 or up. Still keep ZSTDNotFinal for compatibility reasons.

### New Features
* Introduce NewClockCache, which is based on CLOCK algorithm with better concurrent performance in some cases. It can be used to replace the default LRU-based block cache and table cache. To use it, RocksDB need to be linked with TBB lib.
* Change ticker/histogram statistics implementations to accumulate data in thread-local storage, which improves CPU performance by reducing cache coherency costs. Callers of CreateDBStatistics do not need to change anything to use this feature.
* Block cache mid-point insertion, where index and filter block are inserted into LRU block cache with higher priority. The feature can be enabled by setting BlockBasedTableOptions::cache_index_and_filter_blocks_with_high_priority to true and high_pri_pool_ratio > 0 when creating NewLRUCache.

## 4.11.0 (8/1/2016)
### Public API Change
* options.memtable_prefix_bloom_huge_page_tlb_size => memtable_huge_page_size. When it is set, RocksDB will try to allocate memory from huge page for memtable too, rather than just memtable bloom filter.

### New Features
* A tool to migrate DB after options change. See include/rocksdb/utilities/option_change_migration.h.
* Add ReadOptions.background_purge_on_iterator_cleanup. If true, we avoid file deletion when destorying iterators.

## 4.10.0 (7/5/2016)
### Public API Change
* options.memtable_prefix_bloom_bits changes to options.memtable_prefix_bloom_bits_ratio and deprecate options.memtable_prefix_bloom_probes
* enum type CompressionType and PerfLevel changes from char to unsigned char. Value of all PerfLevel shift by one.
* Deprecate options.filter_deletes.

### New Features
* Add avoid_flush_during_recovery option.
* Add a read option background_purge_on_iterator_cleanup to avoid deleting files in foreground when destroying iterators. Instead, a job is scheduled in high priority queue and would be executed in a separate background thread.
* RepairDB support for column families. RepairDB now associates data with non-default column families using information embedded in the SST/WAL files (4.7 or later). For data written by 4.6 or earlier, RepairDB associates it with the default column family.
* Add options.write_buffer_manager which allows users to control total memtable sizes across multiple DB instances.

## 4.9.0 (6/9/2016)
### Public API changes
* Add bottommost_compression option, This option can be used to set a specific compression algorithm for the bottommost level (Last level containing files in the DB).
* Introduce CompactionJobInfo::compression, This field state the compression algorithm used to generate the output files of the compaction.
* Deprecate BlockBaseTableOptions.hash_index_allow_collision=false
* Deprecate options builder (GetOptions()).

### New Features
* Introduce NewSimCache() in rocksdb/utilities/sim_cache.h. This function creates a block cache that is able to give simulation results (mainly hit rate) of simulating block behavior with a configurable cache size.

## 4.8.0 (5/2/2016)
### Public API Change
* Allow preset compression dictionary for improved compression of block-based tables. This is supported for zlib, zstd, and lz4. The compression dictionary's size is configurable via CompressionOptions::max_dict_bytes.
* Delete deprecated classes for creating backups (BackupableDB) and restoring from backups (RestoreBackupableDB). Now, BackupEngine should be used for creating backups, and BackupEngineReadOnly should be used for restorations. For more details, see https://github.com/facebook/rocksdb/wiki/How-to-backup-RocksDB%3F
* Expose estimate of per-level compression ratio via DB property: "rocksdb.compression-ratio-at-levelN".
* Added EventListener::OnTableFileCreationStarted. EventListener::OnTableFileCreated will be called on failure case. User can check creation status via TableFileCreationInfo::status.

### New Features
* Add ReadOptions::readahead_size. If non-zero, NewIterator will create a new table reader which performs reads of the given size.

## 4.7.0 (4/8/2016)
### Public API Change
* rename options compaction_measure_io_stats to report_bg_io_stats and include flush too.
* Change some default options. Now default options will optimize for server-workloads. Also enable slowdown and full stop triggers for pending compaction bytes. These changes may cause sub-optimal performance or significant increase of resource usage. To avoid these risks, users can open existing RocksDB with options extracted from RocksDB option files. See https://github.com/facebook/rocksdb/wiki/RocksDB-Options-File for how to use RocksDB option files. Or you can call Options.OldDefaults() to recover old defaults. DEFAULT_OPTIONS_HISTORY.md will track change history of default options.

