diff --git a/weed/command/backup.go b/weed/command/backup.go
index a6d660006..0f6bed225 100644
--- a/weed/command/backup.go
+++ b/weed/command/backup.go
@@ -121,7 +121,7 @@ func runBackup(cmd *Command, args []string) bool {
 	}
 
 	if v.SuperBlock.CompactionRevision < uint16(stats.CompactRevision) {
-		if err = v.Compact(0, 0); err != nil {
+		if err = v.Compact2(30 * 1024 * 1024 * 1024); err != nil {
 			fmt.Printf("Compact Volume before synchronizing %v\n", err)
 			return true
 		}
diff --git a/weed/command/compact.go b/weed/command/compact.go
index 4a54f5670..85313b749 100644
--- a/weed/command/compact.go
+++ b/weed/command/compact.go
@@ -17,6 +17,9 @@ var cmdCompact = &Command{
   The compacted .dat file is stored as .cpd file.
   The compacted .idx file is stored as .cpx file.
 
+  For method=0, it compacts based on the .dat file, works if .idx file is corrupted.
+  For method=1, it compacts based on the .idx file, works if deletion happened but not written to .dat files.
+
   `,
 }
 
@@ -47,7 +50,7 @@ func runCompact(cmd *Command, args []string) bool {
 			glog.Fatalf("Compact Volume [ERROR] %s\n", err)
 		}
 	} else {
-		if err = v.Compact2(); err != nil {
+		if err = v.Compact2(preallocate); err != nil {
 			glog.Fatalf("Compact Volume [ERROR] %s\n", err)
 		}
 	}
diff --git a/weed/storage/needle_map_memory.go b/weed/storage/needle_map_memory.go
index e4273f1b2..37dee7889 100644
--- a/weed/storage/needle_map_memory.go
+++ b/weed/storage/needle_map_memory.go
@@ -34,14 +34,12 @@ func doLoading(file *os.File, nm *NeedleMap) (*NeedleMap, error) {
 			nm.FileCounter++
 			nm.FileByteCounter = nm.FileByteCounter + uint64(size)
 			oldOffset, oldSize := nm.m.Set(NeedleId(key), offset, size)
-			// glog.V(3).Infoln("reading key", key, "offset", offset*NeedlePaddingSize, "size", size, "oldSize", oldSize)
 			if !oldOffset.IsZero() && oldSize != TombstoneFileSize {
 				nm.DeletionCounter++
 				nm.DeletionByteCounter = nm.DeletionByteCounter + uint64(oldSize)
 			}
 		} else {
 			oldSize := nm.m.Delete(NeedleId(key))
-			// glog.V(3).Infoln("removing key", key, "offset", offset*NeedlePaddingSize, "size", size, "oldSize", oldSize)
 			nm.DeletionCounter++
 			nm.DeletionByteCounter = nm.DeletionByteCounter + uint64(oldSize)
 		}
diff --git a/weed/storage/needle_map_metric_test.go b/weed/storage/needle_map_metric_test.go
new file mode 100644
index 000000000..ae2177a30
--- /dev/null
+++ b/weed/storage/needle_map_metric_test.go
@@ -0,0 +1,31 @@
+package storage
+
+import (
+	"io/ioutil"
+	"math/rand"
+	"testing"
+
+	"github.com/chrislusf/seaweedfs/weed/glog"
+	. "github.com/chrislusf/seaweedfs/weed/storage/types"
+)
+
+func TestFastLoadingNeedleMapMetrics(t *testing.T) {
+
+	idxFile, _ := ioutil.TempFile("", "tmp.idx")
+	nm := NewCompactNeedleMap(idxFile)
+
+	for i := 0; i < 10000; i++ {
+		nm.Put(Uint64ToNeedleId(uint64(i+1)), Uint32ToOffset(uint32(0)), uint32(1))
+		if rand.Float32() < 0.2 {
+			nm.Delete(Uint64ToNeedleId(uint64(rand.Int63n(int64(i))+1)), Uint32ToOffset(uint32(0)))
