技術標籤：HBase原始碼理解hbase

根據前三篇文章，HBase flush主要分三個階段，snapshot，flush，commit，這篇深入HBase MemStore，梳理snapshot的流程。

volatile Section activeSection;
volatile Section snapshotSection;

MemStore 主要包含這兩個Section，寫入的Cell儲存在activeSection中，volatile關鍵字保證執行緒可見性。接下來是snapshot方法：

@Override
  public MemStoreSnapshot snapshot
 
() {
    if (!snapshotSection.getCellSkipListSet().isEmpty()) {
      LOG.warn("Snapshot called again without clearing previous. " +
          "Doing nothing. Another ongoing flush or did we fail last attempt?");
    } else {
      this.snapshotId = EnvironmentEdgeManager.currentTime
 
();
      if (!activeSection.getCellSkipListSet().isEmpty()) {
        snapshotSection = activeSection;
        activeSection = Section.newActiveSection(comparator, conf);
        snapshotSection.getHeapSize().addAndGet(-DEEP_OVERHEAD);
        timeOfOldestEdit = Long.MAX_VALUE;
      }
    }
    MemStoreSnapshot memStoreSnapshot =
 
 new MemStoreSnapshot(this.snapshotId,
        snapshotSection.getCellsCount().get(), snapshotSection.getHeapSize().get(),
        snapshotSection.getTimeRangeTracker(),
        new CollectionBackedScanner(snapshotSection.getCellSkipListSet(), this.comparator),
        this.tagsPresent);
    this.tagsPresent = false;
    return memStoreSnapshot;
  }

snapshot ()方法直接將activeSection的引用賦值給snapshotSection，然後重新構造一個activeSection。舊snapshotSection將會在GC的時候被回收。
Section 是DefaultMemstore中的靜態內部類，其構造方法如下：

private Section(final KeyValue.KVComparator c,
            final Configuration conf, long initHeapSize) {
      this.cellSet = new CellSkipListSet(c);
      this.heapSize = new AtomicLong(initHeapSize);
      this.cellCount = new AtomicInteger(0);
      if (conf != null && conf.getBoolean(USEMSLAB_KEY, USEMSLAB_DEFAULT)) {
        String className = conf.get(MSLAB_CLASS_NAME, HeapMemStoreLAB.class.getName());
        this.allocator = ReflectionUtils.instantiateWithCustomCtor(className,
                new Class[]{Configuration.class}, new Object[]{conf});
      } else {
        this.allocator = null;
      }
    }

其使用一個CellSkipListSet存放cell。分配記憶體涉及到一個類，HeapMemStoreLAB，LAB是local allocation buffer的縮寫。LAB保證memstore中的cell都存在大塊的chunk中，這樣，在flush之後，記憶體是以大塊chunk為單位進行釋放，減少RegionServer中的記憶體碎片。涉及四個超引數：