## 4.6.0 (3/10/2016)
### Public API Changes
* Change default of BlockBasedTableOptions.format_version to 2. It means default DB created by 4.6 or up cannot be opened by RocksDB version 3.9 or earlier.
* Added strict_capacity_limit option to NewLRUCache. If the flag is set to true, insert to cache will fail if no enough capacity can be free. Signature of Cache::Insert() is updated accordingly.
* Tickers [NUMBER_DB_NEXT, NUMBER_DB_PREV, NUMBER_DB_NEXT_FOUND, NUMBER_DB_PREV_FOUND, ITER_BYTES_READ] are not updated immediately. The are updated when the Iterator is deleted.
* Add monotonically increasing counter (DB property "rocksdb.current-super-version-number") that increments upon any change to the LSM tree.

### New Features
* Add CompactionPri::kMinOverlappingRatio, a compaction picking mode friendly to write amplification.
* Deprecate Iterator::IsKeyPinned() and replace it with Iterator::GetProperty() with prop_name="rocksdb.iterator.is.key.pinned"

## 4.5.0 (2/5/2016)
### Public API Changes
* Add a new perf context level between kEnableCount and kEnableTime. Level 2 now does not include timers for mutexes.
* Statistics of mutex operation durations will not be measured by default. If you want to have them enabled, you need to set Statistics::stats_level_ to kAll.
* DBOptions::delete_scheduler and NewDeleteScheduler() are removed, please use DBOptions::sst_file_manager and NewSstFileManager() instead

### New Features
* ldb tool now supports operations to non-default column families.
* Add kPersistedTier to ReadTier.  This option allows Get and MultiGet to read only the persited data and skip mem-tables if writes were done with disableWAL = true.
* Add DBOptions::sst_file_manager. Use NewSstFileManager() in include/rocksdb/sst_file_manager.h to create a SstFileManager that can be used to track the total size of SST files and control the SST files deletion rate.

## 4.4.0 (1/14/2016)
### Public API Changes
* Change names in CompactionPri and add a new one.
* Deprecate options.soft_rate_limit and add options.soft_pending_compaction_bytes_limit.
* If options.max_write_buffer_number > 3, writes will be slowed down when writing to the last write buffer to delay a full stop.
* Introduce CompactionJobInfo::compaction_reason, this field include the reason to trigger the compaction.
* After slow down is triggered, if estimated pending compaction bytes keep increasing, slowdown more.
* Increase default options.delayed_write_rate to 2MB/s.
* Added a new parameter --path to ldb tool. --path accepts the name of either MANIFEST, SST or a WAL file. Either --db or --path can be used when calling ldb.

## 4.3.0 (12/8/2015)
### New Features
* CompactionFilter has new member function called IgnoreSnapshots which allows CompactionFilter to be called even if there are snapshots later than the key.
* RocksDB will now persist options under the same directory as the RocksDB database on successful DB::Open, CreateColumnFamily, DropColumnFamily, and SetOptions.
* Introduce LoadLatestOptions() in rocksdb/utilities/options_util.h.  This function can construct the latest DBOptions / ColumnFamilyOptions used by the specified RocksDB intance.
* Introduce CheckOptionsCompatibility() in rocksdb/utilities/options_util.h.  This function checks whether the input set of options is able to open the specified DB successfully.

### Public API Changes
* When options.db_write_buffer_size triggers, only the column family with the largest column family size will be flushed, not all the column families.

## 4.2.0 (11/9/2015)
### New Features
* Introduce CreateLoggerFromOptions(), this function create a Logger for provided DBOptions.
* Add GetAggregatedIntProperty(), which returns the sum of the GetIntProperty of all the column families.
* Add MemoryUtil in rocksdb/utilities/memory.h.  It currently offers a way to get the memory usage by type from a list rocksdb instances.

