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rocksdb/table/block_based_table_reader.h

678 lines
27 KiB

// Copyright (c) 2011-present, Facebook, Inc. 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).
//
// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file. See the AUTHORS file for names of contributors.
#pragma once
#include <stdint.h>
#include <memory>
#include <set>
#include <string>
#include <utility>
#include <vector>
#include "db/range_tombstone_fragmenter.h"
#include "options/cf_options.h"
#include "rocksdb/options.h"
#include "rocksdb/persistent_cache.h"
#include "rocksdb/statistics.h"
#include "rocksdb/status.h"
#include "rocksdb/table.h"
#include "table/block.h"
#include "table/block_based_table_factory.h"
#include "table/filter_block.h"
#include "table/format.h"
#include "table/persistent_cache_helper.h"
#include "table/table_properties_internal.h"
#include "table/table_reader.h"
#include "table/two_level_iterator.h"
#include "util/coding.h"
#include "util/file_reader_writer.h"
namespace rocksdb {
class BlockHandle;
class Cache;
class FilterBlockReader;
class BlockBasedFilterBlockReader;
class FullFilterBlockReader;
class Footer;
class InternalKeyComparator;
class Iterator;
class RandomAccessFile;
class TableCache;
class TableReader;
class WritableFile;
struct BlockBasedTableOptions;
struct EnvOptions;
struct ReadOptions;
class GetContext;
using std::unique_ptr;
typedef std::vector<std::pair<std::string, std::string>> KVPairBlock;
// A Table is a sorted map from strings to strings. Tables are
// immutable and persistent. A Table may be safely accessed from
// multiple threads without external synchronization.
class BlockBasedTable : public TableReader {
public:
static const std::string kFilterBlockPrefix;
static const std::string kFullFilterBlockPrefix;
static const std::string kPartitionedFilterBlockPrefix;
// The longest prefix of the cache key used to identify blocks.
// For Posix files the unique ID is three varints.
static const size_t kMaxCacheKeyPrefixSize = kMaxVarint64Length * 3 + 1;
// Attempt to open the table that is stored in bytes [0..file_size)
// of "file", and read the metadata entries necessary to allow
// retrieving data from the table.
//
// If successful, returns ok and sets "*table_reader" to the newly opened
// table. The client should delete "*table_reader" when no longer needed.
// If there was an error while initializing the table, sets "*table_reader"
// to nullptr and returns a non-ok status.
//
// @param file must remain live while this Table is in use.
// @param prefetch_index_and_filter_in_cache can be used to disable
// prefetching of
// index and filter blocks into block cache at startup
// @param skip_filters Disables loading/accessing the filter block. Overrides
// prefetch_index_and_filter_in_cache, so filter will be skipped if both
// are set.
static Status Open(const ImmutableCFOptions& ioptions,
const EnvOptions& env_options,
const BlockBasedTableOptions& table_options,
const InternalKeyComparator& internal_key_comparator,
unique_ptr<RandomAccessFileReader>&& file,
uint64_t file_size, unique_ptr<TableReader>* table_reader,
const SliceTransform* prefix_extractor = nullptr,
bool prefetch_index_and_filter_in_cache = true,
bool skip_filters = false, int level = -1,
const bool immortal_table = false,
const SequenceNumber largest_seqno = 0,
TailPrefetchStats* tail_prefetch_stats = nullptr);
bool PrefixMayMatch(const Slice& internal_key,
const ReadOptions& read_options,
const SliceTransform* options_prefix_extractor,
const bool need_upper_bound_check);
// Returns a new iterator over the table contents.
// The result of NewIterator() is initially invalid (caller must
// call one of the Seek methods on the iterator before using it).
// @param skip_filters Disables loading/accessing the filter block
InternalIterator* NewIterator(const ReadOptions&,
const SliceTransform* prefix_extractor,
Arena* arena = nullptr,
bool skip_filters = false,
bool for_compaction = false) override;
InternalIterator* NewRangeTombstoneIterator(
const ReadOptions& read_options) override;
// @param skip_filters Disables loading/accessing the filter block
Status Get(const ReadOptions& readOptions, const Slice& key,
GetContext* get_context, const SliceTransform* prefix_extractor,
bool skip_filters = false) override;
// Pre-fetch the disk blocks that correspond to the key range specified by
// (kbegin, kend). The call will return error status in the event of
// IO or iteration error.
