public ImmutableSegment createImmutableSegmentByMerge(final Configuration conf,
    final CellComparator comparator, MemStoreSegmentsIterator iterator, int numOfCells,
    List<ImmutableSegment> segments, CompactingMemStore.IndexType idxType,
    MemStoreCompactionStrategy.Action action) throws IOException {

    MemStoreLAB memStoreLAB = getMergedMemStoreLAB(conf, segments);
    return createImmutableSegment(conf, comparator, iterator, memStoreLAB, numOfCells, action,
      idxType);

  }
create empty immutable segment for initializations This ImmutableSegment is used as a place
holder for snapshot in Memstore. It won't flush later, So it is not necessary to record the
initial size for it.
@param comparator comparator
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/SegmentFactory.java