### Public API Changes
* CompactionFilter::Context includes information of Column Family ID
* The need-compaction hint given by TablePropertiesCollector::NeedCompact() will be persistent and recoverable after DB recovery. This introduces a breaking format change. If you use this experimental feature, including NewCompactOnDeletionCollectorFactory() in the new version, you may not be able to directly downgrade the DB back to version 4.0 or lower.
* TablePropertiesCollectorFactory::CreateTablePropertiesCollector() now takes an option Context, containing the information of column family ID for the file being written.
* Remove DefaultCompactionFilterFactory.


## 4.1.0 (10/8/2015)
### New Features
* Added single delete operation as a more efficient way to delete keys that have not been overwritten.
* Added experimental AddFile() to DB interface that allow users to add files created by SstFileWriter into an empty Database, see include/rocksdb/sst_file_writer.h and DB::AddFile() for more info.
* Added support for opening SST files with .ldb suffix which enables opening LevelDB databases.
* CompactionFilter now supports filtering of merge operands and merge results.

### Public API Changes
* Added SingleDelete() to the DB interface.
* Added AddFile() to DB interface.
* Added SstFileWriter class.
* CompactionFilter has a new method FilterMergeOperand() that RocksDB applies to every merge operand during compaction to decide whether to filter the operand.
* We removed CompactionFilterV2 interfaces from include/rocksdb/compaction_filter.h. The functionality was deprecated already in version 3.13.

## 4.0.0 (9/9/2015)
### New Features
* Added support for transactions.  See include/rocksdb/utilities/transaction.h for more info.
* DB::GetProperty() now accepts "rocksdb.aggregated-table-properties" and "rocksdb.aggregated-table-properties-at-levelN", in which case it returns aggregated table properties of the target column family, or the aggregated table properties of the specified level N if the "at-level" version is used.
* Add compression option kZSTDNotFinalCompression for people to experiment ZSTD although its format is not finalized.
* We removed the need for LATEST_BACKUP file in BackupEngine. We still keep writing it when we create new backups (because of backward compatibility), but we don't read it anymore.

### Public API Changes
* Removed class Env::RandomRWFile and Env::NewRandomRWFile().
* Renamed DBOptions.num_subcompactions to DBOptions.max_subcompactions to make the name better match the actual functionality of the option.
* Added Equal() method to the Comparator interface that can optionally be overwritten in cases where equality comparisons can be done more efficiently than three-way comparisons.
* Previous 'experimental' OptimisticTransaction class has been replaced by Transaction class.

## 3.13.0 (8/6/2015)
### New Features
* RollbackToSavePoint() in WriteBatch/WriteBatchWithIndex
* Add NewCompactOnDeletionCollectorFactory() in utilities/table_properties_collectors, which allows rocksdb to mark a SST file as need-compaction when it observes at least D deletion entries in any N consecutive entries in that SST file.  Note that this feature depends on an experimental NeedCompact() API --- the result of this API will not persist after DB restart.
* Add DBOptions::delete_scheduler. Use NewDeleteScheduler() in include/rocksdb/delete_scheduler.h to create a DeleteScheduler that can be shared among multiple RocksDB instances to control the file deletion rate of SST files that exist in the first db_path.

### Public API Changes
* Deprecated WriteOptions::timeout_hint_us. We no longer support write timeout. If you really need this option, talk to us and we might consider returning it.
* Deprecated purge_redundant_kvs_while_flush option.
* Removed BackupEngine::NewBackupEngine() and NewReadOnlyBackupEngine() that were deprecated in RocksDB 3.8. Please use BackupEngine::Open() instead.
* Deprecated Compaction Filter V2. We are not aware of any existing use-cases. If you use this filter, your compile will break with RocksDB 3.13. Please let us know if you use it and we'll put it back in RocksDB 3.14.
* Env::FileExists now returns a Status instead of a boolean
* Add statistics::getHistogramString() to print detailed distribution of a histogram metric.
* Add DBOptions::skip_stats_update_on_db_open.  When it is on, DB::Open() will run faster as it skips the random reads required for loading necessary stats from SST files to optimize compaction.

