rocksdb/include/rocksdb/options.h

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

#ifndef STORAGE_ROCKSDB_INCLUDE_OPTIONS_H_
#define STORAGE_ROCKSDB_INCLUDE_OPTIONS_H_

#include <stddef.h>
#include <string>
#include <memory>
#include <vector>
#include <stdint.h>

#include "rocksdb/memtablerep.h"
#include "rocksdb/slice.h"
#include "rocksdb/slice_transform.h"
#include "rocksdb/statistics.h"
#include "rocksdb/table_properties.h"
#include "rocksdb/universal_compaction.h"

namespace rocksdb {

class Cache;
class CompactionFilter;
class CompactionFilterFactory;
class Comparator;
class Env;
class FilterPolicy;
class Logger;
class MergeOperator;
class Snapshot;
class TableFactory;

using std::shared_ptr;

// 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 : 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
};

enum CompactionStyle : char {
  kCompactionStyleLevel       = 0x0, // level based compaction style
  kCompactionStyleUniversal   = 0x1  // Universal compaction style
};

// Compression options for different compression algorithms like Zlib
struct CompressionOptions {
  int window_bits;
  int level;
  int strategy;
  CompressionOptions():window_bits(-14),
                       level(-1),
                       strategy(0){}
  CompressionOptions(int wbits, int lev, int strategy):window_bits(wbits),
                                                       level(lev),
                                                       strategy(strategy){}
};

struct ColumnFamilyOptions {
  // -------------------
  // 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;

  // 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
  // openning the DB in this case.
  // Default: nullptr
  shared_ptr<MergeOperator> merge_operator;

  // 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;

  // 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: a factory that doesn't provide any object
  std::shared_ptr<CompactionFilterFactory> compaction_filter_factory;

  // -------------------
  // 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.
  //
  // Default: 4MB
  size_t write_buffer_size;

  // The maximum number of write buffers that are built up in memory.
  // The default is 2, so that when 1 write buffer is being flushed to
  // storage, new writes can continue to the other write buffer.
  // Default: 2
  int max_write_buffer_number;

  // The minimum number of write buffers that will be merged together
  // before writing to storage.  If set to 1, then
  // all write buffers are fushed to L0 as individual files and this increases
  // read amplification because a get request has to check in all of these
  // files. Also, an in-memory merge may result in writing lesser
  // data to storage if there are duplicate records in each of these
  // individual write buffers.  Default: 1
  int min_write_buffer_number_to_merge;

  // Control over blocks (user data is stored in a set of blocks, and
  // a block is the unit of reading from disk).

  // If non-NULL use the specified cache for blocks.
  // If NULL, rocksdb will automatically create and use an 8MB internal cache.
  // Default: nullptr
  shared_ptr<Cache> block_cache;

  // If non-NULL use the specified cache for compressed blocks.
  // If NULL, rocksdb will not use a compressed block cache.
  // Default: nullptr
  shared_ptr<Cache> block_cache_compressed;

  // Approximate size of user data packed per block.  Note that the
  // block size specified here corresponds to uncompressed data.  The
  // actual size of the unit read from disk may be smaller if
  // compression is enabled.  This parameter can be changed dynamically.
  //
  // Default: 4K
  size_t block_size;

  // Number of keys between restart points for delta encoding of keys.
  // This parameter can be changed dynamically.  Most clients should
  // leave this parameter alone.
  //
  // Default: 16
  int block_restart_interval;


  // Compress blocks using the specified compression algorithm.  This
  // parameter can be changed dynamically.
  //
  // Default: kSnappyCompression, which gives lightweight but fast
  // compression.
  //
  // 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.
  CompressionType compression;

  // Different levels can have different compression policies. There
  // are cases where most lower levels would like to quick compression
  // algorithm while the higher levels (which have more data) use
  // compression algorithms that have better compression but could
  // be slower. This array, if non nullptr, should have an entry for
  // each level of the database. This array, if non nullptr, overides the
  // value specified in the previous field 'compression'. The caller is
  // reponsible for allocating memory and initializing the values in it
  // before invoking Open(). The caller is responsible for freeing this
  // array and it could be freed anytime after the return from Open().
  // This could have been a std::vector but that makes the equivalent
  // java/C api hard to construct.
  std::vector<CompressionType> compression_per_level;

