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diff --git a/include/leveldb/options.h b/include/leveldb/options.h new file mode 100644 index 0000000..fdda718 --- /dev/null +++ b/include/leveldb/options.h @@ -0,0 +1,195 @@ +// 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 <stddef.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 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. + // 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. + // 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-NULL, or to a file stored + // in the same directory as the DB contents if info_log is NULL. + // Default: NULL + Logger* info_log; + + // ------------------- + // 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. + // + // Default: 4MB + size_t write_buffer_size; + + // 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; + + // 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. + // Default: NULL + Cache* block_cache; + + // 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; + + // If non-NULL, use the specified filter policy to reduce disk reads. + // Many applications will benefit from passing the result of + // NewBloomFilterPolicy() here. + // + // Default: NULL + const FilterPolicy* filter_policy; + + // Create an Options object with default values for all fields. + Options(); +}; + +// 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 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 "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 impliicit + // snapshot of the state at the beginning of this read operation. + // Default: NULL + const Snapshot* snapshot; + + ReadOptions() + : verify_checksums(false), + fill_cache(true), + snapshot(NULL) { + } +}; + +// 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 "fsync()". + // + // Default: false + bool sync; + + WriteOptions() + : sync(false) { + } +}; + +} // namespace leveldb + +#endif // STORAGE_LEVELDB_INCLUDE_OPTIONS_H_ |