Status Prefetch(const Slice* begin, const Slice* end) override;
// Given a key, return an approximate byte offset in the file where
// the data for that key begins (or would begin if the key were
// present in the file). The returned value is in terms of file
// bytes, and so includes effects like compression of the underlying data.
// E.g., the approximate offset of the last key in the table will
// be close to the file length.
uint64_t ApproximateOffsetOf(const Slice& key) override;
// Returns true if the block for the specified key is in cache.
// REQUIRES: key is in this table && block cache enabled
bool TEST_KeyInCache(const ReadOptions& options, const Slice& key);
// Set up the table for Compaction. Might change some parameters with
// posix_fadvise
void SetupForCompaction() override;
std::shared_ptr<const TableProperties> GetTableProperties() const override;
size_t ApproximateMemoryUsage() const override;
// convert SST file to a human readable form
Status DumpTable(WritableFile* out_file,
const SliceTransform* prefix_extractor = nullptr) override;
Status VerifyChecksum() override;
void Close() override;
~BlockBasedTable();
bool TEST_filter_block_preloaded() const;
bool TEST_index_reader_preloaded() const;
// IndexReader is the interface that provide the functionality for index
// access.
class IndexReader {
public:
explicit IndexReader(const InternalKeyComparator* icomparator,
Statistics* stats)
: icomparator_(icomparator), statistics_(stats) {}
virtual ~IndexReader() {}
// Create an iterator for index access.
// If iter is null then a new object is created on heap and the callee will
// have the ownership. If a non-null iter is passed in it will be used, and
// the returned value is either the same as iter or a new on-heap object
// that
// wrapps the passed iter. In the latter case the return value would point
// to
// a different object then iter and the callee has the ownership of the
// returned object.
virtual InternalIteratorBase<BlockHandle>* NewIterator(
IndexBlockIter* iter = nullptr, bool total_order_seek = true,
bool fill_cache = true) = 0;
// The size of the index.
virtual size_t size() const = 0;
// Memory usage of the index block
virtual size_t usable_size() const = 0;
// return the statistics pointer
virtual Statistics* statistics() const { return statistics_; }
// Report an approximation of how much memory has been used other than
// memory
// that was allocated in block cache.
virtual size_t ApproximateMemoryUsage() const = 0;
virtual void CacheDependencies(bool /* unused */) {}
// Prefetch all the blocks referenced by this index to the buffer
void PrefetchBlocks(FilePrefetchBuffer* buf);
protected:
const InternalKeyComparator* icomparator_;
private:
Statistics* statistics_;
};
static Slice GetCacheKey(const char* cache_key_prefix,
size_t cache_key_prefix_size,
const BlockHandle& handle, char* cache_key);
// Retrieve all key value pairs from data blocks in the table.
// The key retrieved are internal keys.
Status GetKVPairsFromDataBlocks(std::vector<KVPairBlock>* kv_pair_blocks);
template <class TValue>
struct CachableEntry;
struct Rep;
Rep* get_rep() { return rep_; }
// input_iter: if it is not null, update this one and return it as Iterator
template <typename TBlockIter>
static TBlockIter* NewDataBlockIterator(
Rep* rep, const ReadOptions& ro, const Slice& index_value,
TBlockIter* input_iter = nullptr, bool is_index = false,
bool key_includes_seq = true, bool index_key_is_full = true,
GetContext* get_context = nullptr,
FilePrefetchBuffer* prefetch_buffer = nullptr);
template <typename TBlockIter>
static TBlockIter* NewDataBlockIterator(
Rep* rep, const ReadOptions& ro, const BlockHandle& block_hanlde,
TBlockIter* input_iter = nullptr, bool is_index = false,
bool key_includes_seq = true, bool index_key_is_full = true,
GetContext* get_context = nullptr, Status s = Status(),
FilePrefetchBuffer* prefetch_buffer = nullptr);
class PartitionedIndexIteratorState;
friend class PartitionIndexReader;
protected:
Rep* rep_;
explicit BlockBasedTable(Rep* rep) : rep_(rep) {}
private:
friend class MockedBlockBasedTable;
static std::atomic<uint64_t> next_cache_key_id_;
// If block cache enabled (compressed or uncompressed), looks for the block
// identified by handle in (1) uncompressed cache, (2) compressed cache, and
// then (3) file. If found, inserts into the cache(s) that were searched
// unsuccessfully (e.g., if found in file, will add to both uncompressed and
// compressed caches if they're enabled).