  //different options for compression algorithms
  CompressionOptions compression_opts;

  // If non-nullptr, use the specified filter policy to reduce disk reads.
  // Many applications will benefit from passing the result of
  // NewBloomFilterPolicy() here.
  //
  // Default: nullptr
  const FilterPolicy* filter_policy;

  // 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
  const SliceTransform* prefix_extractor;

  // If true, place whole keys in the filter (not just prefixes).
  // This must generally be true for gets to be efficient.
  //
  // Default: true
  bool whole_key_filtering;

  // Number of levels for this database
  int num_levels;

  // 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.
  int level0_file_num_compaction_trigger;

  // Soft limit on number of level-0 files. We start slowing down writes at this
  // point. A value <0 means that no writing slow down will be triggered by
  // number of files in level-0.
  int level0_slowdown_writes_trigger;

  // Maximum number of level-0 files.  We stop writes at this point.
  int level0_stop_writes_trigger;

  // Maximum level to which a new compacted memtable is pushed if it
  // does not create overlap.  We try to push to level 2 to avoid the
  // relatively expensive level 0=>1 compactions and to avoid some
  // expensive manifest file operations.  We do not push all the way to
  // the largest level since that can generate a lot of wasted disk
  // space if the same key space is being repeatedly overwritten.
  int max_mem_compaction_level;

  // Target file size for compaction.
  // target_file_size_base is per-file size for level-1.
  // Target file size for level L can be calculated by
  // target_file_size_base * (target_file_size_multiplier ^ (L-1))
  // For example, if target_file_size_base is 2MB and
  // target_file_size_multiplier is 10, then each file on level-1 will
  // be 2MB, and each file on level 2 will be 20MB,
  // and each file on level-3 will be 200MB.

  // by default target_file_size_base is 2MB.
  int target_file_size_base;
  // by default target_file_size_multiplier is 1, which means
  // by default files in different levels will have similar size.
  int target_file_size_multiplier;

  // 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 20MB, and if
  // max_bytes_for_level_multiplier is 10, total data size for level-1
  // will be 20MB, total file size for level-2 will be 200MB,
  // and total file size for level-3 will be 2GB.


  // by default 'max_bytes_for_level_base' is 10MB.
  uint64_t max_bytes_for_level_base;
  // by default 'max_bytes_for_level_base' is 10.
  int max_bytes_for_level_multiplier;

  // Different max-size multipliers for different levels.
  // These are multiplied by max_bytes_for_level_multiplier to arrive
  // at the max-size of each level.
  // Default: 1
  std::vector<int> max_bytes_for_level_multiplier_additional;

  // Maximum number of bytes in all compacted files.  We avoid expanding
  // the lower level file set of a compaction if it would make the
  // total compaction cover more than
  // (expanded_compaction_factor * targetFileSizeLevel()) many bytes.
  int expanded_compaction_factor;

  // Maximum number of bytes in all source files to be compacted in a
  // single compaction run. We avoid picking too many files in the
  // source level so that we do not exceed the total source bytes
  // for compaction to exceed
  // (source_compaction_factor * targetFileSizeLevel()) many bytes.
  // Default:1, i.e. pick maxfilesize amount of data as the source of
  // a compaction.
  int source_compaction_factor;

  // Control maximum bytes of overlaps in grandparent (i.e., level+2) before we
  // stop building a single file in a level->level+1 compaction.
  int max_grandparent_overlap_factor;

  // Disable compaction triggered by seek.
  // With bloomfilter and fast storage, a miss on one level
  // is very cheap if the file handle is cached in table cache
  // (which is true if max_open_files is large).
  bool disable_seek_compaction;

