@Override
  protected long keySize() {
    // Need to consider dataSize/keySize of all segments in pipeline and active
    long keySize = getActive().getDataSize();
    for (Segment segment : this.pipeline.getSegments()) {
      keySize += segment.getDataSize();
    }
    return keySize;
  }
Push the current active memstore segment into the pipeline and create a snapshot of the tail of
current compaction pipeline Snapshot must be cleared by call to {@link #clearSnapshot}.
{@link #clearSnapshot(long)}.
@return {@link MemStoreSnapshot}
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/CompactingMemStore.java