options.h

// 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 <stddef.h>
#include <stdint.h>
#include <limits>
#include <memory>
#include <string>
#include <unordered_map>
#include <vector>

#include "rocksdb/advanced_options.h"
#include "rocksdb/comparator.h"
#include "rocksdb/env.h"
#include "rocksdb/file_checksum.h"
#include "rocksdb/listener.h"
#include "rocksdb/universal_compaction.h"
#include "rocksdb/version.h"
#include "rocksdb/write_buffer_manager.h"

#ifdef max
#undef max
#endif

namespace ROCKSDB_NAMESPACE {

class Cache;
class CompactionFilter;
class CompactionFilterFactory;
class Comparator;
class ConcurrentTaskLimiter;
class Env;
enum InfoLogLevel : unsigned char;
class SstFileManager;
class FilterPolicy;
class Logger;
class MergeOperator;
class Snapshot;
class MemTableRepFactory;
class RateLimiter;
class Slice;
class Statistics;
class InternalKeyComparator;
class WalFilter;
class FileSystem;

enum class CpuPriority {
  kIdle = 0,
  kLow = 1,
  kNormal = 2,
  kHigh = 3,
};

// DB contents are stored in a set of blocks, each of which holds a
// sequence of key,value pairs.  Each block may be compressed before
// being stored in a file.  The following enum describes which
// compression method (if any) is used to compress a block.
enum CompressionType : unsigned char {
  // NOTE: do not change the values of existing entries, as these are
  // part of the persistent format on disk.
  kNoCompression = 0x0,
  kSnappyCompression = 0x1,
  kZlibCompression = 0x2,
  kBZip2Compression = 0x3,
  kLZ4Compression = 0x4,
  kLZ4HCCompression = 0x5,
  kXpressCompression = 0x6,
  kZSTD = 0x7,

  // Only use kZSTDNotFinalCompression if you have to use ZSTD lib older than
  // 0.8.0 or consider a possibility of downgrading the service or copying
  // the database files to another service running with an older version of
  // RocksDB that doesn't have kZSTD. Otherwise, you should use kZSTD. We will
  // eventually remove the option from the public API.
  kZSTDNotFinalCompression = 0x40,

  // kDisableCompressionOption is used to disable some compression options.
  kDisableCompressionOption = 0xff,
};

struct Options;
struct DbPath;

struct ColumnFamilyOptions : public AdvancedColumnFamilyOptions {
  // The function recovers options to a previous version. Only 4.6 or later
  // versions are supported.
  ColumnFamilyOptions* OldDefaults(int rocksdb_major_version = 4,
                                   int rocksdb_minor_version = 6);

  // Some functions that make it easier to optimize RocksDB
  // Use this if your DB is very small (like under 1GB) and you don't want to
  // spend lots of memory for memtables.
  // An optional cache object is passed in to be used as the block cache
  ColumnFamilyOptions* OptimizeForSmallDb(
      std::shared_ptr<Cache>* cache = nullptr);

  // Use this if you don't need to keep the data sorted, i.e. you'll never use
  // an iterator, only Put() and Get() API calls
  //
  // Not supported in ROCKSDB_LITE
  ColumnFamilyOptions* OptimizeForPointLookup(uint64_t block_cache_size_mb);

  // Default values for some parameters in ColumnFamilyOptions are not
  // optimized for heavy workloads and big datasets, which means you might
  // observe write stalls under some conditions. As a starting point for tuning
  // RocksDB options, use the following two functions:
  // * OptimizeLevelStyleCompaction -- optimizes level style compaction
  // * OptimizeUniversalStyleCompaction -- optimizes universal style compaction
  // Universal style compaction is focused on reducing Write Amplification
  // Factor for big data sets, but increases Space Amplification. You can learn
  // more about the different styles here:
  // https://github.com/facebook/rocksdb/wiki/Rocksdb-Architecture-Guide
  // Make sure to also call IncreaseParallelism(), which will provide the
  // biggest performance gains.
  // Note: we might use more memory than memtable_memory_budget during high
  // write rate period
  //
  // OptimizeUniversalStyleCompaction is not supported in ROCKSDB_LITE
  ColumnFamilyOptions* OptimizeLevelStyleCompaction(
      uint64_t memtable_memory_budget = 512 * 1024 * 1024);
  ColumnFamilyOptions* OptimizeUniversalStyleCompaction(
      uint64_t memtable_memory_budget = 512 * 1024 * 1024);

  // -------------------
  // Parameters that affect behavior

  // Comparator used to define the order of keys in the table.
  // Default: a comparator that uses lexicographic byte-wise ordering
  //
  // REQUIRES: The client must ensure that the comparator supplied
  // here has the same name and orders keys *exactly* the same as the
  // comparator provided to previous open calls on the same DB.
  const Comparator* comparator = BytewiseComparator();

