LevelDB场景分析1--整体结构分析

基本用法

数据结构

class DBImpl : public DB

{

private:

    struct CompactionState;

    struct Writer;// Information kept for every waiting writer

    Env* const env_; // 文件，目录，日志，Schedule线程

    const InternalKeyComparator internal_comparator_;

    const InternalFilterPolicy internal_filter_policy_; // 提高随机读的性能

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

    bool owns_info_log_;

    bool owns_cache_;

    const std::string dbname_;

    // table_cache_ provides its own synchronization

    TableCache* table_cache_;

    // Lock over the persistent DB state.  Non-NULL iff successfully acquired.

    FileLock* db_lock_;

    // State below is protected by mutex_

    port::Mutex mutex_;

    port::AtomicPointer shutting_down_;

    port::CondVar bg_cv_;          // Signalled when background work finishes

    MemTable* mem_;

    MemTable* imm_;                // Memtable being compacted

    port::AtomicPointer has_imm_;  // So bg thread can detect non-NULL imm_

    WritableFile* logfile_;

    uint64_t logfile_number_;

    log::Writer* log_;

    uint32_t seed_;                // For sampling.

    // Queue of writers.

    std::deque<Writer*> writers_;

    WriteBatch* tmp_batch_;

    SnapshotList snapshots_;

    // Set of table files to protect from deletion because they are

    // part of ongoing compactions.

    std::set<uint64_t> pending_outputs_;

    // Has a background compaction been scheduled or is running?

    bool bg_compaction_scheduled_;

    ManualCompaction* manual_compaction_;

    VersionSet* versions_;

    // Have we encountered a background error in paranoid mode?

    Status bg_error_;

CompactionStats stats_[config::kNumLevels];

};

红色部分成员是需要重点关注的。

主要场景

public: 

Status DB::Open(const Options& options, const std::string& dbname, DB** dbptr) 

// Implementations of the DB interface

  virtual Status Put(const WriteOptions&, const Slice& key, const Slice& value);

  virtual Status Delete(const WriteOptions&, const Slice& key);

  virtual Status Write(const WriteOptions& options, WriteBatch* updates);

  virtual Status Get(const ReadOptions& options,

                     const Slice& key,

                     std::string* value);

  virtual Iterator* NewIterator(const ReadOptions&);

  virtual const Snapshot* GetSnapshot();

  virtual void ReleaseSnapshot(const Snapshot* snapshot);

  virtual bool GetProperty(const Slice& property, std::string* value);

  virtual void GetApproximateSizes(const Range* range, int n, uint64_t* sizes);

  virtual void CompactRange(const Slice* begin, const Slice* end);

  // Extra methods (for testing) that are not in the public DB interface

    // Record a sample of bytes read at the specified internal key.

  // Samples are taken approximately once every config::kReadBytesPeriod

  // bytes.

  void RecordReadSample(Slice key);

private:

Iterator* NewInternalIterator(const ReadOptions&,

                                SequenceNumber* latest_snapshot,

                                uint32_t* seed);

  Status NewDB();

  // Recover the descriptor from persistent storage.  May do a significant

  // amount of work to recover recently logged updates.  Any changes to

  // be made to the descriptor are added to *edit.

  Status Recover(VersionEdit* edit) EXCLUSIVE_LOCKS_REQUIRED(mutex_);

  void MaybeIgnoreError(Status* s) const;

  // Delete any unneeded files and stale in-memory entries.

  void DeleteObsoleteFiles();

  // Compact the in-memory write buffer to disk.  Switches to a new

  // log-file/memtable and writes a new descriptor iff successful.

  // Errors are recorded in bg_error_.

  void CompactMemTable() EXCLUSIVE_LOCKS_REQUIRED(mutex_);

  Status RecoverLogFile(uint64_t log_number,

                        VersionEdit* edit,

                        SequenceNumber* max_sequence)

      EXCLUSIVE_LOCKS_REQUIRED(mutex_);

  Status WriteLevel0Table(MemTable* mem, VersionEdit* edit, Version* base)

      EXCLUSIVE_LOCKS_REQUIRED(mutex_);

  Status MakeRoomForWrite(bool force /* compact even if there is room? */)

      EXCLUSIVE_LOCKS_REQUIRED(mutex_);

  WriteBatch* BuildBatchGroup(Writer** last_writer);

  void RecordBackgroundError(const Status& s);

  void MaybeScheduleCompaction() EXCLUSIVE_LOCKS_REQUIRED(mutex_);

  static void BGWork(void* db);

  void BackgroundCall();

  void  BackgroundCompaction() EXCLUSIVE_LOCKS_REQUIRED(mutex_);

  void CleanupCompaction(CompactionState* compact)

      EXCLUSIVE_LOCKS_REQUIRED(mutex_);

  Status DoCompactionWork(CompactionState* compact)

      EXCLUSIVE_LOCKS_REQUIRED(mutex_);

  Status OpenCompactionOutputFile(CompactionState* compact);

  Status FinishCompactionOutputFile(CompactionState* compact, Iterator* input);

  Status InstallCompactionResults(CompactionState* compact)

      EXCLUSIVE_LOCKS_REQUIRED(mutex_);

综合介绍

LevelDB主要有以下几个部分的内存开销：memtable,immutable table,table cache,block cache

memtable和immutable table的大小由 options_.write_buffer_size决定。

table cache由max_open_files决定数量。

block cache最大可以传入 size_t的大小。

由于使用了mmap，因此需要尽量使内存大于数据规模。否则可能造成随机读爆慢。