//
// @param block_entry value is set to the uncompressed block if found. If
// in uncompressed block cache, also sets cache_handle to reference that
// block.
static Status MaybeLoadDataBlockToCache(FilePrefetchBuffer* prefetch_buffer,
Rep* rep, const ReadOptions& ro,
const BlockHandle& handle,
Slice compression_dict,
CachableEntry<Block>* block_entry,
bool is_index = false,
GetContext* get_context = nullptr);
// For the following two functions:
// if `no_io == true`, we will not try to read filter/index from sst file
// were they not present in cache yet.
CachableEntry<FilterBlockReader> GetFilter(
const SliceTransform* prefix_extractor = nullptr,
FilePrefetchBuffer* prefetch_buffer = nullptr, bool no_io = false,
GetContext* get_context = nullptr) const;
virtual CachableEntry<FilterBlockReader> GetFilter(
FilePrefetchBuffer* prefetch_buffer, const BlockHandle& filter_blk_handle,
const bool is_a_filter_partition, bool no_io, GetContext* get_context,
const SliceTransform* prefix_extractor = nullptr) const;
// Get the iterator from the index reader.
// If input_iter is not set, return new Iterator
// If input_iter is set, update it and return it as Iterator
//
// Note: ErrorIterator with Status::Incomplete shall be returned if all the
// following conditions are met:
// 1. We enabled table_options.cache_index_and_filter_blocks.
// 2. index is not present in block cache.
// 3. We disallowed any io to be performed, that is, read_options ==
// kBlockCacheTier
InternalIteratorBase<BlockHandle>* NewIndexIterator(
const ReadOptions& read_options, bool need_upper_bound_check = false,
IndexBlockIter* input_iter = nullptr,
CachableEntry<IndexReader>* index_entry = nullptr,
GetContext* get_context = nullptr);
// Read block cache from block caches (if set): block_cache and
// block_cache_compressed.
// On success, Status::OK with be returned and @block will be populated with
// pointer to the block as well as its block handle.
// @param compression_dict Data for presetting the compression library's
// dictionary.
static Status GetDataBlockFromCache(
const Slice& block_cache_key, const Slice& compressed_block_cache_key,
Cache* block_cache, Cache* block_cache_compressed,
const ImmutableCFOptions& ioptions, const ReadOptions& read_options,
BlockBasedTable::CachableEntry<Block>* block, uint32_t format_version,
const Slice& compression_dict, size_t read_amp_bytes_per_bit,
bool is_index = false, GetContext* get_context = nullptr,
MemoryAllocator* allocator = nullptr);
// Put a raw block (maybe compressed) to the corresponding block caches.
// This method will perform decompression against raw_block if needed and then
// populate the block caches.
// On success, Status::OK will be returned; also @block will be populated with
// uncompressed block and its cache handle.
//
// REQUIRES: raw_block is heap-allocated. PutDataBlockToCache() will be
// responsible for releasing its memory if error occurs.
// @param compression_dict Data for presetting the compression library's
// dictionary.
static Status PutDataBlockToCache(
const Slice& block_cache_key, const Slice& compressed_block_cache_key,
Cache* block_cache, Cache* block_cache_compressed,
const ReadOptions& read_options, const ImmutableCFOptions& ioptions,
CachableEntry<Block>* block, Block* raw_block, uint32_t format_version,
const Slice& compression_dict, size_t read_amp_bytes_per_bit,
bool is_index = false, Cache::Priority pri = Cache::Priority::LOW,
GetContext* get_context = nullptr, MemoryAllocator* allocator = nullptr);
// Calls (*handle_result)(arg, ...) repeatedly, starting with the entry found
// after a call to Seek(key), until handle_result returns false.
// May not make such a call if filter policy says that key is not present.
friend class TableCache;
friend class BlockBasedTableBuilder;
void ReadMeta(const Footer& footer);
// Figure the index type, update it in rep_, and also return it.
BlockBasedTableOptions::IndexType UpdateIndexType();
// Create a index reader based on the index type stored in the table.
