10225101483
/
XOY-Leveldb


								// 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_LEVELDB_INCLUDE_OPTIONS_H_

								#define STORAGE_LEVELDB_INCLUDE_OPTIONS_H_


								#include <cstddef>


								#include "leveldb/export.h"


								namespace leveldb {


								class Cache;

								class Comparator;

								class Env;

								class FilterPolicy;

								class Logger;

								class Snapshot;


								// 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 {

								  // NOTE: do not change the values of existing entries, as these are

								  // part of the persistent format on disk.

								  kNoCompression = 0x0,

								  kSnappyCompression = 0x1

								};


								// Options to control the behavior of a database (passed to DB::Open)

								struct LEVELDB_EXPORT Options {

								  // Create an Options object with default values for all fields.

								  Options();


								  // -------------------

								  // 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;


								  // If true, the database will be created if it is missing.

								  bool create_if_missing = false;


								  // If true, an error is raised if the database already exists.

								  bool error_if_exists = false;


								  // 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.

								  bool paranoid_checks = false;


								  // 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-null, or to a file stored

								  // in the same directory as the DB contents if info_log is null.

								  Logger* info_log = 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 two 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.

								  size_t write_buffer_size = 4 * 1024 * 1024;


								  // 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).

								  int max_open_files = 1000;


								  // 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, leveldb will automatically create and use an 8MB internal cache.

								  Cache* block_cache = nullptr;


								  // 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.

								  size_t block_size = 4 * 1024;


								  // Number of keys between restart points for delta encoding of keys.

								  // This parameter can be changed dynamically.  Most clients should

								  // leave this parameter alone.

								  int block_restart_interval = 16;


								  // Leveldb will write up to this amount of bytes to a file before

								  // switching to a new one.

								  // Most clients should leave this parameter alone.  However if your

								  // filesystem is more efficient with larger files, you could

								  // consider increasing the value.  The downside will be longer

								  // compactions and hence longer latency/performance hiccups.

								  // Another reason to increase this parameter might be when you are

								  // initially populating a large database.

								  size_t max_file_size = 2 * 1024 * 1024;


								  // 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 = kSnappyCompression;


								  // EXPERIMENTAL: If true, append to existing MANIFEST and log files

								  // when a database is opened.  This can significantly speed up open.

								  //

								  // Default: currently false, but may become true later.

								  bool reuse_logs = false;


								  // If non-null, use the specified filter policy to reduce disk reads.

								  // Many applications will benefit from passing the result of

								  // NewBloomFilterPolicy() here.

								  const FilterPolicy* filter_policy = nullptr;

								};


								// Options that control read operations

								struct LEVELDB_EXPORT ReadOptions {

								  ReadOptions() = default;


								  // If true, all data read from underlying storage will be

								  // verified against corresponding checksums.

								  bool verify_checksums = false;


								  // Should the data read for this iteration be cached in memory?

								  // Callers may wish to set this field to false for bulk scans.

								  bool fill_cache = true;


								  // If "snapshot" is non-null, 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 null, use an implicit

								  // snapshot of the state at the beginning of this read operation.

								  const Snapshot* snapshot = nullptr;

								};


								// Options that control write operations

								struct LEVELDB_EXPORT WriteOptions {

								  WriteOptions() = default;


								  // 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 "fsync()".

								  bool sync = false;

								};


								}  // namespace leveldb


								#endif  // STORAGE_LEVELDB_INCLUDE_OPTIONS_H_