Store the checkpoint we read last, so that we don't keep reading the same checkpoint on each tick.

Signed-off-by: Callum Styan <callumstyan@gmail.com>

storage/remote/wal_watcher.go

@@ -99,6 +99,7 @@ type WALWatcher struct {
 	writer         writeTo
 	logger         log.Logger
 	walDir         string
+	lastCheckpoint string
 
 	startTime int64
 
@@ -183,6 +184,7 @@ func (w *WALWatcher) run() error {
 			return errors.Wrap(err, "readCheckpoint")
 		}
 	}
+	w.lastCheckpoint = lastCheckpoint
 
 	currentSegment, err := w.findSegmentForIndex(nextIndex)
 	if err != nil {
@@ -342,9 +344,10 @@ func (w *WALWatcher) garbageCollectSeries(segmentNum int) error {
 		return errors.Wrap(err, "tsdb.LastCheckpoint")
 	}
 
-	if dir == "" {
+	if dir == "" || dir == w.lastCheckpoint {
 		return nil
 	}
+	w.lastCheckpoint = dir
 
 	index, err := checkpointNum(dir)
 	if err != nil {