// Optionally, user can pass a preloaded meta_index_iter for the index that
// need to access extra meta blocks for index construction. This parameter
// helps avoid re-reading meta index block if caller already created one.
Status CreateIndexReader(
FilePrefetchBuffer* prefetch_buffer, IndexReader** index_reader,
InternalIterator* preloaded_meta_index_iter = nullptr,
const int level = -1);
bool FullFilterKeyMayMatch(
const ReadOptions& read_options, FilterBlockReader* filter,
const Slice& user_key, const bool no_io,
const SliceTransform* prefix_extractor = nullptr) const;
// Read the meta block from sst.
static Status ReadMetaBlock(Rep* rep, FilePrefetchBuffer* prefetch_buffer,
std::unique_ptr<Block>* meta_block,
std::unique_ptr<InternalIterator>* iter);
Status VerifyChecksumInBlocks(InternalIteratorBase<Slice>* index_iter);
Status VerifyChecksumInBlocks(InternalIteratorBase<BlockHandle>* index_iter);
// Create the filter from the filter block.
virtual FilterBlockReader* ReadFilter(
FilePrefetchBuffer* prefetch_buffer, const BlockHandle& filter_handle,
const bool is_a_filter_partition,
const SliceTransform* prefix_extractor = nullptr) const;
static void SetupCacheKeyPrefix(Rep* rep, uint64_t file_size);
// Generate a cache key prefix from the file
static void GenerateCachePrefix(Cache* cc,
RandomAccessFile* file, char* buffer, size_t* size);
static void GenerateCachePrefix(Cache* cc,
WritableFile* file, char* buffer, size_t* size);
// Helper functions for DumpTable()
Status DumpIndexBlock(WritableFile* out_file);
Status DumpDataBlocks(WritableFile* out_file);
void DumpKeyValue(const Slice& key, const Slice& value,
WritableFile* out_file);
// No copying allowed
explicit BlockBasedTable(const TableReader&) = delete;
void operator=(const TableReader&) = delete;
friend class PartitionedFilterBlockReader;
friend class PartitionedFilterBlockTest;
InternalIterator* NewUnfragmentedRangeTombstoneIterator(
const ReadOptions& read_options);
};
// Maitaning state of a two-level iteration on a partitioned index structure
class BlockBasedTable::PartitionedIndexIteratorState
: public TwoLevelIteratorState {
public:
PartitionedIndexIteratorState(
BlockBasedTable* table,
std::unordered_map<uint64_t, CachableEntry<Block>>* block_map,
const bool index_key_includes_seq, const bool index_key_is_full);
InternalIteratorBase<BlockHandle>* NewSecondaryIterator(
const BlockHandle& index_value) override;
private:
// Don't own table_
BlockBasedTable* table_;
std::unordered_map<uint64_t, CachableEntry<Block>>* block_map_;
bool index_key_includes_seq_;
bool index_key_is_full_;
};
// CachableEntry represents the entries that *may* be fetched from block cache.
// field `value` is the item we want to get.
// field `cache_handle` is the cache handle to the block cache. If the value
// was not read from cache, `cache_handle` will be nullptr.
template <class TValue>
struct BlockBasedTable::CachableEntry {
CachableEntry(TValue* _value, Cache::Handle* _cache_handle)
: value(_value), cache_handle(_cache_handle) {}
CachableEntry() : CachableEntry(nullptr, nullptr) {}
void Release(Cache* cache, bool force_erase = false) {
if (cache_handle) {
cache->Release(cache_handle, force_erase);
value = nullptr;
cache_handle = nullptr;
}
}
bool IsSet() const { return cache_handle != nullptr; }
TValue* value = nullptr;
// if the entry is from the cache, cache_handle will be populated.