+		}
+	}
+
+	mm, _ := newNeedleMapMetricFromIndexFile(idxFile)
+
+	glog.V(0).Infof("FileCount expected %d actual %d", nm.FileCount(), mm.FileCount())
+	glog.V(0).Infof("DeletedSize expected %d actual %d", nm.DeletedSize(), mm.DeletedSize())
+	glog.V(0).Infof("ContentSize expected %d actual %d", nm.ContentSize(), mm.ContentSize())
+	glog.V(0).Infof("DeletedCount expected %d actual %d", nm.DeletedCount(), mm.DeletedCount())
+	glog.V(0).Infof("MaxFileKey expected %d actual %d", nm.MaxFileKey(), mm.MaxFileKey())
+}
diff --git a/weed/storage/store_vacuum.go b/weed/storage/store_vacuum.go
index b1f1a6277..5dacb71bf 100644
--- a/weed/storage/store_vacuum.go
+++ b/weed/storage/store_vacuum.go
@@ -16,7 +16,7 @@ func (s *Store) CheckCompactVolume(volumeId needle.VolumeId) (float64, error) {
 }
 func (s *Store) CompactVolume(vid needle.VolumeId, preallocate int64, compactionBytePerSecond int64) error {
 	if v := s.findVolume(vid); v != nil {
-		return v.Compact(preallocate, compactionBytePerSecond)
+		return v.Compact2(preallocate) // compactionBytePerSecond
 	}
 	return fmt.Errorf("volume id %d is not found during compact", vid)
 }
diff --git a/weed/storage/volume_loading.go b/weed/storage/volume_loading.go
index d87a58302..a09939447 100644
--- a/weed/storage/volume_loading.go
+++ b/weed/storage/volume_loading.go
@@ -75,7 +75,7 @@ func (v *Volume) load(alsoLoadIndex bool, createDatIfMissing bool, needleMapKind
 	if err == nil && alsoLoadIndex {
 		var indexFile *os.File
 		if v.noWriteOrDelete {
-			glog.V(1).Infoln("open to read file", fileName+".idx")
+			glog.V(0).Infoln("open to read file", fileName+".idx")
 			if indexFile, err = os.OpenFile(fileName+".idx", os.O_RDONLY, 0644); err != nil {
 				return fmt.Errorf("cannot read Volume Index %s.idx: %v", fileName, err)
 			}
diff --git a/weed/storage/volume_vacuum.go b/weed/storage/volume_vacuum.go
index 704f1f4ef..434b5989d 100644
--- a/weed/storage/volume_vacuum.go
+++ b/weed/storage/volume_vacuum.go
@@ -20,9 +20,19 @@ func (v *Volume) garbageLevel() float64 {
 	if v.ContentSize() == 0 {
 		return 0
 	}
-	return float64(v.DeletedSize()) / float64(v.ContentSize())
+	deletedSize := v.DeletedSize()
+	fileSize := v.ContentSize()
+	if v.DeletedCount() > 0 && v.DeletedSize() == 0 {
+		// this happens for .sdx converted back to normal .idx
+		// where deleted entry size is missing
+		datFileSize, _, _ := v.FileStat()
+		deletedSize = datFileSize - fileSize - super_block.SuperBlockSize
+		fileSize = datFileSize
+	}
+	return float64(deletedSize) / float64(fileSize)
 }
 
+// compact a volume based on deletions in .dat files
 func (v *Volume) Compact(preallocate int64, compactionBytePerSecond int64) error {
 
 	if v.MemoryMapMaxSizeMb != 0 { //it makes no sense to compact in memory
@@ -45,7 +55,8 @@ func (v *Volume) Compact(preallocate int64, compactionBytePerSecond int64) error
 	return v.copyDataAndGenerateIndexFile(filePath+".cpd", filePath+".cpx", preallocate, compactionBytePerSecond)
 }
 
