protected long indexEntryOnHeapSize(boolean onHeap) {
    // in most cases index is allocated on-heap
    // override this method when it is not always the case, e.g., in CCM
    return indexEntrySize();
  }
@return The increase in heap size because of this cell addition. This includes this cell POJO's
        heap size itself and additional overhead because of addition on to CSLM.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/Segment.java