## 3.12.0 (7/2/2015)
### New Features
* Added experimental support for optimistic transactions.  See include/rocksdb/utilities/optimistic_transaction.h for more info.
* Added a new way to report QPS from db_bench (check out --report_file and --report_interval_seconds)
* Added a cache for individual rows. See DBOptions::row_cache for more info.
* Several new features on EventListener (see include/rocksdb/listener.h):
 - OnCompationCompleted() now returns per-compaction job statistics, defined in include/rocksdb/compaction_job_stats.h.
 - Added OnTableFileCreated() and OnTableFileDeleted().
* Add compaction_options_universal.enable_trivial_move to true, to allow trivial move while performing universal compaction. Trivial move will happen only when all the input files are non overlapping.

### Public API changes
* EventListener::OnFlushCompleted() now passes FlushJobInfo instead of a list of parameters.
* DB::GetDbIdentity() is now a const function.  If this function is overridden in your application, be sure to also make GetDbIdentity() const to avoid compile error.
* Move listeners from ColumnFamilyOptions to DBOptions.
* Add max_write_buffer_number_to_maintain option
* DB::CompactRange()'s parameter reduce_level is changed to change_level, to allow users to move levels to lower levels if allowed. It can be used to migrate a DB from options.level_compaction_dynamic_level_bytes=false to options.level_compaction_dynamic_level_bytes.true.
* Change default value for options.compaction_filter_factory and options.compaction_filter_factory_v2 to nullptr instead of DefaultCompactionFilterFactory and DefaultCompactionFilterFactoryV2.
* If CancelAllBackgroundWork is called without doing a flush after doing loads with WAL disabled, the changes which haven't been flushed before the call to CancelAllBackgroundWork will be lost.
* WBWIIterator::Entry() now returns WriteEntry instead of `const WriteEntry&`
* options.hard_rate_limit is deprecated.
* When options.soft_rate_limit or options.level0_slowdown_writes_trigger is triggered, the way to slow down writes is changed to: write rate to DB is limited to to options.delayed_write_rate.
* DB::GetApproximateSizes() adds a parameter to allow the estimation to include data in mem table, with default to be not to include. It is now only supported in skip list mem table.
* DB::CompactRange() now accept CompactRangeOptions instead of multiple parameters. CompactRangeOptions is defined in include/rocksdb/options.h.
* CompactRange() will now skip bottommost level compaction for level based compaction if there is no compaction filter, bottommost_level_compaction is introduced in CompactRangeOptions to control when it's possible to skip bottommost level compaction. This mean that if you want the compaction to produce a single file you need to set bottommost_level_compaction to BottommostLevelCompaction::kForce.
* Add Cache.GetPinnedUsage() to get the size of memory occupied by entries that are in use by the system.
* DB:Open() will fail if the compression specified in Options is not linked with the binary. If you see this failure, recompile RocksDB with compression libraries present on your system. Also, previously our default compression was snappy. This behavior is now changed. Now, the default compression is snappy only if it's available on the system. If it isn't we change the default to kNoCompression.
* We changed how we account for memory used in block cache. Previously, we only counted the sum of block sizes currently present in block cache. Now, we count the actual memory usage of the blocks. For example, a block of size 4.5KB will use 8KB memory with jemalloc. This might decrease your memory usage and possibly decrease performance. Increase block cache size if you see this happening after an upgrade.
* Add BackupEngineImpl.options_.max_background_operations to specify the maximum number of operations that may be performed in parallel. Add support for parallelized backup and restore.
* Add DB::SyncWAL() that does a WAL sync without blocking writers.

## 3.11.0 (5/19/2015)
### New Features
* Added a new API Cache::SetCapacity(size_t capacity) to dynamically change the maximum configured capacity of the cache. If the new capacity is less than the existing cache usage, the implementation will try to lower the usage by evicting the necessary number of elements following a strict LRU policy.
* Added an experimental API for handling flashcache devices (blacklists background threads from caching their reads) -- NewFlashcacheAwareEnv
* If universal compaction is used and options.num_levels > 1, compact files are tried to be stored in none-L0 with smaller files based on options.target_file_size_base. The limitation of DB size when using universal compaction is greatly mitigated by using more levels. You can set num_levels = 1 to make universal compaction behave as before. If you set num_levels > 1 and want to roll back to a previous version, you need to compact all files to a big file in level 0 (by setting target_file_size_base to be large and CompactRange(<cf_handle>, nullptr, nullptr, true, 0) and reopen the DB with the same version to rewrite the manifest, and then you can open it using previous releases.
* More information about rocksdb background threads are available in Env::GetThreadList(), including the number of bytes read / written by a compaction job, mem-table size and current number of bytes written by a flush job and many more.  Check include/rocksdb/thread_status.h for more detail.

