mirror of https://github.com/facebook/rocksdb.git
272 lines
13 KiB
C++
272 lines
13 KiB
C++
// Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved.
|
|
// This source code is licensed under both the GPLv2 (found in the
|
|
// COPYING file in the root directory) and Apache 2.0 License
|
|
// (found in the LICENSE.Apache file in the root directory).
|
|
|
|
#include "cache/cache_key.h"
|
|
|
|
#include <algorithm>
|
|
#include <atomic>
|
|
|
|
#include "rocksdb/cache.h"
|
|
#include "table/unique_id_impl.h"
|
|
#include "util/hash.h"
|
|
#include "util/math.h"
|
|
|
|
namespace ROCKSDB_NAMESPACE {
|
|
|
|
// Value space plan for CacheKey:
|
|
//
|
|
// session_etc64_ | offset_etc64_ | Only generated by
|
|
// ---------------+---------------+------------------------------------------
|
|
// 0 | 0 | Reserved for "empty" CacheKey()
|
|
// 0 | > 0, < 1<<63 | CreateUniqueForCacheLifetime
|
|
// 0 | >= 1<<63 | CreateUniqueForProcessLifetime
|
|
// > 0 | any | OffsetableCacheKey.WithOffset
|
|
|
|
CacheKey CacheKey::CreateUniqueForCacheLifetime(Cache *cache) {
|
|
// +1 so that we can reserve all zeros for "unset" cache key
|
|
uint64_t id = cache->NewId() + 1;
|
|
// Ensure we don't collide with CreateUniqueForProcessLifetime
|
|
assert((id >> 63) == 0U);
|
|
return CacheKey(0, id);
|
|
}
|
|
|
|
CacheKey CacheKey::CreateUniqueForProcessLifetime() {
|
|
// To avoid colliding with CreateUniqueForCacheLifetime, assuming
|
|
// Cache::NewId counts up from zero, here we count down from UINT64_MAX.
|
|
// If this ever becomes a point of contention, we could use CoreLocalArray.
|
|
static std::atomic<uint64_t> counter{UINT64_MAX};
|
|
uint64_t id = counter.fetch_sub(1, std::memory_order_relaxed);
|
|
// Ensure we don't collide with CreateUniqueForCacheLifetime
|
|
assert((id >> 63) == 1U);
|
|
return CacheKey(0, id);
|
|
}
|
|
|
|
// Value plan for CacheKeys from OffsetableCacheKey, assuming that
|
|
// db_session_ids are generated from a base_session_id and
|
|
// session_id_counter (by SemiStructuredUniqueIdGen+EncodeSessionId
|
|
// in DBImpl::GenerateDbSessionId):
|
|
//
|
|
// Conceptual inputs:
|
|
// db_id (unstructured, from GenerateRawUniqueId or equiv)
|
|
// * could be shared between cloned DBs but rare
|
|
// * could be constant, if session id suffices
|
|
// base_session_id (unstructured, from GenerateRawUniqueId)
|
|
// session_id_counter (structured)
|
|
// * usually much smaller than 2**24
|
|
// file_number (structured)
|
|
// * usually smaller than 2**24
|
|
// offset_in_file (structured, might skip lots of values)
|
|
// * usually smaller than 2**32
|
|
// max_offset determines placement of file_number to prevent
|
|
// overlapping with offset
|
|
//
|
|
// Outputs come from bitwise-xor of the constituent pieces, low bits on left:
|
|
//
|
|
// |------------------------- session_etc64 -------------------------|
|
|
// | +++++++++++++++ base_session_id (lower 64 bits) +++++++++++++++ |
|
|
// |-----------------------------------------------------------------|
|
|
// | session_id_counter ...| |
|
|
// |-----------------------------------------------------------------|
|
|
// | | ... file_number |
|
|
// | | overflow & meta |
|
|
// |-----------------------------------------------------------------|
|
|
//
|
|
//
|
|
// |------------------------- offset_etc64 --------------------------|
|
|
// | hash of: ++++++++++++++++++++++++++++++++++++++++++++++++++++++ |
|
|
// | * base_session_id (upper ~39 bits) |
|
|
// | * db_id (~122 bits entropy) |
|
|
// |-----------------------------------------------------------------|
|
|
// | offset_in_file ............... | |
|
|
// |-----------------------------------------------------------------|
|
|
// | | file_number, 0-3 |
|
|
// | | lower bytes |
|
|
// |-----------------------------------------------------------------|
|
|
//
|
|
// Based on max_offset, a maximal number of bytes 0..3 is chosen for
|
|
// including from lower bits of file_number in offset_etc64. The choice
|
|
// is encoded in two bits of metadata going into session_etc64, though
|
|
// the common case of 3 bytes is encoded as 0 so that session_etc64
|
|
// is unmodified by file_number concerns in the common case.
|
|
//
|
|
// There is nothing preventing "file number overflow & meta" from meeting
|
|
// and overlapping with session_id_counter, but reaching such a case requires
|
|
// an intractable combination of large file offsets (thus at least some large
|
|
// files), large file numbers (thus large number of files generated), and
|
|
// large number of session IDs generated in a single process. A trillion each
|
|
// (2**40) of session ids, offsets, and file numbers comes to 120 bits.
