隨著執行時間的增加，memtable會慢慢 轉化成 sstable。

sstable會越來越多 我們就需要進行整合 compact

程式碼會在寫入查詢key值 db寫入時等多出位置呼叫MaybeScheduleCompaction () 

檢測是否需要進行compact

 1 void DBImpl::MaybeScheduleCompaction() {
 2   mutex_.AssertHeld();
 3   if (bg_compaction_scheduled_) {
 4     // Already scheduled
 5   } else if (shutting_down_.Acquire_Load()) {
 
 6     // DB is being deleted; no more background compactions
 7   } else if (imm_ == NULL &&
 8              manual_compaction_ == NULL &&
 9              !versions_->NeedsCompaction()) {
10     // No work to be done
11   } else {
12     bg_compaction_scheduled_ = true;
13     env_->Schedule(&DBImpl::BGWork, this
 
);
14   }
15 }
16 
17 void DBImpl::BGWork(void* db) {
18   reinterpret_cast<DBImpl*>(db)->BackgroundCall();
19 }
20 
21 void DBImpl::BackgroundCall() {
22   MutexLock l(&mutex_);
23   assert(bg_compaction_scheduled_);
24   if (!shutting_down_.Acquire_Load()) {
25     BackgroundCompaction();
 
26   }
27   bg_compaction_scheduled_ = false;
28 
29   // Previous compaction may have produced too many files in a level,
30   // so reschedule another compaction if needed.
31   MaybeScheduleCompaction();
32   bg_cv_.SignalAll();
33 }

實際進行compact的函式是 void DBImpl::BackgroundCompaction()