### Public API changes
* TablePropertiesCollector::AddUserKey() is added to replace TablePropertiesCollector::Add(). AddUserKey() exposes key type, sequence number and file size up to now to users.
* DBOptions::bytes_per_sync used to apply to both WAL and table files. As of 3.11 it applies only to table files. If you want to use this option to sync WAL in the background, please use wal_bytes_per_sync

## 3.10.0 (3/24/2015)
### New Features
* GetThreadStatus() is now able to report detailed thread status, including:
 - Thread Operation including flush and compaction.
 - The stage of the current thread operation.
 - The elapsed time in micros since the current thread operation started.
 More information can be found in include/rocksdb/thread_status.h.  In addition, when running db_bench with --thread_status_per_interval, db_bench will also report thread status periodically.
* Changed the LRU caching algorithm so that referenced blocks (by iterators) are never evicted. This change made parameter removeScanCountLimit obsolete. Because of that NewLRUCache doesn't take three arguments anymore. table_cache_remove_scan_limit option is also removed
* By default we now optimize the compilation for the compilation platform (using -march=native). If you want to build portable binary, use 'PORTABLE=1' before the make command.
* We now allow level-compaction to place files in different paths by
  specifying them in db_paths along with the target_size.
  Lower numbered levels will be placed earlier in the db_paths and higher
  numbered levels will be placed later in the db_paths vector.
* Potentially big performance improvements if you're using RocksDB with lots of column families (100-1000)
* Added BlockBasedTableOptions.format_version option, which allows user to specify which version of block based table he wants. As a general guideline, newer versions have more features, but might not be readable by older versions of RocksDB.
* Added new block based table format (version 2), which you can enable by setting BlockBasedTableOptions.format_version = 2. This format changes how we encode size information in compressed blocks and should help with memory allocations if you're using Zlib or BZip2 compressions.
* MemEnv (env that stores data in memory) is now available in default library build. You can create it by calling NewMemEnv().
* Add SliceTransform.SameResultWhenAppended() to help users determine it is safe to apply prefix bloom/hash.
* Block based table now makes use of prefix bloom filter if it is a full fulter.
* Block based table remembers whether a whole key or prefix based bloom filter is supported in SST files. Do a sanity check when reading the file with users' configuration.
* Fixed a bug in ReadOnlyBackupEngine that deleted corrupted backups in some cases, even though the engine was ReadOnly
* options.level_compaction_dynamic_level_bytes, a feature to allow RocksDB to pick dynamic base of bytes for levels. With this feature turned on, we will automatically adjust max bytes for each level. The goal of this feature is to have lower bound on size amplification. For more details, see comments in options.h.
* Added an abstract base class WriteBatchBase for write batches
* Fixed a bug where we start deleting files of a dropped column families even if there are still live references to it

### Public API changes
* Deprecated skip_log_error_on_recovery and table_cache_remove_scan_count_limit options.
* Logger method logv with log level parameter is now virtual

### RocksJava
* Added compression per level API.
* MemEnv is now available in RocksJava via RocksMemEnv class.
* lz4 compression is now included in rocksjava static library when running `make rocksdbjavastatic`.
* Overflowing a size_t when setting rocksdb options now throws an IllegalArgumentException, which removes the necessity for a developer to catch these Exceptions explicitly.

## 3.9.0 (12/8/2014)

### New Features
* Add rocksdb::GetThreadList(), which in the future will return the current status of all
  rocksdb-related threads.  We will have more code instruments in the following RocksDB
  releases.
* Change convert function in rocksdb/utilities/convenience.h to return Status instead of boolean.
  Also add support for nested options in convert function

### Public API changes
* New API to create a checkpoint added. Given a directory name, creates a new
  database which is an image of the existing database.
* New API LinkFile added to Env. If you implement your own Env class, an
  implementation of the API LinkFile will have to be provided.
* MemTableRep takes MemTableAllocator instead of Arena

### Improvements
* RocksDBLite library now becomes smaller and will be compiled with -fno-exceptions flag.

## 3.8.0 (11/14/2014)

### Public API changes
* BackupEngine::NewBackupEngine() was deprecated; please use BackupEngine::Open() from now on.
* BackupableDB/RestoreBackupableDB have new GarbageCollect() methods, which will clean up files from corrupt and obsolete backups.
* BackupableDB/RestoreBackupableDB have new GetCorruptedBackups() methods which list corrupt backups.