  // REQUIRES: The client must provide a merge operator if Merge operation
  // needs to be accessed. Calling Merge on a DB without a merge operator
  // would result in Status::NotSupported. The client must ensure that the
  // merge operator supplied here has the same name and *exactly* the same
  // semantics as the merge operator provided to previous open calls on
  // the same DB. The only exception is reserved for upgrade, where a DB
  // previously without a merge operator is introduced to Merge operation
  // for the first time. It's necessary to specify a merge operator when
  // opening the DB in this case.
  // Default: nullptr
  std::shared_ptr<MergeOperator> merge_operator = nullptr;

  // A single CompactionFilter instance to call into during compaction.
  // Allows an application to modify/delete a key-value during background
  // compaction.
  //
  // If the client requires a new compaction filter to be used for different
  // compaction runs, it can specify compaction_filter_factory instead of this
  // option.  The client should specify only one of the two.
  // compaction_filter takes precedence over compaction_filter_factory if
  // client specifies both.
  //
  // If multithreaded compaction is being used, the supplied CompactionFilter
  // instance may be used from different threads concurrently and so should be
  // thread-safe.
  //
  // Default: nullptr
  const CompactionFilter* compaction_filter = nullptr;

  // This is a factory that provides compaction filter objects which allow
  // an application to modify/delete a key-value during background compaction.
  //
  // A new filter will be created on each compaction run.  If multithreaded
  // compaction is being used, each created CompactionFilter will only be used
  // from a single thread and so does not need to be thread-safe.
  //
  // Default: nullptr
  std::shared_ptr<CompactionFilterFactory> compaction_filter_factory = nullptr;

  // -------------------
  // Parameters that affect performance

  // Amount of data to build up in memory (backed by an unsorted log
  // on disk) before converting to a sorted on-disk file.
  //
  // Larger values increase performance, especially during bulk loads.
  // Up to max_write_buffer_number write buffers may be held in memory
  // at the same time,
  // so you may wish to adjust this parameter to control memory usage.
  // Also, a larger write buffer will result in a longer recovery time
  // the next time the database is opened.
  //
  // Note that write_buffer_size is enforced per column family.
  // See db_write_buffer_size for sharing memory across column families.
  //
  // Default: 64MB
  //
  // Dynamically changeable through SetOptions() API
  size_t write_buffer_size = 64 << 20;

  // Compress blocks using the specified compression algorithm.
  //
  // Default: kSnappyCompression, if it's supported. If snappy is not linked
  // with the library, the default is kNoCompression.
  //
  // Typical speeds of kSnappyCompression on an Intel(R) Core(TM)2 2.4GHz:
  //    ~200-500MB/s compression
  //    ~400-800MB/s decompression
  //
  // Note that these speeds are significantly faster than most
  // persistent storage speeds, and therefore it is typically never
  // worth switching to kNoCompression.  Even if the input data is
  // incompressible, the kSnappyCompression implementation will
  // efficiently detect that and will switch to uncompressed mode.
  //
  // If you do not set `compression_opts.level`, or set it to
  // `CompressionOptions::kDefaultCompressionLevel`, we will attempt to pick the
  // default corresponding to `compression` as follows:
  //
  // - kZSTD: 3
  // - kZlibCompression: Z_DEFAULT_COMPRESSION (currently -1)
  // - kLZ4HCCompression: 0
  // - For all others, we do not specify a compression level
  //
  // Dynamically changeable through SetOptions() API
  CompressionType compression;

  // Compression algorithm that will be used for the bottommost level that
  // contain files.
  //
  // Default: kDisableCompressionOption (Disabled)
  CompressionType bottommost_compression = kDisableCompressionOption;

  // different options for compression algorithms used by bottommost_compression
  // if it is enabled. To enable it, please see the definition of
  // CompressionOptions.
  CompressionOptions bottommost_compression_opts;

  // different options for compression algorithms
  CompressionOptions compression_opts;

  // Number of files to trigger level-0 compaction. A value <0 means that
  // level-0 compaction will not be triggered by number of files at all.
  //
  // Default: 4
  //
  // Dynamically changeable through SetOptions() API
  int level0_file_num_compaction_trigger = 4;

  // If non-nullptr, use the specified function to determine the
  // prefixes for keys.  These prefixes will be placed in the filter.
  // Depending on the workload, this can reduce the number of read-IOP
  // cost for scans when a prefix is passed via ReadOptions to
  // db.NewIterator().  For prefix filtering to work properly,
  // "prefix_extractor" and "comparator" must be such that the following
  // properties hold:
  //
  // 1) key.starts_with(prefix(key))
  // 2) Compare(prefix(key), key) <= 0.
  // 3) If Compare(k1, k2) <= 0, then Compare(prefix(k1), prefix(k2)) <= 0
  // 4) prefix(prefix(key)) == prefix(key)
  //
  // Default: nullptr
  std::shared_ptr<const SliceTransform> prefix_extractor = nullptr;

