@Override
  public MemStoreSize getFlushableSize() {
    MemStoreSize mss = getSnapshotSize();
    return mss.getDataSize() > 0 ? mss : getActive().getMemStoreSize();
  }
Creates a snapshot of the current memstore. Snapshot must be cleared by call to
{@link #clearSnapshot(long)}
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/DefaultMemStore.java