### Cleanup
* Bunch of code cleanup, some extra warnings turned on (-Wshadow, -Wshorten-64-to-32, -Wnon-virtual-dtor)

### New features
* CompactFiles and EventListener, although they are still in experimental state
* Full ColumnFamily support in RocksJava.

## 3.7.0 (11/6/2014)
### Public API changes
* Introduce SetOptions() API to allow adjusting a subset of options dynamically online
* Introduce 4 new convenient functions for converting Options from string: GetColumnFamilyOptionsFromMap(), GetColumnFamilyOptionsFromString(), GetDBOptionsFromMap(), GetDBOptionsFromString()
* Remove WriteBatchWithIndex.Delete() overloads using SliceParts
* When opening a DB, if options.max_background_compactions is larger than the existing low pri pool of options.env, it will enlarge it. Similarly, options.max_background_flushes is larger than the existing high pri pool of options.env, it will enlarge it.

## 3.6.0 (10/7/2014)
### Disk format changes
* If you're using RocksDB on ARM platforms and you're using default bloom filter, there is a disk format change you need to be aware of. There are three steps you need to do when you convert to new release: 1. turn off filter policy, 2. compact the whole database, 3. turn on filter policy

### Behavior changes
* We have refactored our system of stalling writes.  Any stall-related statistics' meanings are changed. Instead of per-write stall counts, we now count stalls per-epoch, where epochs are periods between flushes and compactions. You'll find more information in our Tuning Perf Guide once we release RocksDB 3.6.
* When disableDataSync=true, we no longer sync the MANIFEST file.
* Add identity_as_first_hash property to CuckooTable. SST file needs to be rebuilt to be opened by reader properly.

### Public API changes
* Change target_file_size_base type to uint64_t from int.
* Remove allow_thread_local. This feature was proved to be stable, so we are turning it always-on.

## 3.5.0 (9/3/2014)
### New Features
* Add include/utilities/write_batch_with_index.h, providing a utility class to query data out of WriteBatch when building it.
* Move BlockBasedTable related options to BlockBasedTableOptions from Options. Change corresponding JNI interface. Options affected include:
  no_block_cache, block_cache, block_cache_compressed, block_size, block_size_deviation, block_restart_interval, filter_policy, whole_key_filtering. filter_policy is changed to shared_ptr from a raw pointer.
* Remove deprecated options: disable_seek_compaction and db_stats_log_interval
* OptimizeForPointLookup() takes one parameter for block cache size. It now builds hash index, bloom filter, and block cache.

### Public API changes
* The Prefix Extractor used with V2 compaction filters is now passed user key to SliceTransform::Transform instead of unparsed RocksDB key.

## 3.4.0 (8/18/2014)
### New Features
* Support Multiple DB paths in universal style compactions
* Add feature of storing plain table index and bloom filter in SST file.
* CompactRange() will never output compacted files to level 0. This used to be the case when all the compaction input files were at level 0.
* Added iterate_upper_bound to define the extent upto which the forward iterator will return entries. This will prevent iterating over delete markers and overwritten entries for edge cases where you want to break out the iterator anyways. This may improve performance in case there are a large number of delete markers or overwritten entries.

### Public API changes
* DBOptions.db_paths now is a vector of a DBPath structure which indicates both of path and target size
* NewPlainTableFactory instead of bunch of parameters now accepts PlainTableOptions, which is defined in include/rocksdb/table.h
* Moved include/utilities/*.h to include/rocksdb/utilities/*.h
* Statistics APIs now take uint32_t as type instead of Tickers. Also make two access functions getTickerCount and histogramData const
* Add DB property rocksdb.estimate-num-keys, estimated number of live keys in DB.
* Add DB::GetIntProperty(), which returns DB properties that are integer as uint64_t.
* The Prefix Extractor used with V2 compaction filters is now passed user key to SliceTransform::Transform instead of unparsed RocksDB key.