  // Control maximum total data size for a level.
  // max_bytes_for_level_base is the max total for level-1.
  // Maximum number of bytes for level L can be calculated as
  // (max_bytes_for_level_base) * (max_bytes_for_level_multiplier ^ (L-1))
  // For example, if max_bytes_for_level_base is 200MB, and if
  // max_bytes_for_level_multiplier is 10, total data size for level-1
  // will be 200MB, total file size for level-2 will be 2GB,
  // and total file size for level-3 will be 20GB.
  //
  // Default: 256MB.
  //
  // Dynamically changeable through SetOptions() API
  uint64_t max_bytes_for_level_base = 256 * 1048576;

  // Deprecated.
  uint64_t snap_refresh_nanos = 0;

  // Disable automatic compactions. Manual compactions can still
  // be issued on this column family
  //
  // Dynamically changeable through SetOptions() API
  bool disable_auto_compactions = false;

  // This is a factory that provides TableFactory objects.
  // Default: a block-based table factory that provides a default
  // implementation of TableBuilder and TableReader with default
  // BlockBasedTableOptions.
  std::shared_ptr<TableFactory> table_factory;

  // A list of paths where SST files for this column family
  // can be put into, with its target size. Similar to db_paths,
  // newer data is placed into paths specified earlier in the
  // vector while older data gradually moves to paths specified
  // later in the vector.
  // Note that, if a path is supplied to multiple column
  // families, it would have files and total size from all
  // the column families combined. User should provision for the
  // total size(from all the column families) in such cases.
  //
  // If left empty, db_paths will be used.
  // Default: empty
  std::vector<DbPath> cf_paths;

  // Compaction concurrent thread limiter for the column family.
  // If non-nullptr, use given concurrent thread limiter to control
  // the max outstanding compaction tasks. Limiter can be shared with
  // multiple column families across db instances.
  //
  // Default: nullptr
  std::shared_ptr<ConcurrentTaskLimiter> compaction_thread_limiter = nullptr;

  // Create ColumnFamilyOptions with default values for all fields
  ColumnFamilyOptions();
  // Create ColumnFamilyOptions from Options
  explicit ColumnFamilyOptions(const Options& options);

  void Dump(Logger* log) const;
};

enum class WALRecoveryMode : char {
  // Original levelDB recovery
  // We tolerate incomplete record in trailing data on all logs
  // Use case : This is legacy behavior
  kTolerateCorruptedTailRecords = 0x00,
  // Recover from clean shutdown
  // We don't expect to find any corruption in the WAL
  // Use case : This is ideal for unit tests and rare applications that
  // can require high consistency guarantee
  kAbsoluteConsistency = 0x01,
  // Recover to point-in-time consistency (default)
  // We stop the WAL playback on discovering WAL inconsistency
  // Use case : Ideal for systems that have disk controller cache like
  // hard disk, SSD without super capacitor that store related data
  kPointInTimeRecovery = 0x02,
  // Recovery after a disaster
  // We ignore any corruption in the WAL and try to salvage as much data as
  // possible
  // Use case : Ideal for last ditch effort to recover data or systems that
  // operate with low grade unrelated data
  kSkipAnyCorruptedRecords = 0x03,
};

struct DbPath {
  std::string path;
  uint64_t target_size;  // Target size of total files under the path, in byte.

  DbPath() : target_size(0) {}
  DbPath(const std::string& p, uint64_t t) : path(p), target_size(t) {}
};

struct DBOptions {
  // The function recovers options to the option as in version 4.6.
  DBOptions* OldDefaults(int rocksdb_major_version = 4,
                         int rocksdb_minor_version = 6);

  // Some functions that make it easier to optimize RocksDB

  // Use this if your DB is very small (like under 1GB) and you don't want to
  // spend lots of memory for memtables.
  // An optional cache object is passed in for the memory of the
  // memtable to cost to
  DBOptions* OptimizeForSmallDb(std::shared_ptr<Cache>* cache = nullptr);

#ifndef ROCKSDB_LITE
  // By default, RocksDB uses only one background thread for flush and
  // compaction. Calling this function will set it up such that total of
  // `total_threads` is used. Good value for `total_threads` is the number of
  // cores. You almost definitely want to call this function if your system is
  // bottlenecked by RocksDB.
  DBOptions* IncreaseParallelism(int total_threads = 16);
#endif  // ROCKSDB_LITE

  // If true, the database will be created if it is missing.
  // Default: false
  bool create_if_missing = false;

  // If true, missing column families will be automatically created.
  // Default: false
  bool create_missing_column_families = false;

  // If true, an error is raised if the database already exists.
  // Default: false
  bool error_if_exists = false;

  // If true, RocksDB will aggressively check consistency of the data.
  // Also, if any of the  writes to the database fails (Put, Delete, Merge,
  // Write), the database will switch to read-only mode and fail all other
  // Write operations.
  // In most cases you want this to be set to true.
  // Default: true
  bool paranoid_checks = true;

