HBASE-21721 FSHLog : reduce write#syncs() times (#2217)

Signed-off-by: stack <stack@apache.org>
Signed-off-by: Anoop <anoopsamjohn@apache.org>

Author: cuibo01

hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/wal/FSHLog.java

@@ -636,8 +636,12 @@ public void run() {
           Throwable lastException = null;
           try {
             TraceUtil.addTimelineAnnotation("syncing writer");
+            long unSyncedFlushSeq = highestUnsyncedTxid;
             writer.sync(sf.isForceSync());
             TraceUtil.addTimelineAnnotation("writer synced");
+            if (unSyncedFlushSeq > currentSequence) {
+              currentSequence = unSyncedFlushSeq;
+            }
             currentSequence = updateHighestSyncedSequence(currentSequence);
           } catch (IOException e) {
             LOG.error("Error syncing, request close of WAL", e);