## 3.3.0 (7/10/2014)
### New Features
* Added JSON API prototype.
* HashLinklist reduces performance outlier caused by skewed bucket by switching data in the bucket from linked list to skip list. Add parameter threshold_use_skiplist in NewHashLinkListRepFactory().
* RocksDB is now able to reclaim storage space more effectively during the compaction process.  This is done by compensating the size of each deletion entry by the 2X average value size, which makes compaction to be triggered by deletion entries more easily.
* Add TimeOut API to write.  Now WriteOptions have a variable called timeout_hint_us.  With timeout_hint_us set to non-zero, any write associated with this timeout_hint_us may be aborted when it runs longer than the specified timeout_hint_us, and it is guaranteed that any write completes earlier than the specified time-out will not be aborted due to the time-out condition.
* Add a rate_limiter option, which controls total throughput of flush and compaction. The throughput is specified in bytes/sec. Flush always has precedence over compaction when available bandwidth is constrained.

### Public API changes
* Removed NewTotalOrderPlainTableFactory because it is not used and implemented semantically incorrect.

## 3.2.0 (06/20/2014)

### Public API changes
* We removed seek compaction as a concept from RocksDB because:
1) It makes more sense for spinning disk workloads, while RocksDB is primarily designed for flash and memory,
2) It added some complexity to the important code-paths,
3) None of our internal customers were really using it.
Because of that, Options::disable_seek_compaction is now obsolete. It is still a parameter in Options, so it does not break the build, but it does not have any effect. We plan to completely remove it at some point, so we ask users to please remove this option from your code base.
* Add two parameters to NewHashLinkListRepFactory() for logging on too many entries in a hash bucket when flushing.
* Added new option BlockBasedTableOptions::hash_index_allow_collision. When enabled, prefix hash index for block-based table will not store prefix and allow hash collision, reducing memory consumption.

### New Features
* PlainTable now supports a new key encoding: for keys of the same prefix, the prefix is only written once. It can be enabled through encoding_type parameter of NewPlainTableFactory()
* Add AdaptiveTableFactory, which is used to convert from a DB of PlainTable to BlockBasedTabe, or vise versa. It can be created using NewAdaptiveTableFactory()

### Performance Improvements
* Tailing Iterator re-implemeted with ForwardIterator + Cascading Search Hint , see ~20% throughput improvement.

## 3.1.0 (05/21/2014)

### Public API changes
* Replaced ColumnFamilyOptions::table_properties_collectors with ColumnFamilyOptions::table_properties_collector_factories

### New Features
* Hash index for block-based table will be materialized and reconstructed more efficiently. Previously hash index is constructed by scanning the whole table during every table open.
* FIFO compaction style

## 3.0.0 (05/05/2014)

### Public API changes
* Added _LEVEL to all InfoLogLevel enums
* Deprecated ReadOptions.prefix and ReadOptions.prefix_seek. Seek() defaults to prefix-based seek when Options.prefix_extractor is supplied. More detail is documented in https://github.com/facebook/rocksdb/wiki/Prefix-Seek-API-Changes
* MemTableRepFactory::CreateMemTableRep() takes info logger as an extra parameter.

### New Features
* Column family support
* Added an option to use different checksum functions in BlockBasedTableOptions
* Added ApplyToAllCacheEntries() function to Cache

## 2.8.0 (04/04/2014)

* Removed arena.h from public header files.
* By default, checksums are verified on every read from database
* Change default value of several options, including: paranoid_checks=true, max_open_files=5000, level0_slowdown_writes_trigger=20, level0_stop_writes_trigger=24, disable_seek_compaction=true, max_background_flushes=1 and allow_mmap_writes=false
* Added is_manual_compaction to CompactionFilter::Context
* Added "virtual void WaitForJoin()" in class Env. Default operation is no-op.
* Removed BackupEngine::DeleteBackupsNewerThan() function
* Added new option -- verify_checksums_in_compaction
* Changed Options.prefix_extractor from raw pointer to shared_ptr (take ownership)
  Changed HashSkipListRepFactory and HashLinkListRepFactory constructor to not take SliceTransform object (use Options.prefix_extractor implicitly)
* Added Env::GetThreadPoolQueueLen(), which returns the waiting queue length of thread pools
* Added a command "checkconsistency" in ldb tool, which checks
  if file system state matches DB state (file existence and file sizes)
* Separate options related to block based table to a new struct BlockBasedTableOptions.
* WriteBatch has a new function Count() to return total size in the batch, and Data() now returns a reference instead of a copy
* Add more counters to perf context.
* Supports several more DB properties: compaction-pending, background-errors and cur-size-active-mem-table.

