195 lines
6.7 KiB
C++
195 lines
6.7 KiB
C++
// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file. See the AUTHORS file for names of contributors.
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#ifndef STORAGE_LEVELDB_INCLUDE_OPTIONS_H_
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#define STORAGE_LEVELDB_INCLUDE_OPTIONS_H_
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#include <stddef.h>
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namespace leveldb {
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class Cache;
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class Comparator;
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class Env;
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class FilterPolicy;
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class Logger;
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class Snapshot;
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// DB contents are stored in a set of blocks, each of which holds a
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// sequence of key,value pairs. Each block may be compressed before
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// being stored in a file. The following enum describes which
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// compression method (if any) is used to compress a block.
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enum CompressionType {
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// NOTE: do not change the values of existing entries, as these are
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// part of the persistent format on disk.
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kNoCompression = 0x0,
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kSnappyCompression = 0x1
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};
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// Options to control the behavior of a database (passed to DB::Open)
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struct Options {
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// -------------------
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// Parameters that affect behavior
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// Comparator used to define the order of keys in the table.
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// Default: a comparator that uses lexicographic byte-wise ordering
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//
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// REQUIRES: The client must ensure that the comparator supplied
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// here has the same name and orders keys *exactly* the same as the
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// comparator provided to previous open calls on the same DB.
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const Comparator* comparator;
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// If true, the database will be created if it is missing.
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// Default: false
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bool create_if_missing;
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// If true, an error is raised if the database already exists.
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// Default: false
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bool error_if_exists;
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// If true, the implementation will do aggressive checking of the
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// data it is processing and will stop early if it detects any
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// errors. This may have unforeseen ramifications: for example, a
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// corruption of one DB entry may cause a large number of entries to
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// become unreadable or for the entire DB to become unopenable.
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// Default: false
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bool paranoid_checks;
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// Use the specified object to interact with the environment,
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// e.g. to read/write files, schedule background work, etc.
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// Default: Env::Default()
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Env* env;
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// Any internal progress/error information generated by the db will
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// be written to info_log if it is non-NULL, or to a file stored
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// in the same directory as the DB contents if info_log is NULL.
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// Default: NULL
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Logger* info_log;
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// -------------------
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// Parameters that affect performance
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// Amount of data to build up in memory (backed by an unsorted log
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// on disk) before converting to a sorted on-disk file.
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//
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// Larger values increase performance, especially during bulk loads.
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// Up to two write buffers may be held in memory at the same time,
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// so you may wish to adjust this parameter to control memory usage.
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// Also, a larger write buffer will result in a longer recovery time
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// the next time the database is opened.
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//
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// Default: 4MB
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size_t write_buffer_size;
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// Number of open files that can be used by the DB. You may need to
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// increase this if your database has a large working set (budget
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// one open file per 2MB of working set).
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//
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// Default: 1000
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int max_open_files;
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// Control over blocks (user data is stored in a set of blocks, and
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// a block is the unit of reading from disk).
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// If non-NULL, use the specified cache for blocks.
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// If NULL, leveldb will automatically create and use an 8MB internal cache.
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// Default: NULL
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Cache* block_cache;
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// Approximate size of user data packed per block. Note that the
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// block size specified here corresponds to uncompressed data. The
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// actual size of the unit read from disk may be smaller if
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// compression is enabled. This parameter can be changed dynamically.
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//
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// Default: 4K
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size_t block_size;
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// Number of keys between restart points for delta encoding of keys.
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// This parameter can be changed dynamically. Most clients should
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// leave this parameter alone.
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//
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// Default: 16
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int block_restart_interval;
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// Compress blocks using the specified compression algorithm. This
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// parameter can be changed dynamically.
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//
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// Default: kSnappyCompression, which gives lightweight but fast
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// compression.
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//
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// Typical speeds of kSnappyCompression on an Intel(R) Core(TM)2 2.4GHz:
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// ~200-500MB/s compression
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// ~400-800MB/s decompression
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// Note that these speeds are significantly faster than most
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// persistent storage speeds, and therefore it is typically never
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// worth switching to kNoCompression. Even if the input data is
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// incompressible, the kSnappyCompression implementation will
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// efficiently detect that and will switch to uncompressed mode.
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CompressionType compression;
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// If non-NULL, use the specified filter policy to reduce disk reads.
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// Many applications will benefit from passing the result of
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// NewBloomFilterPolicy() here.
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//
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// Default: NULL
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const FilterPolicy* filter_policy;
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// Create an Options object with default values for all fields.
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Options();
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};
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// Options that control read operations
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struct ReadOptions {
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// If true, all data read from underlying storage will be
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// verified against corresponding checksums.
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// Default: false
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bool verify_checksums;
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// Should the data read for this iteration be cached in memory?
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// Callers may wish to set this field to false for bulk scans.
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// Default: true
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bool fill_cache;
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// If "snapshot" is non-NULL, read as of the supplied snapshot
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// (which must belong to the DB that is being read and which must
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// not have been released). If "snapshot" is NULL, use an impliicit
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// snapshot of the state at the beginning of this read operation.
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// Default: NULL
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const Snapshot* snapshot;
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ReadOptions()
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: verify_checksums(false),
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fill_cache(true),
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snapshot(NULL) {
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}
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};
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// Options that control write operations
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struct WriteOptions {
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// If true, the write will be flushed from the operating system
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// buffer cache (by calling WritableFile::Sync()) before the write
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// is considered complete. If this flag is true, writes will be
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// slower.
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//
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// If this flag is false, and the machine crashes, some recent
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// writes may be lost. Note that if it is just the process that
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// crashes (i.e., the machine does not reboot), no writes will be
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// lost even if sync==false.
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//
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// In other words, a DB write with sync==false has similar
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// crash semantics as the "write()" system call. A DB write
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// with sync==true has similar crash semantics to a "write()"
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// system call followed by "fsync()".
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//
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// Default: false
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bool sync;
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WriteOptions()
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: sync(false) {
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}
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};
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} // namespace leveldb
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#endif // STORAGE_LEVELDB_INCLUDE_OPTIONS_H_
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