leveldb 学习记录(八) compact

随着运行时间的增加，memtable会慢慢 转化成 sstable。

sstable会越来越多 我们就需要进行整合 compact

代码会在写入查询key值 db写入时等多出位置调用MaybeScheduleCompaction () 

检测是否需要进行compact

void DBImpl::MaybeScheduleCompaction() {
  mutex_.AssertHeld();
  if (bg_compaction_scheduled_) {
    // Already scheduled
  } else if (shutting_down_.Acquire_Load()) {
    // DB is being deleted; no more background compactions
  } else if (imm_ == NULL &&
             manual_compaction_ == NULL &&
             !versions_->NeedsCompaction()) {
    // No work to be done
  } else {
    bg_compaction_scheduled_ = true;
    env_->Schedule(&DBImpl::BGWork, this);
  }
}

void DBImpl::BGWork(void* db) {
  reinterpret_cast<DBImpl*>(db)->BackgroundCall();
}

void DBImpl::BackgroundCall() {
  MutexLock l(&mutex_);
  assert(bg_compaction_scheduled_);
  if (!shutting_down_.Acquire_Load()) {
    BackgroundCompaction();
  }
  bg_compaction_scheduled_ = false;

  // Previous compaction may have produced too many files in a level,
  // so reschedule another compaction if needed.
  MaybeScheduleCompaction();
  bg_cv_.SignalAll();
}

实际进行compact的函数是 void DBImpl::BackgroundCompaction() 

1 手动触发情况下 会填写class DBImpl下的一个变量 ManualCompaction manual_compaction_

struct ManualCompaction {
    int level;
    bool done;
    const InternalKey* begin;   // NULL means beginning of key range
    const InternalKey* end;     // NULL means end of key range
    InternalKey tmp_storage;    // Used to keep track of compaction progress
  };

为了避免外部指定的 key-range 过大，一次 compact 过多的 sstable 文件， manual_compaction 可能不会一次做完，所以有 done 来标

识是否已经全部完成， tmp_storage 保存上一次 compact 到的 end-key，即下一次的 startkey。
指定的beg end KEY会赋值到 versions_中，以便后面进行compact。  versions_->CompactRange(m->level, m->begin, m->end);

2 通过 versions_->PickCompaction() 选择需要compact的level 和 key range

Compaction* VersionSet::PickCompaction() {
  Compaction* c;
  int level;

  // We prefer compactions triggered by too much data in a level over
  // the compactions triggered by seeks.
  const bool size_compaction = (current_->compaction_score_ >= 1);
  const bool seek_compaction = (current_->file_to_compact_ != NULL);
  if (size_compaction) {
    level = current_->compaction_level_;
    assert(level >= 0);
    assert(level+1 < config::kNumLevels);
    c = new Compaction(level);

    // Pick the first file that comes after compact_pointer_[level]
    for (size_t i = 0; i < current_->files_[level].size(); i++) {
      FileMetaData* f = current_->files_[level][i];
      if (compact_pointer_[level].empty() ||
          icmp_.Compare(f->largest.Encode(), compact_pointer_[level]) > 0) {
        c->inputs_[0].push_back(f);
        break;
      }
    }
    if (c->inputs_[0].empty()) {
      // Wrap-around to the beginning of the key space
      c->inputs_[0].push_back(current_->files_[level][0]);
    }
  } else if (seek_compaction) {
    level = current_->file_to_compact_level_;
    c = new Compaction(level);
    c->inputs_[0].push_back(current_->file_to_compact_);
  } else {
    return NULL;
  }

  c->input_version_ = current_;
  c->input_version_->Ref();

  // Files in level 0 may overlap each other, so pick up all overlapping ones
  if (level == 0) {
    InternalKey smallest, largest;
    GetRange(c->inputs_[0], &smallest, &largest);
    // Note that the next call will discard the file we placed in
    // c->inputs_[0] earlier and replace it with an overlapping set
    // which will include the picked file.
    current_->GetOverlappingInputs(0, &smallest, &largest, &c->inputs_[0]);
    assert(!c->inputs_[0].empty());
  }

  SetupOtherInputs(c);

  return c;
}

PickCompaction函数中 根据 文件尺寸和被seek多次 来确认compact的文件

使用 c = new Compaction(level)   记录要compact的level和文件指针

 todo  实际的compact操作  CompactMemTable()  DoCompactionWork()

参考

《leveldb实现解析》 淘宝 那岩