); if (!status.ok()) { RecordBackgroundError(status); } VersionSet::LevelSummaryStorage tmp; Log(options_.info_log, "Moved #%lld to level-%d %lld bytes %s: %s\n", static_cast
(f->number), c->level() + 1, static_cast
(f->file_size), status.ToString().c_str(), versions_->LevelSummary(&tmp)); } else { CompactionState* compact = new CompactionState(c); status = DoCompactionWork(compact); if (!status.ok()) { RecordBackgroundError(status); } CleanupCompaction(compact); c->ReleaseInputs(); DeleteObsoleteFiles(); } delete c; if (status.ok()) { // Done } else if (shutting_down_.Acquire_Load()) { // Ignore compaction errors found during shutting down } else { Log(options_.info_log, "Compaction error: %s", status.ToString().c_str()); } if (is_manual) { ManualCompaction* m = manual_compaction_; if (!status.ok()) { m->done = true; } if (!m->done) { // We only compacted part of the requested range. Update *m // to the range that is left to be compacted. m->tmp_storage = manual_end; m->begin = &m->tmp_storage; } manual_compaction_ = NULL; } }
1.如果存在immutable memtable，将其dump成sstable，完成返回。
2.如果是外部触发的compact，根据manual_compaction指定的level/start_key/end_key，选出compaction（VersionSet::CompactRange()）
3.如果不是manual compact，则根据db当前状态，选出compaction（VersionSet::PickCompaction()），考虑到level sstable的均衡性，提高查找效率。class compaction用于记录compact信息，包括compact的level和输入sstable文件等等，参见version_set.h。
4.对于非manual compact并且选出的sstable都处于level-n且不会造成过多的GrandparentOverrlap（Compaction::IsTrivialMove()），简单处理，将这些sstable推到level-n+1，更新db元信息即可(VersionSet::LogAndApply())。
5.其他情况，则一律根据确定出的Compaction，做具体的compact处理（DBImpl::DoCompactionWork()），最后做异常情况的清理（DBImpl::CleanupCompaction()）。
DBimpl::DoCompactionWork()，实际的compact过程就是对多个已经排序的sstable做一次merge排序，丢弃掉相同的Key以及删除的数据。
Status DBImpl::DoCompactionWork(CompactionState* compact) { const uint64_t start_micros = env_->NowMicros(); //immutable compact时计时用 int64_t imm_micros = 0; // Micros spent doing imm_ compactions Log(options_.info_log, "Compacting %d@%d + %d@%d files", compact->compaction->num_input_files(0), compact->compaction->level(), compact->compaction->num_input_files(1), compact->compaction->level() + 1); assert(versions_->NumLevelFiles(compact->compaction->level()) > 0); assert(compact->builder == NULL); assert(compact->outfile == NULL); if (snapshots_.empty()) { compact->smallest_snapshot = versions_->LastSequence(); } else { compact->smallest_snapshot = snapshots_.oldest()->number_; } // Release mutex while we're actually doing the compaction work mutex_.Unlock(); //将选出的compaction中的sstable构造MergingIterator //对于level-0做归并排序，其他level的sstable做一个连接他们的iterator Iterator* input = versions_->MakeInputIterator(compact->compaction); //定位到每一个sstable的first，后面将遍历input sstable的entry input->SeekToFirst(); Status status; ParsedInternalKey ikey; std::string current_user_key; bool has_current_user_key = false; SequenceNumber last_sequence_for_key = kMaxSequenceNumber; for (; input->Valid() && !shutting_down_.Acquire_Load(); ) { // Prioritize immutable compaction work //优先完成immutable的compact if (has_imm_.NoBarrier_Load() != NULL) { const uint64_t imm_start = env_->NowMicros(); mutex_.Lock(); if (imm_ != NULL) { CompactMemTable(); bg_cv_.SignalAll(); // Wakeup MakeRoomForWrite() if necessary } mutex_.Unlock(); imm_micros +