  // Puts are delayed 0-1 ms when any level has a compaction score that exceeds
  // soft_rate_limit. This is ignored when == 0.0.
  // CONSTRAINT: soft_rate_limit <= hard_rate_limit. If this constraint does not
  // hold, RocksDB will set soft_rate_limit = hard_rate_limit
  // Default: 0 (disabled)
  double soft_rate_limit;

  // Puts are delayed 1ms at a time when any level has a compaction score that
  // exceeds hard_rate_limit. This is ignored when <= 1.0.
  // Default: 0 (disabled)
  double hard_rate_limit;

  // Max time a put will be stalled when hard_rate_limit is enforced. If 0, then
  // there is no limit.
  // Default: 1000
  unsigned int rate_limit_delay_max_milliseconds;

  // Disable block cache. If this is set to true,
  // then no block cache should be used, and the block_cache should
  // point to a nullptr object.
  // Default: false
  bool no_block_cache;

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

  // During data eviction of table's LRU cache, it would be inefficient
  // to strictly follow LRU because this piece of memory will not really
  // be released unless its refcount falls to zero. Instead, make two
  // passes: the first pass will release items with refcount = 1,
  // and if not enough space releases after scanning the number of
  // elements specified by this parameter, we will remove items in LRU
  // order.
  int table_cache_remove_scan_count_limit;

  // Disable automatic compactions. Manual compactions can still
  // be issued on this column family
  bool disable_auto_compactions;

  // Purge duplicate/deleted keys when a memtable is flushed to storage.
  // Default: true
  bool purge_redundant_kvs_while_flush;

  // This is used to close a block before it reaches the configured
  // 'block_size'. If the percentage of free space in the current block is less
  // than this specified number and adding a new record to the block will
  // exceed the configured block size, then this block will be closed and the
  // new record will be written to the next block.
  // Default is 10.
  int block_size_deviation;

  // The compaction style. Default: kCompactionStyleLevel
  CompactionStyle compaction_style;

  // The options needed to support Universal Style compactions
  CompactionOptionsUniversal compaction_options_universal;

  // Use KeyMayExist API to filter deletes when this is true.
  // If KeyMayExist returns false, i.e. the key definitely does not exist, then
  // the delete is a noop. KeyMayExist only incurs in-memory look up.
  // This optimization avoids writing the delete to storage when appropriate.
  // Default: false
  bool filter_deletes;

  // An iteration->Next() sequentially skips over keys with the same
  // user-key unless this option is set. This number specifies the number
  // of keys (with the same userkey) that will be sequentially
  // skipped before a reseek is issued.
  // Default: 8
  uint64_t max_sequential_skip_in_iterations;

  // This is a factory that provides MemTableRep objects.
  // Default: a factory that provides a skip-list-based implementation of
  // MemTableRep.
  std::shared_ptr<MemTableRepFactory> memtable_factory;

  // This is a factory that provides TableFactory objects.
  // Default: a factory that provides a default implementation of
  // Table and TableBuilder.
  std::shared_ptr<TableFactory> table_factory;

  // This option allows user to to collect their own interested statistics of
  // the tables.
  // Default: emtpy vector -- no user-defined statistics collection will be
  // performed.
  std::vector<std::shared_ptr<TablePropertiesCollector>>
    table_properties_collectors;

  // Allows thread-safe inplace updates. Requires Updates iff
  // * key exists in current memtable
  // * new sizeof(new_value) <= sizeof(old_value)
  // * old_value for that key is a put i.e. kTypeValue
  // Default: false.
  bool inplace_update_support;

  // Number of locks used for inplace update
  // Default: 10000, if inplace_update_support = true, else 0.
  size_t inplace_update_num_locks;

  // Create ColumnFamilyOptions with default values for all fields
  ColumnFamilyOptions();
};

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

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

  // If true, the implementation will do aggressive checking of the
  // data it is processing and will stop early if it detects any
  // errors.  This may have unforeseen ramifications: for example, a
  // corruption of one DB entry may cause a large number of entries to
  // become unreadable or for the entire DB to become unopenable.
  // 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.
  // Default: false
  bool paranoid_checks;

