syncer(dm): fix log flood and performance when frequently call Snapsh…

…ot (#4744)

close #4619

dm/syncer/checkpoint.go

@@ -243,7 +243,7 @@ type CheckPoint interface {
 	// SaveGlobalPointForcibly saves the global binlog stream's checkpoint forcibly.
 	SaveGlobalPointForcibly(location binlog.Location)
 
-	// Snapshot make a snapshot of current checkpoint
+	// Snapshot make a snapshot of current checkpoint. If returns nil, it means nothing has changed since last call.
 	Snapshot(isSyncFlush bool) *SnapshotInfo
 
 	// FlushPointsExcept flushes the global checkpoint and tables'
@@ -376,6 +376,7 @@ func (cp *RemoteCheckPoint) Snapshot(isSyncFlush bool) *SnapshotInfo {
 	// make snapshot is visit in single thread, so depend on rlock should be enough
 	cp.snapshotSeq++
 	id := cp.snapshotSeq
+	cp.lastSnapshotCreationTime = time.Now()
 
 	tableCheckPoints := make(map[string]map[string]tablePoint, len(cp.points))
 	for s, tableCps := range cp.points {
@@ -412,7 +413,6 @@ func (cp *RemoteCheckPoint) Snapshot(isSyncFlush bool) *SnapshotInfo {
 	}
 
 	cp.snapshots = append(cp.snapshots, snapshot)
-	cp.lastSnapshotCreationTime = time.Now()
 	return &SnapshotInfo{
 		id:        id,
 		globalPos: globalPoint.location,

dm/syncer/syncer.go

@@ -1168,7 +1168,7 @@ func (s *Syncer) flushCheckPoints() error {
 	snapshotInfo, exceptTables, shardMetaSQLs, shardMetaArgs := s.createCheckpointSnapshot(true)
 
 	if snapshotInfo == nil {
-		s.tctx.L().Info("checkpoint has no change, skip sync flush checkpoint")
+		s.tctx.L().Debug("checkpoint has no change, skip sync flush checkpoint")
 		return nil
 	}
 
@@ -1204,7 +1204,7 @@ func (s *Syncer) flushCheckPointsAsync(asyncFlushJob *job) {
 	snapshotInfo, exceptTables, shardMetaSQLs, shardMetaArgs := s.createCheckpointSnapshot(false)
 
 	if snapshotInfo == nil {
-		s.tctx.L().Info("checkpoint has no change, skip async flush checkpoint", zap.Int64("job seq", asyncFlushJob.flushSeq))
+		s.tctx.L().Debug("checkpoint has no change, skip async flush checkpoint", zap.Int64("job seq", asyncFlushJob.flushSeq))
 		return
 	}