  // Use the specified object to interact with the environment,
  // e.g. to read/write files, schedule background work, etc. In the near
  // future, support for doing storage operations such as read/write files
  // through env will be deprecated in favor of file_system (see below)
  // Default: Env::Default()
  Env* env = Env::Default();

  // Use to control write rate of flush and compaction. Flush has higher
  // priority than compaction. Rate limiting is disabled if nullptr.
  // If rate limiter is enabled, bytes_per_sync is set to 1MB by default.
  // Default: nullptr
  std::shared_ptr<RateLimiter> rate_limiter = nullptr;

  // Use to track SST files and control their file deletion rate.
  //
  // Features:
  //  - Throttle the deletion rate of the SST files.
  //  - Keep track the total size of all SST files.
  //  - Set a maximum allowed space limit for SST files that when reached
  //    the DB wont do any further flushes or compactions and will set the
  //    background error.
  //  - Can be shared between multiple dbs.
  // Limitations:
  //  - Only track and throttle deletes of SST files in
  //    first db_path (db_name if db_paths is empty).
  //
  // Default: nullptr
  std::shared_ptr<SstFileManager> sst_file_manager = nullptr;

  // Any internal progress/error information generated by the db will
  // be written to info_log if it is non-nullptr, or to a file stored
  // in the same directory as the DB contents if info_log is nullptr.
  // Default: nullptr
  std::shared_ptr<Logger> info_log = nullptr;

#ifdef NDEBUG
  InfoLogLevel info_log_level = INFO_LEVEL;
#else
  InfoLogLevel info_log_level = DEBUG_LEVEL;
#endif  // NDEBUG

  // Number of open files that can be used by the DB.  You may need to
  // increase this if your database has a large working set. Value -1 means
  // files opened are always kept open. You can estimate number of files based
  // on target_file_size_base and target_file_size_multiplier for level-based
  // compaction. For universal-style compaction, you can usually set it to -1.
  //
  // Default: -1
  //
  // Dynamically changeable through SetDBOptions() API.
  int max_open_files = -1;

  // If max_open_files is -1, DB will open all files on DB::Open(). You can
  // use this option to increase the number of threads used to open the files.
  // Default: 16
  int max_file_opening_threads = 16;

  // Once write-ahead logs exceed this size, we will start forcing the flush of
  // column families whose memtables are backed by the oldest live WAL file
  // (i.e. the ones that are causing all the space amplification). If set to 0
  // (default), we will dynamically choose the WAL size limit to be
  // [sum of all write_buffer_size * max_write_buffer_number] * 4
  // This option takes effect only when there are more than one column family as
  // otherwise the wal size is dictated by the write_buffer_size.
  //
  // Default: 0
  //
  // Dynamically changeable through SetDBOptions() API.
  uint64_t max_total_wal_size = 0;

  // If non-null, then we should collect metrics about database operations
  std::shared_ptr<Statistics> statistics = nullptr;

  // By default, writes to stable storage use fdatasync (on platforms
  // where this function is available). If this option is true,
  // fsync is used instead.
  //
  // fsync and fdatasync are equally safe for our purposes and fdatasync is
  // faster, so it is rarely necessary to set this option. It is provided
  // as a workaround for kernel/filesystem bugs, such as one that affected
  // fdatasync with ext4 in kernel versions prior to 3.7.
  bool use_fsync = false;

  // A list of paths where SST files can be put into, with its target size.
  // Newer data is placed into paths specified earlier in the vector while
  // older data gradually moves to paths specified later in the vector.
  //
  // For example, you have a flash device with 10GB allocated for the DB,
  // as well as a hard drive of 2TB, you should config it to be:
  //   [{"/flash_path", 10GB}, {"/hard_drive", 2TB}]
  //
  // The system will try to guarantee data under each path is close to but
  // not larger than the target size. But current and future file sizes used
  // by determining where to place a file are based on best-effort estimation,
  // which means there is a chance that the actual size under the directory
  // is slightly more than target size under some workloads. User should give
  // some buffer room for those cases.
  //
  // If none of the paths has sufficient room to place a file, the file will
  // be placed to the last path anyway, despite to the target size.
  //
  // Placing newer data to earlier paths is also best-efforts. User should
  // expect user files to be placed in higher levels in some extreme cases.
  //
  // If left empty, only one path will be used, which is db_name passed when
  // opening the DB.
  // Default: empty
  std::vector<DbPath> db_paths;

  // This specifies the info LOG dir.
  // If it is empty, the log files will be in the same dir as data.
  // If it is non empty, the log files will be in the specified dir,
  // and the db data dir's absolute path will be used as the log file
  // name's prefix.
  std::string db_log_dir = "";

  // This specifies the absolute dir path for write-ahead logs (WAL).
  // If it is empty, the log files will be in the same dir as data,
  //   dbname is used as the data dir by default
  // If it is non empty, the log files will be in kept the specified dir.
  // When destroying the db,
  //   all log files in wal_dir and the dir itself is deleted
  std::string wal_dir = "";