  // Use the specified object to interact with the environment,
  // e.g. to read/write files, schedule background work, etc.
  // Default: Env::Default()
  Env* env;

  // 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
  shared_ptr<Logger> info_log;

  // 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 (budget
  // one open file per 2MB of working set).
  //
  // Default: 1000
  int max_open_files;

  // If non-null, then we should collect metrics about database operations
  // Statistics objects should not be shared between DB instances as
  // it does not use any locks to prevent concurrent updates.
  shared_ptr<Statistics> statistics;

  // If true, then the contents of data files are not synced
  // to stable storage. Their contents remain in the OS buffers till the
  // OS decides to flush them. This option is good for bulk-loading
  // of data. Once the bulk-loading is complete, please issue a
  // sync to the OS to flush all dirty buffesrs to stable storage.
  // Default: false
  bool disableDataSync;

  // If true, then every store to stable storage will issue a fsync.
  // If false, then every store to stable storage will issue a fdatasync.
  // This parameter should be set to true while storing data to
  // filesystem like ext3 that can lose files after a reboot.
  // Default: false
  bool use_fsync;

  // This number controls how often a new scribe log about
  // db deploy stats is written out.
  // -1 indicates no logging at all.
  // Default value is 1800 (half an hour).
  int db_stats_log_interval;

  // 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
  uint64_t delete_obsolete_files_period_micros;

  // Maximum number of concurrent background jobs, submitted to
  // the default LOW priority thread pool
  // Default: 1
  int max_background_compactions;

  // Maximum number of concurrent background memtable flush jobs, submitted to
  // the HIGH priority thread pool.
  // By default, all background jobs (major compaction and memtable flush) go
  // to the LOW priority pool. If this option is set to a positive number,
  // memtable flush jobs will be submitted to the HIGH priority pool.
  // It is important when the same Env is shared by multiple db instances.
  // Without a separate pool, long running major compaction jobs could
  // potentially block memtable flush jobs of other db instances, leading to
  // unnecessary Put stalls.
  // Default: 0
  int max_background_flushes;

  // 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;

  // 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)
  size_t log_file_time_to_roll;

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

  // manifest file is rolled over on reaching this limit.
  // The older manifest file be deleted.
  // The default value is MAX_INT so that roll-over does not take place.
  uint64_t max_manifest_file_size;

  // size of one block in arena memory allocation.
  // If <= 0, a proper value is automatically calculated (usually 1/10 of
  // writer_buffer_size).
  //
  // Default: 0
  size_t arena_block_size;

  // 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_secondsi / 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;
  uint64_t WAL_size_limit_MB;

  // 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;

  // Data being read from file storage may be buffered in the OS
  // Default: true
  bool allow_os_buffer;

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

  // Allow the OS to mmap file for writing. Default: true
  bool allow_mmap_writes;

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

  // Skip log corruption error on recovery (If client is ok with
  // losing most recent changes)
  // Default: false
  bool skip_log_error_on_recovery;

  // if not zero, dump rocksdb.stats to LOG every stats_dump_period_sec
  // Default: 3600 (1 hour)
  unsigned int stats_dump_period_sec;

  // 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;

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

  // 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;

  // Allows OS to incrementally sync files to disk while they are being
  // written, asynchronously, in the background.
  // Issue one request for every bytes_per_sync written. 0 turns it off.
  // Default: 0
  uint64_t bytes_per_sync;

  // Create DBOptions with default values for all fields
  DBOptions();
};

// Options to control the behavior of a database (passed to DB::Open)
struct Options : public DBOptions, public ColumnFamilyOptions {
  // Create an Options object with default values for all fields.
  Options() :
    DBOptions(),
    ColumnFamilyOptions() {}

  Options(const DBOptions& db_options,
          const ColumnFamilyOptions& column_family_options)
      : DBOptions(db_options), ColumnFamilyOptions(column_family_options) {}

  void Dump(Logger* log) const;