Cache::Handle* cache_handle = nullptr;
};
struct BlockBasedTable::Rep {
Rep(const ImmutableCFOptions& _ioptions, const EnvOptions& _env_options,
const BlockBasedTableOptions& _table_opt,
const InternalKeyComparator& _internal_comparator, bool skip_filters,
int _level, const bool _immortal_table)
: ioptions(_ioptions),
env_options(_env_options),
table_options(_table_opt),
filter_policy(skip_filters ? nullptr : _table_opt.filter_policy.get()),
internal_comparator(_internal_comparator),
filter_type(FilterType::kNoFilter),
index_type(BlockBasedTableOptions::IndexType::kBinarySearch),
hash_index_allow_collision(false),
whole_key_filtering(_table_opt.whole_key_filtering),
prefix_filtering(true),
range_del_handle(BlockHandle::NullBlockHandle()),
global_seqno(kDisableGlobalSequenceNumber),
level(_level),
immortal_table(_immortal_table) {}
const ImmutableCFOptions& ioptions;
const EnvOptions& env_options;
const BlockBasedTableOptions table_options;
const FilterPolicy* const filter_policy;
const InternalKeyComparator& internal_comparator;
Status status;
unique_ptr<RandomAccessFileReader> file;
char cache_key_prefix[kMaxCacheKeyPrefixSize];
size_t cache_key_prefix_size = 0;
char persistent_cache_key_prefix[kMaxCacheKeyPrefixSize];
size_t persistent_cache_key_prefix_size = 0;
char compressed_cache_key_prefix[kMaxCacheKeyPrefixSize];
size_t compressed_cache_key_prefix_size = 0;
uint64_t dummy_index_reader_offset =
0; // ID that is unique for the block cache.
PersistentCacheOptions persistent_cache_options;
// Footer contains the fixed table information
Footer footer;
// index_reader and filter will be populated and used only when
// options.block_cache is nullptr; otherwise we will get the index block via
// the block cache.
unique_ptr<IndexReader> index_reader;
unique_ptr<FilterBlockReader> filter;
enum class FilterType {
kNoFilter,
kFullFilter,
kBlockFilter,
kPartitionedFilter,
};
FilterType filter_type;
BlockHandle filter_handle;
std::shared_ptr<const TableProperties> table_properties;
// Block containing the data for the compression dictionary. We take ownership
// for the entire block struct, even though we only use its Slice member. This
// is easier because the Slice member depends on the continued existence of
// another member ("allocation").
std::unique_ptr<const BlockContents> compression_dict_block;
BlockBasedTableOptions::IndexType index_type;
bool hash_index_allow_collision;
bool whole_key_filtering;
bool prefix_filtering;
// TODO(kailiu) It is very ugly to use internal key in table, since table
// module should not be relying on db module. However to make things easier
// and compatible with existing code, we introduce a wrapper that allows
// block to extract prefix without knowing if a key is internal or not.
unique_ptr<SliceTransform> internal_prefix_transform;
std::shared_ptr<const SliceTransform> table_prefix_extractor;
// only used in level 0 files when pin_l0_filter_and_index_blocks_in_cache is
// true or in all levels when pin_top_level_index_and_filter is set in
// combination with partitioned index/filters: then we do use the LRU cache,
// but we always keep the filter & index block's handle checked out here (=we
// don't call Release()), plus the parsed out objects the LRU cache will never
// push flush them out, hence they're pinned
CachableEntry<FilterBlockReader> filter_entry;
CachableEntry<IndexReader> index_entry;
// range deletion meta-block is pinned through reader's lifetime when LRU
// cache is enabled.
CachableEntry<Block> range_del_entry;
BlockHandle range_del_handle;
std::shared_ptr<const FragmentedRangeTombstoneList> fragmented_range_dels;
// If global_seqno is used, all Keys in this file will have the same
// seqno with value `global_seqno`.
//
// A value of kDisableGlobalSequenceNumber means that this feature is disabled
// and every key have it's own seqno.
SequenceNumber global_seqno;
// the level when the table is opened, could potentially change when trivial
// move is involved
int level;
// If false, blocks in this file are definitely all uncompressed. Knowing this
// before reading individual blocks enables certain optimizations.
