Long startCacheFlush(final byte[] encodedRegionName, final Set<byte[]> families) {
    Map<byte[], Long> familytoSeq = new HashMap<>();
    for (byte[] familyName : families) {
      familytoSeq.put(familyName, HConstants.NO_SEQNUM);
    }
    return startCacheFlush(encodedRegionName, familytoSeq);
  }
@param encodedRegionName Region to flush.
@param families          Families to flush. May be a subset of all families in the region.
@return Returns {@link HConstants#NO_SEQNUM} if we are flushing the whole region OR if we are
        flushing a subset of all families but there are no edits in those families not being
        flushed; in other words, this is effectively same as a flush of all of the region
        though we were passed a subset of regions. Otherwise, it returns the sequence id of the
        oldest/lowest outstanding edit.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/wal/SequenceIdAccounting.java