  // Set appropriate parameters for bulk loading.
  // The reason that this is a function that returns "this" instead of a
  // constructor is to enable chaining of multiple similar calls in the future.
  //
  // All data will be in level 0 without any automatic compaction.
  // It's recommended to manually call CompactRange(NULL, NULL) before reading
  // from the database, because otherwise the read can be very slow.
  Options* PrepareForBulkLoad();
};

//
// An application can issue a read request (via Get/Iterators) and specify
// if that read should process data that ALREADY resides on a specified cache
// level. For example, if an application specifies kBlockCacheTier then the
// Get call will process data that is already processed in the memtable or
// the block cache. It will not page in data from the OS cache or data that
// resides in storage.
enum ReadTier {
  kReadAllTier    = 0x0, // data in memtable, block cache, OS cache or storage
  kBlockCacheTier = 0x1  // data in memtable or block cache
};

// Options that control read operations
struct ReadOptions {
  // If true, all data read from underlying storage will be
  // verified against corresponding checksums.
  // Default: false
  bool verify_checksums;

  // Should the "data block"/"index block"/"filter block" read for this
  // iteration be cached in memory?
  // Callers may wish to set this field to false for bulk scans.
  // Default: true
  bool fill_cache;

  // If this option is set and memtable implementation allows, Seek
  // might only return keys with the same prefix as the seek-key
  bool prefix_seek;

  // If "snapshot" is non-nullptr, read as of the supplied snapshot
  // (which must belong to the DB that is being read and which must
  // not have been released).  If "snapshot" is nullptr, use an impliicit
  // snapshot of the state at the beginning of this read operation.
  // Default: nullptr
  const Snapshot* snapshot;

  // If "prefix" is non-nullptr, and ReadOptions is being passed to
  // db.NewIterator, only return results when the key begins with this
  // prefix.  This field is ignored by other calls (e.g., Get).
  // Options.prefix_extractor must also be set, and
  // prefix_extractor.InRange(prefix) must be true.  The iterator
  // returned by NewIterator when this option is set will behave just
  // as if the underlying store did not contain any non-matching keys,
  // with two exceptions.  Seek() only accepts keys starting with the
  // prefix, and SeekToLast() is not supported.  prefix filter with this
  // option will sometimes reduce the number of read IOPs.
  // Default: nullptr
  const Slice* prefix;

  // Specify if this read request should process data that ALREADY
  // resides on a particular cache. If the required data is not
  // found at the specified cache, then Status::Incomplete is returned.
  // Default: kReadAllTier
  ReadTier read_tier;

  ReadOptions()
      : verify_checksums(false),
        fill_cache(true),
        prefix_seek(false),
        snapshot(nullptr),
        prefix(nullptr),
        read_tier(kReadAllTier) {
  }
  ReadOptions(bool cksum, bool cache) :
              verify_checksums(cksum), fill_cache(cache),
              prefix_seek(false), snapshot(nullptr), prefix(nullptr),
              read_tier(kReadAllTier) {
  }
};

// Options that control write operations
struct WriteOptions {
  // If true, the write will be flushed from the operating system
  // buffer cache (by calling WritableFile::Sync()) before the write
  // is considered complete.  If this flag is true, writes will be
  // slower.
  //
  // If this flag is false, and the machine crashes, some recent
  // writes may be lost.  Note that if it is just the process that
  // crashes (i.e., the machine does not reboot), no writes will be
  // lost even if sync==false.
  //
  // In other words, a DB write with sync==false has similar
  // crash semantics as the "write()" system call.  A DB write
  // with sync==true has similar crash semantics to a "write()"
  // system call followed by "fdatasync()".
  //
  // Default: false
  bool sync;

  // If true, writes will not first go to the write ahead log,
  // and the write may got lost after a crash.
  bool disableWAL;

  WriteOptions()
      : sync(false),
        disableWAL(false) {
  }
};

// Options that control flush operations
struct FlushOptions {
  // If true, the flush will wait until the flush is done.
  // Default: true
  bool wait;

  FlushOptions()
      : wait(true) {
  }
};

}  // namespace rocksdb

#endif  // STORAGE_ROCKSDB_INCLUDE_OPTIONS_H_