  // The periodicity when obsolete files get deleted. The default
  // value is 6 hours. The files that get out of scope by compaction
  // process will still get automatically delete on every compaction,
  // regardless of this setting
  //
  // Default: 6 hours
  //
  // Dynamically changeable through SetDBOptions() API.
  uint64_t delete_obsolete_files_period_micros = 6ULL * 60 * 60 * 1000000;

  // Maximum number of concurrent background jobs (compactions and flushes).
  //
  // Default: 2
  //
  // Dynamically changeable through SetDBOptions() API.
  int max_background_jobs = 2;

  // NOT SUPPORTED ANYMORE: RocksDB automatically decides this based on the
  // value of max_background_jobs. This option is ignored.
  //
  // Dynamically changeable through SetDBOptions() API.
  int base_background_compactions = -1;

  // NOT SUPPORTED ANYMORE: RocksDB automatically decides this based on the
  // value of max_background_jobs. For backwards compatibility we will set
  // `max_background_jobs = max_background_compactions + max_background_flushes`
  // in the case where user sets at least one of `max_background_compactions` or
  // `max_background_flushes` (we replace -1 by 1 in case one option is unset).
  //
  // Maximum number of concurrent background compaction jobs, submitted to
  // the default LOW priority thread pool.
  //
  // If you're increasing this, also consider increasing number of threads in
  // LOW priority thread pool. For more information, see
  // Env::SetBackgroundThreads
  //
  // Default: -1
  //
  // Dynamically changeable through SetDBOptions() API.
  int max_background_compactions = -1;

  // This value represents the maximum number of threads that will
  // concurrently perform a compaction job by breaking it into multiple,
  // smaller ones that are run simultaneously.
  // Default: 1 (i.e. no subcompactions)
  uint32_t max_subcompactions = 1;

  // NOT SUPPORTED ANYMORE: RocksDB automatically decides this based on the
  // value of max_background_jobs. For backwards compatibility we will set
  // `max_background_jobs = max_background_compactions + max_background_flushes`
  // in the case where user sets at least one of `max_background_compactions` or
  // `max_background_flushes`.
  //
  // Maximum number of concurrent background memtable flush jobs, submitted by
  // default to the HIGH priority thread pool. If the HIGH priority thread pool
  // is configured to have zero threads, flush jobs will share the LOW priority
  // thread pool with compaction jobs.
  //
  // It is important to use both thread pools when the same Env is shared by
  // multiple db instances. Without a separate pool, long running compaction
  // jobs could potentially block memtable flush jobs of other db instances,
  // leading to unnecessary Put stalls.
  //
  // If you're increasing this, also consider increasing number of threads in
  // HIGH priority thread pool. For more information, see
  // Env::SetBackgroundThreads
  // Default: -1
  int max_background_flushes = -1;

  // Specify the maximal size of the info log file. If the log file
  // is larger than `max_log_file_size`, a new info log file will
  // be created.
  // If max_log_file_size == 0, all logs will be written to one
  // log file.
  size_t max_log_file_size = 0;

  // Time for the info log file to roll (in seconds).
  // If specified with non-zero value, log file will be rolled
  // if it has been active longer than `log_file_time_to_roll`.
  // Default: 0 (disabled)
  // Not supported in ROCKSDB_LITE mode!
  size_t log_file_time_to_roll = 0;

  // Maximal info log files to be kept.
  // Default: 1000
  size_t keep_log_file_num = 1000;

  // Recycle log files.
  // If non-zero, we will reuse previously written log files for new
  // logs, overwriting the old data.  The value indicates how many
  // such files we will keep around at any point in time for later
  // use.  This is more efficient because the blocks are already
  // allocated and fdatasync does not need to update the inode after
  // each write.
  // Default: 0
  size_t recycle_log_file_num = 0;

  // manifest file is rolled over on reaching this limit.
  // The older manifest file be deleted.
  // The default value is 1GB so that the manifest file can grow, but not
  // reach the limit of storage capacity.
  uint64_t max_manifest_file_size = 1024 * 1024 * 1024;

  // Number of shards used for table cache.
  int table_cache_numshardbits = 6;

  // NOT SUPPORTED ANYMORE
  // int table_cache_remove_scan_count_limit;

  // The following two fields affect how archived logs will be deleted.
  // 1. If both set to 0, logs will be deleted asap and will not get into
  //    the archive.
  // 2. If WAL_ttl_seconds is 0 and WAL_size_limit_MB is not 0,
  //    WAL files will be checked every 10 min and if total size is greater
  //    then WAL_size_limit_MB, they will be deleted starting with the
  //    earliest until size_limit is met. All empty files will be deleted.
  // 3. If WAL_ttl_seconds is not 0 and WAL_size_limit_MB is 0, then
  //    WAL files will be checked every WAL_ttl_seconds / 2 and those that
  //    are older than WAL_ttl_seconds will be deleted.
  // 4. If both are not 0, WAL files will be checked every 10 min and both
  //    checks will be performed with ttl being first.
  uint64_t WAL_ttl_seconds = 0;
  uint64_t WAL_size_limit_MB = 0;