-func (v *Volume) Compact2() error {
+// compact a volume based on deletions in .idx files
+func (v *Volume) Compact2(preallocate int64) error {
 
 	if v.MemoryMapMaxSizeMb != 0 { //it makes no sense to compact in memory
 		return nil
@@ -58,8 +69,10 @@ func (v *Volume) Compact2() error {
 	}()
 
 	filePath := v.FileName()
+	v.lastCompactIndexOffset = v.IndexFileSize()
+	v.lastCompactRevision = v.SuperBlock.CompactionRevision
 	glog.V(3).Infof("creating copies for volume %d ...", v.Id)
-	return v.copyDataBasedOnIndexFile(filePath+".cpd", filePath+".cpx")
+	return v.copyDataBasedOnIndexFile(filePath+".cpd", filePath+".cpx", preallocate)
 }
 
 func (v *Volume) CommitCompact() error {
@@ -140,6 +153,7 @@ func fetchCompactRevisionFromDatFile(datBackend backend.BackendStorageFile) (com
 	return superBlock.CompactionRevision, nil
 }
 
+// if old .dat and .idx files are updated, this func tries to apply the same changes to new files accordingly
 func (v *Volume) makeupDiff(newDatFileName, newIdxFileName, oldDatFileName, oldIdxFileName string) (err error) {
 	var indexSize int64
 
@@ -150,6 +164,7 @@ func (v *Volume) makeupDiff(newDatFileName, newIdxFileName, oldDatFileName, oldI
 	oldDatBackend := backend.NewDiskFile(oldDatFile)
 	defer oldDatBackend.Close()
 
+	// skip if the old .idx file has not changed
 	if indexSize, err = verifyIndexFileIntegrity(oldIdxFile); err != nil {
 		return fmt.Errorf("verifyIndexFileIntegrity %s failed: %v", oldIdxFileName, err)
 	}
@@ -157,6 +172,7 @@ func (v *Volume) makeupDiff(newDatFileName, newIdxFileName, oldDatFileName, oldI
 		return nil
 	}
 
+	// fail if the old .dat file has changed to a new revision
 	oldDatCompactRevision, err := fetchCompactRevisionFromDatFile(oldDatBackend)
 	if err != nil {
 		return fmt.Errorf("fetchCompactRevisionFromDatFile src %s failed: %v", oldDatFile.Name(), err)
@@ -337,14 +353,14 @@ func (v *Volume) copyDataAndGenerateIndexFile(dstName, idxName string, prealloca
 	return
 }
 
-func (v *Volume) copyDataBasedOnIndexFile(dstName, idxName string) (err error) {
+func (v *Volume) copyDataBasedOnIndexFile(dstName, idxName string, preallocate int64) (err error) {
 	var (
-		dst, oldIndexFile *os.File
+		dstDatBackend backend.BackendStorageFile
+		oldIndexFile  *os.File
 	)
-	if dst, err = os.OpenFile(dstName, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0644); err != nil {
+	if dstDatBackend, err = createVolumeFile(dstName, preallocate, 0); err != nil {
 		return
 	}
-	dstDatBackend := backend.NewDiskFile(dst)
 	defer dstDatBackend.Close()
 
 	if oldIndexFile, err = os.OpenFile(v.FileName()+".idx", os.O_RDONLY, 0644); err != nil {
@@ -357,7 +373,7 @@ func (v *Volume) copyDataBasedOnIndexFile(dstName, idxName string) (err error) {
 	now := uint64(time.Now().Unix())
 
 	v.SuperBlock.CompactionRevision++
-	dst.Write(v.SuperBlock.Bytes())
+	dstDatBackend.WriteAt(v.SuperBlock.Bytes(), 0)
 	newOffset := int64(v.SuperBlock.BlockSize())
 
 	idx2.WalkIndexFile(oldIndexFile, func(key NeedleId, offset Offset, size uint32) error {
diff --git a/weed/storage/volume_vacuum_test.go b/weed/storage/volume_vacuum_test.go
index 07b9f70c1..95f43d6ec 100644
--- a/weed/storage/volume_vacuum_test.go
+++ b/weed/storage/volume_vacuum_test.go
@@ -84,7 +84,7 @@ func TestCompaction(t *testing.T) {
 	}
 
 	startTime := time.Now()
-	v.Compact(0, 1024*1024)
+	v.Compact2(0)
 	speed := float64(v.ContentSize()) / time.Now().Sub(startTime).Seconds()
 	t.Logf("compaction speed: %.2f bytes/s", speed)