### New Features
* If we find one truncated record at the end of the MANIFEST or WAL files,
  we will ignore it. We assume that writers of these records were interrupted
  and that we can safely ignore it.
* A new SST format "PlainTable" is added, which is optimized for memory-only workloads. It can be created through NewPlainTableFactory() or NewTotalOrderPlainTableFactory().
* A new mem table implementation hash linked list optimizing for the case that there are only few keys for each prefix, which can be created through NewHashLinkListRepFactory().
* Merge operator supports a new function PartialMergeMulti() to allow users to do partial merges against multiple operands.
* Now compaction filter has a V2 interface. It buffers the kv-pairs sharing the same key prefix, process them in batches, and return the batched results back to DB. The new interface uses a new structure CompactionFilterContext for the same purpose as CompactionFilter::Context in V1.
* Geo-spatial support for locations and radial-search.

## 2.7.0 (01/28/2014)

### Public API changes

* Renamed `StackableDB::GetRawDB()` to `StackableDB::GetBaseDB()`.
* Renamed `WriteBatch::Data()` `const std::string& Data() const`.
* Renamed class `TableStats` to `TableProperties`.
* Deleted class `PrefixHashRepFactory`. Please use `NewHashSkipListRepFactory()` instead.
* Supported multi-threaded `EnableFileDeletions()` and `DisableFileDeletions()`.
* Added `DB::GetOptions()`.
* Added `DB::GetDbIdentity()`.

### New Features

* Added [BackupableDB](https://github.com/facebook/rocksdb/wiki/How-to-backup-RocksDB%3F)
* Implemented [TailingIterator](https://github.com/facebook/rocksdb/wiki/Tailing-Iterator), a special type of iterator that
  doesn't create a snapshot (can be used to read newly inserted data)
  and is optimized for doing sequential reads.
* Added property block for table, which allows (1) a table to store
  its metadata and (2) end user to collect and store properties they
  are interested in.
* Enabled caching index and filter block in block cache (turned off by default).
* Supported error report when doing manual compaction.
* Supported additional Linux platform flavors and Mac OS.
* Put with `SliceParts` - Variant of `Put()` that gathers output like `writev(2)`
* Bug fixes and code refactor for compatibility with upcoming Column
  Family feature.

### Performance Improvements

* Huge benchmark performance improvements by multiple efforts. For example, increase in readonly QPS from about 530k in 2.6 release to 1.1 million in 2.7 [1]
* Speeding up a way RocksDB deleted obsolete files - no longer listing the whole directory under a lock -- decrease in p99
* Use raw pointer instead of shared pointer for statistics: [5b825d](https://github.com/facebook/rocksdb/commit/5b825d6964e26ec3b4bb6faa708ebb1787f1d7bd) -- huge increase in performance -- shared pointers are slow
* Optimized locking for `Get()` -- [1fdb3f](https://github.com/facebook/rocksdb/commit/1fdb3f7dc60e96394e3e5b69a46ede5d67fb976c) -- 1.5x QPS increase for some workloads
* Cache speedup - [e8d40c3](https://github.com/facebook/rocksdb/commit/e8d40c31b3cca0c3e1ae9abe9b9003b1288026a9)
* Implemented autovector, which allocates first N elements on stack. Most of vectors in RocksDB are small. Also, we never want to allocate heap objects while holding a mutex. -- [c01676e4](https://github.com/facebook/rocksdb/commit/c01676e46d3be08c3c140361ef1f5884f47d3b3c)
* Lots of efforts to move malloc, memcpy and IO outside of locks