|
|
// With two bits of metadata and byte granularity, this is on the verge of
|
|
// overlap, but even in the overlap case, it doesn't seem likely that
|
|
// a file from billions of files or session ids ago will still be live
|
|
// or cached.
|
|
//
|
|
// In fact, if our SST files are all < 4TB (see
|
|
// BlockBasedTable::kMaxFileSizeStandardEncoding), then SST files generated
|
|
// in a single process are guaranteed to have unique cache keys, unless/until
|
|
// number session ids * max file number = 2**86, e.g. 1 trillion DB::Open in
|
|
// a single process and 64 trillion files generated. Even at that point, to
|
|
// see a collision we would need a miraculous re-synchronization of session
|
|
// id and file number, along with a live file or stale cache entry from
|
|
// trillions of files ago.
|
|
//
|
|
// How https://github.com/pdillinger/unique_id applies here:
|
|
// Every bit of output always includes "unstructured" uniqueness bits and
|
|
// often combines with "structured" uniqueness bits. The "unstructured" bits
|
|
// change infrequently: only when we cannot guarantee our state tracking for
|
|
// "structured" uniqueness hasn't been cloned. Using a static
|
|
// SemiStructuredUniqueIdGen for db_session_ids, this means we only get an
|
|
// "all new" session id when a new process uses RocksDB. (Between processes,
|
|
// we don't know if a DB or other persistent storage has been cloned.) Within
|
|
// a process, only the session_lower of the db_session_id changes
|
|
// incrementally ("structured" uniqueness).
|
|
//
|
|
// This basically means that our offsets, counters and file numbers allow us
|
|
// to do somewhat "better than random" (birthday paradox) while in the
|
|
// degenerate case of completely new session for each tiny file, we still
|
|
// have strong uniqueness properties from the birthday paradox, with ~103
|
|
// bit session IDs or up to 128 bits entropy with different DB IDs sharing a
|
|
// cache.
|
|
//
|
|
// More collision probability analysis:
|
|
// Suppose a RocksDB host generates (generously) 2 GB/s (10TB data, 17 DWPD)
|
|
// with average process/session lifetime of (pessimistically) 4 minutes.
|
|
// In 180 days (generous allowable data lifespan), we generate 31 million GB
|
|
// of data, or 2^55 bytes, and 2^16 "all new" session IDs.
|
|
//
|
|
// First, suppose this is in a single DB (lifetime 180 days):
|
|
// 128 bits cache key size
|
|
// - 55 <- ideal size for byte offsets + file numbers
|
|
// - 2 <- bits for offsets and file numbers not exactly powers of two
|
|
// - 2 <- bits for file number encoding metadata
|
|
// + 2 <- bits saved not using byte offsets in BlockBasedTable::GetCacheKey
|
|
// ----
|
|
// 71 <- bits remaining for distinguishing session IDs
|
|
// The probability of a collision in 71 bits of session ID data is less than
|
|
// 1 in 2**(71 - (2 * 16)), or roughly 1 in a trillion. And this assumes all
|
|
// data from the last 180 days is in cache for potential collision, and that
|
|
// cache keys under each session id exhaustively cover the remaining 57 bits
|
|
// while in reality they'll only cover a small fraction of it.
|
|
//
|
|
// Although data could be transferred between hosts, each host has its own
|
|
// cache and we are already assuming a high rate of "all new" session ids.
|
|
// So this doesn't really change the collision calculation. Across a fleet
|
|
// of 1 million, each with <1 in a trillion collision possibility,
|
|
// fleetwide collision probability is <1 in a million.
|
|
//
|
|
// Now suppose we have many DBs per host, say 2**10, with same host-wide write
|
|
// rate and process/session lifetime. File numbers will be ~10 bits smaller
|
|
// and we will have 2**10 times as many session IDs because of simultaneous
|
|
// lifetimes. So now collision chance is less than 1 in 2**(81 - (2 * 26)),
|
|
// or roughly 1 in a billion.
|
|
//
|
|
// Suppose instead we generated random or hashed cache keys for each
|
|
// (compressed) block. For 1KB compressed block size, that is 2^45 cache keys
|
|
// in 180 days. Collision probability is more easily estimated at roughly
|
|
// 1 in 2**(128 - (2 * 45)) or roughly 1 in a trillion (assuming all
|
|
// data from the last 180 days is in cache, but NOT the other assumption
|
|
// for the 1 in a trillion estimate above).
|
|
//
|
|
// Conclusion: Burning through session IDs, particularly "all new" IDs that
|
|
// only arise when a new process is started, is the only way to have a
|
|
// plausible chance of cache key collision. When processes live for hours
|
|
// or days, the chance of a cache key collision seems more plausibly due
|
|
// to bad hardware than to bad luck in random session ID data.