  // Number of bytes to preallocate (via fallocate) the manifest
  // files.  Default is 4mb, which is reasonable to reduce random IO
  // as well as prevent overallocation for mounts that preallocate
  // large amounts of data (such as xfs's allocsize option).
  size_t manifest_preallocation_size = 4 * 1024 * 1024;

  // Allow the OS to mmap file for reading sst tables. Default: false
  bool allow_mmap_reads = false;

  // Allow the OS to mmap file for writing.
  // DB::SyncWAL() only works if this is set to false.
  // Default: false
  bool allow_mmap_writes = false;

  // Enable direct I/O mode for read/write
  // they may or may not improve performance depending on the use case
  //
  // Files will be opened in "direct I/O" mode
  // which means that data r/w from the disk will not be cached or
  // buffered. The hardware buffer of the devices may however still
  // be used. Memory mapped files are not impacted by these parameters.

  // Use O_DIRECT for user and compaction reads.
  // When true, we also force new_table_reader_for_compaction_inputs to true.
  // Default: false
  // Not supported in ROCKSDB_LITE mode!
  bool use_direct_reads = false;

  // Use O_DIRECT for writes in background flush and compactions.
  // Default: false
  // Not supported in ROCKSDB_LITE mode!
  bool use_direct_io_for_flush_and_compaction = false;

  // If false, fallocate() calls are bypassed
  bool allow_fallocate = true;

  // Disable child process inherit open files. Default: true
  bool is_fd_close_on_exec = true;

  // NOT SUPPORTED ANYMORE -- this options is no longer used
  bool skip_log_error_on_recovery = false;

  // if not zero, dump rocksdb.stats to LOG every stats_dump_period_sec
  //
  // Default: 600 (10 min)
  //
  // Dynamically changeable through SetDBOptions() API.
  unsigned int stats_dump_period_sec = 600;

  // if not zero, dump rocksdb.stats to RocksDB every stats_persist_period_sec
  // Default: 600
  unsigned int stats_persist_period_sec = 600;

  // If true, automatically persist stats to a hidden column family (column
  // family name: ___rocksdb_stats_history___) every
  // stats_persist_period_sec seconds; otherwise, write to an in-memory
  // struct. User can query through `GetStatsHistory` API.
  // If user attempts to create a column family with the same name on a DB
  // which have previously set persist_stats_to_disk to true, the column family
  // creation will fail, but the hidden column family will survive, as well as
  // the previously persisted statistics.
  // When peristing stats to disk, the stat name will be limited at 100 bytes.
  // Default: false
  bool persist_stats_to_disk = false;

  // if not zero, periodically take stats snapshots and store in memory, the
  // memory size for stats snapshots is capped at stats_history_buffer_size
  // Default: 1MB
  size_t stats_history_buffer_size = 1024 * 1024;

  // If set true, will hint the underlying file system that the file
  // access pattern is random, when a sst file is opened.
  // Default: true
  bool advise_random_on_open = true;

  // Amount of data to build up in memtables across all column
  // families before writing to disk.
  //
  // This is distinct from write_buffer_size, which enforces a limit
  // for a single memtable.
  //
  // This feature is disabled by default. Specify a non-zero value
  // to enable it.
  //
  // Default: 0 (disabled)
  size_t db_write_buffer_size = 0;

  // The memory usage of memtable will report to this object. The same object
  // can be passed into multiple DBs and it will track the sum of size of all
  // the DBs. If the total size of all live memtables of all the DBs exceeds
  // a limit, a flush will be triggered in the next DB to which the next write
  // is issued.
  //
  // If the object is only passed to one DB, the behavior is the same as
  // db_write_buffer_size. When write_buffer_manager is set, the value set will
  // override db_write_buffer_size.
  //
  // This feature is disabled by default. Specify a non-zero value
  // to enable it.
  //
  // Default: null
  std::shared_ptr<WriteBufferManager> write_buffer_manager = nullptr;

  // Specify the file access pattern once a compaction is started.
  // It will be applied to all input files of a compaction.
  // Default: NORMAL
  enum AccessHint { NONE, NORMAL, SEQUENTIAL, WILLNEED };
  AccessHint access_hint_on_compaction_start = NORMAL;

  // If true, always create a new file descriptor and new table reader
  // for compaction inputs. Turn this parameter on may introduce extra
  // memory usage in the table reader, if it allocates extra memory
  // for indexes. This will allow file descriptor prefetch options
  // to be set for compaction input files and not to impact file
  // descriptors for the same file used by user queries.
  // Suggest to enable BlockBasedTableOptions.cache_index_and_filter_blocks
  // for this mode if using block-based table.
  //
  // Default: false
  // This flag has no affect on the behavior of compaction and plan to delete
  // in the future.
  bool new_table_reader_for_compaction_inputs = false;

