private boolean worthPreFlushing() {
    return this.memStoreSizing.getDataSize()
        > this.conf.getLong("hbase.hregion.preclose.flush.size", 1024 * 1024 * 5);
  }
Returns True if its worth doing a flush before we put up the close flag.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/HRegion.java