|
|
//
|
|
OffsetableCacheKey::OffsetableCacheKey(const std::string &db_id,
|
|
const std::string &db_session_id,
|
|
uint64_t file_number,
|
|
uint64_t max_offset) {
|
|
#ifndef NDEBUG
|
|
max_offset_ = max_offset;
|
|
#endif
|
|
// Closely related to GetSstInternalUniqueId, but only need 128 bits and
|
|
// need to include an offset within the file.
|
|
// See also https://github.com/pdillinger/unique_id for background.
|
|
uint64_t session_upper = 0; // Assignment to appease clang-analyze
|
|
uint64_t session_lower = 0; // Assignment to appease clang-analyze
|
|
{
|
|
Status s = DecodeSessionId(db_session_id, &session_upper, &session_lower);
|
|
if (!s.ok()) {
|
|
// A reasonable fallback in case malformed
|
|
Hash2x64(db_session_id.data(), db_session_id.size(), &session_upper,
|
|
&session_lower);
|
|
}
|
|
}
|
|
|
|
// Hash the session upper (~39 bits entropy) and DB id (120+ bits entropy)
|
|
// for more global uniqueness entropy.
|
|
// (It is possible that many DBs descended from one common DB id are copied
|
|
// around and proliferate, in which case session id is critical, but it is
|
|
// more common for different DBs to have different DB ids.)
|
|
uint64_t db_hash = Hash64(db_id.data(), db_id.size(), session_upper);
|
|
|
|
// This establishes the db+session id part of the cache key.
|
|
//
|
|
// Exactly preserve (in common cases; see modifiers below) session lower to
|
|
// ensure that session ids generated during the same process lifetime are
|
|
// guaranteed unique.
|
|
//
|
|
// We put this first for CommonPrefixSlice(), so that a small-ish set of
|
|
// cache key prefixes to cover entries relevant to any DB.
|
|
session_etc64_ = session_lower;
|
|
// This provides extra entopy in case of different DB id or process
|
|
// generating a session id, but is also partly/variably obscured by
|
|
// file_number and offset (see below).
|
|
offset_etc64_ = db_hash;
|
|
|
|
// Into offset_etc64_ we are (eventually) going to pack & xor in an offset and
|
|
// a file_number, but we might need the file_number to overflow into
|
|
// session_etc64_. (There must only be one session_etc64_ value per
|
|
// file, and preferably shared among many files.)
|
|
//
|
|
// Figure out how many bytes of file_number we are going to be able to
|
|
// pack in with max_offset, though our encoding will only support packing
|
|
// in up to 3 bytes of file_number. (16M file numbers is enough for a new
|
|
// file number every second for half a year.)
|
|
int file_number_bytes_in_offset_etc =
|
|
(63 - FloorLog2(max_offset | 0x100000000U)) / 8;
|
|
int file_number_bits_in_offset_etc = file_number_bytes_in_offset_etc * 8;
|
|
|
|
// Assert two bits of metadata
|
|
assert(file_number_bytes_in_offset_etc >= 0 &&
|
|
file_number_bytes_in_offset_etc <= 3);
|
|
// Assert we couldn't have used a larger allowed number of bytes (shift
|
|
// would chop off bytes).
|
|
assert(file_number_bytes_in_offset_etc == 3 ||
|
|
(max_offset << (file_number_bits_in_offset_etc + 8) >>
|
|
(file_number_bits_in_offset_etc + 8)) != max_offset);
|
|
|
|
uint64_t mask = (uint64_t{1} << (file_number_bits_in_offset_etc)) - 1;
|
|
// Pack into high bits of etc so that offset can go in low bits of etc
|
|
// TODO: could be EndianSwapValue?
|
|
uint64_t offset_etc_modifier = ReverseBits(file_number & mask);
|
|
assert(offset_etc_modifier << file_number_bits_in_offset_etc == 0U);
|
|
|
|
// Overflow and 3 - byte count (likely both zero) go into session_id part
|
|
uint64_t session_etc_modifier =
|
|
(file_number >> file_number_bits_in_offset_etc << 2) |
|
|
static_cast<uint64_t>(3 - file_number_bytes_in_offset_etc);
|
|
// Packed into high bits to minimize interference with session id counter.
|
|
session_etc_modifier = ReverseBits(session_etc_modifier);
|
|
|
|
// Assert session_id part is only modified in extreme cases
|
|
assert(session_etc_modifier == 0 || file_number > /*3 bytes*/ 0xffffffU ||
|
|
max_offset > /*5 bytes*/ 0xffffffffffU);
|
|
|
|
// Xor in the modifiers
|
|
session_etc64_ ^= session_etc_modifier;
|
|
offset_etc64_ ^= offset_etc_modifier;
|
|
|
|
// Although DBImpl guarantees (in recent versions) that session_lower is not
|
|
// zero, that's not entirely sufficient to guarantee that session_etc64_ is
|
|
// not zero (so that the 0 case can be used by CacheKey::CreateUnique*)
|
|
if (session_etc64_ == 0U) {
|
|
session_etc64_ = session_upper | 1U;
|
|
}
|
|
assert(session_etc64_ != 0);
|
|
}
|
|
|
|
} // namespace ROCKSDB_NAMESPACE
|