  // If non-zero, we perform bigger reads when doing compaction. If you're
  // running RocksDB on spinning disks, you should set this to at least 2MB.
  // That way RocksDB's compaction is doing sequential instead of random reads.
  //
  // When non-zero, we also force new_table_reader_for_compaction_inputs to
  // true.
  //
  // Default: 0
  //
  // Dynamically changeable through SetDBOptions() API.
  size_t compaction_readahead_size = 0;

  // This is a maximum buffer size that is used by WinMmapReadableFile in
  // unbuffered disk I/O mode. We need to maintain an aligned buffer for
  // reads. We allow the buffer to grow until the specified value and then
  // for bigger requests allocate one shot buffers. In unbuffered mode we
  // always bypass read-ahead buffer at ReadaheadRandomAccessFile
  // When read-ahead is required we then make use of compaction_readahead_size
  // value and always try to read ahead. With read-ahead we always
  // pre-allocate buffer to the size instead of growing it up to a limit.
  //
  // This option is currently honored only on Windows
  //
  // Default: 1 Mb
  //
  // Special value: 0 - means do not maintain per instance buffer. Allocate
  //                per request buffer and avoid locking.
  size_t random_access_max_buffer_size = 1024 * 1024;

  // This is the maximum buffer size that is used by WritableFileWriter.
  // On Windows, we need to maintain an aligned buffer for writes.
  // We allow the buffer to grow until it's size hits the limit in buffered
  // IO and fix the buffer size when using direct IO to ensure alignment of
  // write requests if the logical sector size is unusual
  //
  // Default: 1024 * 1024 (1 MB)
  //
  // Dynamically changeable through SetDBOptions() API.
  size_t writable_file_max_buffer_size = 1024 * 1024;

  // Use adaptive mutex, which spins in the user space before resorting
  // to kernel. This could reduce context switch when the mutex is not
  // heavily contended. However, if the mutex is hot, we could end up
  // wasting spin time.
  // Default: false
  bool use_adaptive_mutex = false;

  // Create DBOptions with default values for all fields
  DBOptions();
  // Create DBOptions from Options
  explicit DBOptions(const Options& options);

  void Dump(Logger* log) const;

  // Allows OS to incrementally sync files to disk while they are being
  // written, asynchronously, in the background. This operation can be used
  // to smooth out write I/Os over time. Users shouldn't rely on it for
  // persistency guarantee.
  // Issue one request for every bytes_per_sync written. 0 turns it off.
  //
  // You may consider using rate_limiter to regulate write rate to device.
  // When rate limiter is enabled, it automatically enables bytes_per_sync
  // to 1MB.
  //
  // This option applies to table files
  //
  // Default: 0, turned off
  //
  // Note: DOES NOT apply to WAL files. See wal_bytes_per_sync instead
  // Dynamically changeable through SetDBOptions() API.
  uint64_t bytes_per_sync = 0;

  // Same as bytes_per_sync, but applies to WAL files
  //
  // Default: 0, turned off
  //
  // Dynamically changeable through SetDBOptions() API.
  uint64_t wal_bytes_per_sync = 0;

  // When true, guarantees WAL files have at most `wal_bytes_per_sync`
  // bytes submitted for writeback at any given time, and SST files have at most
  // `bytes_per_sync` bytes pending writeback at any given time. This can be
  // used to handle cases where processing speed exceeds I/O speed during file
  // generation, which can lead to a huge sync when the file is finished, even
  // with `bytes_per_sync` / `wal_bytes_per_sync` properly configured.
  //
  //  - If `sync_file_range` is supported it achieves this by waiting for any
  //    prior `sync_file_range`s to finish before proceeding. In this way,
  //    processing (compression, etc.) can proceed uninhibited in the gap
  //    between `sync_file_range`s, and we block only when I/O falls behind.
  //  - Otherwise the `WritableFile::Sync` method is used. Note this mechanism
  //    always blocks, thus preventing the interleaving of I/O and processing.
  //
  // Note: Enabling this option does not provide any additional persistence
  // guarantees, as it may use `sync_file_range`, which does not write out
  // metadata.
  //
  // Default: false
  bool strict_bytes_per_sync = false;

  // A vector of EventListeners whose callback functions will be called
  // when specific RocksDB event happens.
  std::vector<std::shared_ptr<EventListener>> listeners;

  // If true, then the status of the threads involved in this DB will
  // be tracked and available via GetThreadList() API.
  //
  // Default: false
  bool enable_thread_tracking = false;

