diff --git a/archive.go b/archive.go
index 4746ad2..aef367a 100644
--- a/archive.go
+++ b/archive.go
@@ -20,12 +20,34 @@ import (
 	"sync/atomic"
 )
 
+// Whenever changed, update MarshalJSON as well!
 type CheckpointMetrics struct {
 	Frequency int64   `json:"frequency"`
 	Start     int64   `json:"start"`
 	Data      []Float `json:"data"`
 }
 
+func (cm *CheckpointMetrics) MarshalJSON() ([]byte, error) {
+	buf := make([]byte, 0, 128+len(cm.Data)*8)
+	buf = append(buf, `{"frequency":`...)
+	buf = strconv.AppendInt(buf, cm.Frequency, 10)
+	buf = append(buf, `,"start":`...)
+	buf = strconv.AppendInt(buf, cm.Start, 10)
+	buf = append(buf, `,"data":[`...)
+	for i, x := range cm.Data {
+		if i != 0 {
+			buf = append(buf, ',')
+		}
+		if x.IsNaN() {
+			buf = append(buf, `null`...)
+		} else {
+			buf = strconv.AppendFloat(buf, float64(x), 'f', -1, 32)
+		}
+	}
+	buf = append(buf, `]}`...)
+	return buf, nil
+}
+
 type CheckpointFile struct {
 	From     int64                         `json:"from"`
 	To       int64                         `json:"to"`
@@ -76,7 +98,7 @@ func (m *MemoryStore) ToCheckpoint(dir string, from, to int64) (int, error) {
 						continue
 					}
 
-					log.Printf("checkpointing %#v failed: %s", workItem.selector, err.Error())
+					log.Printf("error while checkpointing %#v: %s", workItem.selector, err.Error())
 					atomic.AddInt32(&errs, 1)
 				} else {
 					atomic.AddInt32(&n, 1)
@@ -216,8 +238,7 @@ func (m *MemoryStore) FromCheckpoint(dir string, from int64) (int, error) {
 				nn, err := lvl.fromCheckpoint(filepath.Join(dir, host[0], host[1]), from, m)
 				if err != nil {
 					log.Fatalf("error while loading checkpoints: %s", err.Error())
-					atomic.AddInt32(&errs, int32(nn))
-					continue
+					atomic.AddInt32(&errs, 1)
 				}
 				atomic.AddInt32(&n, int32(nn))
 			}
@@ -436,26 +457,57 @@ func ArchiveCheckpoints(checkpointsDir, archiveDir string, from int64) (int, err
 		return 0, err
 	}
 
-	n := 0
+	type workItem struct {
+		cdir, adir    string
+		cluster, host string
+	}
+
+	var wg sync.WaitGroup
+	n, errs := int32(0), int32(0)
+	work := make(chan workItem, NumWorkers*2)
+
+	wg.Add(NumWorkers)
+	for worker := 0; worker < NumWorkers; worker++ {
+		go func() {
+			defer wg.Done()
+			for workItem := range work {
+				m, err := archiveCheckpoints(workItem.cdir, workItem.adir, from)
+				if err != nil {
+					log.Printf("error while archiving %s/%s: %s", workItem.cluster, workItem.host, err.Error())
+					atomic.AddInt32(&errs, 1)
+				}
+				atomic.AddInt32(&n, int32(m))
+			}
+		}()
+	}
+
 	for _, de1 := range entries1 {
-		entries2, err := os.ReadDir(filepath.Join(checkpointsDir, de1.Name()))
-		if err != nil {
-			return n, err
+		entries2, e := os.ReadDir(filepath.Join(checkpointsDir, de1.Name()))
+		if e != nil {
+			err = e
 		}
 
 		for _, de2 := range entries2 {
 			cdir := filepath.Join(checkpointsDir, de1.Name(), de2.Name())
 			adir := filepath.Join(archiveDir, de1.Name(), de2.Name())
-			m, err := archiveCheckpoints(cdir, adir, from)
-			n += m
-			if err != nil {
-				return n, err
+			work <- workItem{
+				adir: adir, cdir: cdir,
+				cluster: de1.Name(), host: de2.Name(),
 			}
-
 		}
 	}
 
-	return n, nil
+	close(work)
+	wg.Wait()
+
+	if err != nil {
+		return int(n), err
+	}
+
+	if errs > 0 {
+		return int(n), fmt.Errorf("%d errors happend while archiving (%d successes)", errs, n)
+	}
+	return int(n), nil
 }
 
 // Helper function for `ArchiveCheckpoints`.