bool blocks_maybe_compressed = true;
bool closed = false;
const bool immortal_table;
};
template <class TBlockIter, typename TValue = Slice>
class BlockBasedTableIterator : public InternalIteratorBase<TValue> {
public:
BlockBasedTableIterator(BlockBasedTable* table,
const ReadOptions& read_options,
const InternalKeyComparator& icomp,
InternalIteratorBase<BlockHandle>* index_iter,
bool check_filter, bool need_upper_bound_check,
const SliceTransform* prefix_extractor, bool is_index,
bool key_includes_seq = true,
bool index_key_is_full = true,
bool for_compaction = false)
: table_(table),
read_options_(read_options),
icomp_(icomp),
index_iter_(index_iter),
pinned_iters_mgr_(nullptr),
block_iter_points_to_real_block_(false),
check_filter_(check_filter),
need_upper_bound_check_(need_upper_bound_check),
prefix_extractor_(prefix_extractor),
is_index_(is_index),
key_includes_seq_(key_includes_seq),
index_key_is_full_(index_key_is_full),
for_compaction_(for_compaction) {}
~BlockBasedTableIterator() { delete index_iter_; }
void Seek(const Slice& target) override;
void SeekForPrev(const Slice& target) override;
void SeekToFirst() override;
void SeekToLast() override;
void Next() override;
void Prev() override;
bool Valid() const override {
return !is_out_of_bound_ && block_iter_points_to_real_block_ &&
block_iter_.Valid();
}
Slice key() const override {
assert(Valid());
return block_iter_.key();
}
TValue value() const override {
assert(Valid());
return block_iter_.value();
}
Status status() const override {
if (!index_iter_->status().ok()) {
return index_iter_->status();
} else if (block_iter_points_to_real_block_) {
return block_iter_.status();
} else {
return Status::OK();
}
}
bool IsOutOfBound() override { return is_out_of_bound_; }
void SetPinnedItersMgr(PinnedIteratorsManager* pinned_iters_mgr) override {
pinned_iters_mgr_ = pinned_iters_mgr;
}
bool IsKeyPinned() const override {
return pinned_iters_mgr_ && pinned_iters_mgr_->PinningEnabled() &&
block_iter_points_to_real_block_ && block_iter_.IsKeyPinned();
}
bool IsValuePinned() const override {
// BlockIter::IsValuePinned() is always true. No need to check
return pinned_iters_mgr_ && pinned_iters_mgr_->PinningEnabled() &&
block_iter_points_to_real_block_;
}
bool CheckPrefixMayMatch(const Slice& ikey) {
if (check_filter_ &&
!table_->PrefixMayMatch(ikey, read_options_, prefix_extractor_,
need_upper_bound_check_)) {
// TODO remember the iterator is invalidated because of prefix
// match. This can avoid the upper level file iterator to falsely
// believe the position is the end of the SST file and move to
// the first key of the next file.
ResetDataIter();
return false;
}
return true;
}
void ResetDataIter() {
if (block_iter_points_to_real_block_) {
if (pinned_iters_mgr_ != nullptr && pinned_iters_mgr_->PinningEnabled()) {
block_iter_.DelegateCleanupsTo(pinned_iters_mgr_);
}
block_iter_.Invalidate(Status::OK());
block_iter_points_to_real_block_ = false;
}
}
void SavePrevIndexValue() {
if (block_iter_points_to_real_block_) {
// Reseek. If they end up with the same data block, we shouldn't re-fetch
// the same data block.
prev_index_value_ = index_iter_->value();
}
}
void InitDataBlock();
void FindKeyForward();
void FindKeyBackward();
private:
BlockBasedTable* table_;
const ReadOptions read_options_;
const InternalKeyComparator& icomp_;
InternalIteratorBase<BlockHandle>* index_iter_;
PinnedIteratorsManager* pinned_iters_mgr_;
TBlockIter block_iter_;
bool block_iter_points_to_real_block_;
bool is_out_of_bound_ = false;
bool check_filter_;
// TODO(Zhongyi): pick a better name
bool need_upper_bound_check_;
const SliceTransform* prefix_extractor_;
// If the blocks over which we iterate are index blocks
bool is_index_;
// If the keys in the blocks over which we iterate include 8 byte sequence
bool key_includes_seq_;
bool index_key_is_full_;
// If this iterator is created for compaction
bool for_compaction_;
BlockHandle prev_index_value_;
static const size_t kInitReadaheadSize = 8 * 1024;
// Found that 256 KB readahead size provides the best performance, based on
// experiments.
static const size_t kMaxReadaheadSize;
size_t readahead_size_ = kInitReadaheadSize;
size_t readahead_limit_ = 0;
int num_file_reads_ = 0;
std::unique_ptr<FilePrefetchBuffer> prefetch_buffer_;
};
} // namespace rocksdb