  // The limited write rate to DB if soft_pending_compaction_bytes_limit or
  // level0_slowdown_writes_trigger is triggered, or we are writing to the
  // last mem table allowed and we allow more than 3 mem tables. It is
  // calculated using size of user write requests before compression.
  // RocksDB may decide to slow down more if the compaction still
  // gets behind further.
  // If the value is 0, we will infer a value from `rater_limiter` value
  // if it is not empty, or 16MB if `rater_limiter` is empty. Note that
  // if users change the rate in `rate_limiter` after DB is opened,
  // `delayed_write_rate` won't be adjusted.
  //
  // Unit: byte per second.
  //
  // Default: 0
  //
  // Dynamically changeable through SetDBOptions() API.
  uint64_t delayed_write_rate = 0;

  // By default, a single write thread queue is maintained. The thread gets
  // to the head of the queue becomes write batch group leader and responsible
  // for writing to WAL and memtable for the batch group.
  //
  // If enable_pipelined_write is true, separate write thread queue is
  // maintained for WAL write and memtable write. A write thread first enter WAL
  // writer queue and then memtable writer queue. Pending thread on the WAL
  // writer queue thus only have to wait for previous writers to finish their
  // WAL writing but not the memtable writing. Enabling the feature may improve
  // write throughput and reduce latency of the prepare phase of two-phase
  // commit.
  //
  // Default: false
  bool enable_pipelined_write = false;

  // Setting unordered_write to true trades higher write throughput with
  // relaxing the immutability guarantee of snapshots. This violates the
  // repeatability one expects from ::Get from a snapshot, as well as
  // ::MultiGet and Iterator's consistent-point-in-time view property.
  // If the application cannot tolerate the relaxed guarantees, it can implement
  // its own mechanisms to work around that and yet benefit from the higher
  // throughput. Using TransactionDB with WRITE_PREPARED write policy and
  // two_write_queues=true is one way to achieve immutable snapshots despite
  // unordered_write.
  //
  // By default, i.e., when it is false, rocksdb does not advance the sequence
  // number for new snapshots unless all the writes with lower sequence numbers
  // are already finished. This provides the immutability that we except from
  // snapshots. Moreover, since Iterator and MultiGet internally depend on
  // snapshots, the snapshot immutability results into Iterator and MultiGet
  // offering consistent-point-in-time view. If set to true, although
  // Read-Your-Own-Write property is still provided, the snapshot immutability
  // property is relaxed: the writes issued after the snapshot is obtained (with
  // larger sequence numbers) will be still not visible to the reads from that
  // snapshot, however, there still might be pending writes (with lower sequence
  // number) that will change the state visible to the snapshot after they are
  // landed to the memtable.
  //
  // Default: false
  bool unordered_write = false;

  // If true, allow multi-writers to update mem tables in parallel.
  // Only some memtable_factory-s support concurrent writes; currently it
  // is implemented only for SkipListFactory.  Concurrent memtable writes
  // are not compatible with inplace_update_support or filter_deletes.
  // It is strongly recommended to set enable_write_thread_adaptive_yield
  // if you are going to use this feature.
  //
  // Default: true
  bool allow_concurrent_memtable_write = true;

  // If true, threads synchronizing with the write batch group leader will
  // wait for up to write_thread_max_yield_usec before blocking on a mutex.
  // This can substantially improve throughput for concurrent workloads,
  // regardless of whether allow_concurrent_memtable_write is enabled.
  //
  // Default: true
  bool enable_write_thread_adaptive_yield = true;

  // The maximum limit of number of bytes that are written in a single batch
  // of WAL or memtable write. It is followed when the leader write size
  // is larger than 1/8 of this limit.
  //
  // Default: 1 MB
  uint64_t max_write_batch_group_size_bytes = 1 << 20;

  // The maximum number of microseconds that a write operation will use
  // a yielding spin loop to coordinate with other write threads before
  // blocking on a mutex.  (Assuming write_thread_slow_yield_usec is
  // set properly) increasing this value is likely to increase RocksDB
  // throughput at the expense of increased CPU usage.
  //
  // Default: 100
  uint64_t write_thread_max_yield_usec = 100;

  // The latency in microseconds after which a std::this_thread::yield
  // call (sched_yield on Linux) is considered to be a signal that
  // other processes or threads would like to use the current core.
  // Increasing this makes writer threads more likely to take CPU
  // by spinning, which will show up as an increase in the number of
  // involuntary context switches.
  //
  // Default: 3
  uint64_t write_thread_slow_yield_usec = 3;

  // If true, then DB::Open() will not update the statistics used to optimize
  // compaction decision by loading table properties from many files.
  // Turning off this feature will improve DBOpen time especially in
  // disk environment.
  //
  // Default: false
  bool skip_stats_update_on_db_open = false;

  // If true, then DB::Open() will not fetch and check sizes of all sst files.
  // This may significantly speed up startup if there are many sst files,
  // especially when using non-default Env with expensive GetFileSize().
  // We'll still check that all required sst files exist.
  // If paranoid_checks is false, this option is ignored, and sst files are
  // not checked at all.
  //
  // Default: false
  bool skip_checking_sst_file_sizes_on_db_open = false;