public Map<byte[], List<byte[]>> rollWal(long now) throws IOException {
      this.lastRollTime = now;
      // reset the flag in front to avoid missing roll request before we return from rollWriter.
      this.rollRequest.set(false);
      return wal.rollWriter(true);
    }
Independently control the roll of each wal. When use multiwal, can avoid all wal roll together.
see HBASE-24665 for detail
hbase-server/src/main/java/org/apache/hadoop/hbase/wal/AbstractWALRoller.java