avoid writing the same file

go/storage/needle_read_write.go

@@ -130,9 +130,11 @@ func (n *Needle) Read(r io.Reader, size uint32, version Version) (ret int, err e
 		n.readNeedleHeader(bytes)
 		n.Data = bytes[NeedleHeaderSize : NeedleHeaderSize+size]
 		checksum := util.BytesToUint32(bytes[NeedleHeaderSize+size : NeedleHeaderSize+size+NeedleChecksumSize])
-		if checksum != NewCRC(n.Data).Value() {
+		newChecksum := NewCRC(n.Data)
+		if checksum != newChecksum.Value() {
 			return 0, errors.New("CRC error! Data On Disk Corrupted!")
 		}
+		n.Checksum = newChecksum
 		return
 	case Version2:
 		if size == 0 {
@@ -151,9 +153,11 @@ func (n *Needle) Read(r io.Reader, size uint32, version Version) (ret int, err e
 		}
 		n.readNeedleDataVersion2(bytes[NeedleHeaderSize : NeedleHeaderSize+int(n.Size)])
 		checksum := util.BytesToUint32(bytes[NeedleHeaderSize+n.Size : NeedleHeaderSize+n.Size+NeedleChecksumSize])
-		if checksum != NewCRC(n.Data).Value() {
+		newChecksum := NewCRC(n.Data)
+		if checksum != newChecksum.Value() {
 			return 0, errors.New("CRC error! Data On Disk Corrupted!")
 		}
+		n.Checksum = newChecksum
 		return
 	}
 	return 0, fmt.Errorf("Unsupported Version! (%d)", version)

go/storage/volume.go

@@ -158,6 +158,27 @@ func (v *Volume) NeedToReplicate() bool {
 	return v.ReplicaType.GetCopyCount() > 1
 }
 
+func (v *Volume) isFileUnchanged(n *Needle) bool {
+	nv, ok := v.nm.Get(n.Id)
+	if ok && nv.Offset > 0 {
+		if _, err := v.dataFile.Seek(int64(nv.Offset)*NeedlePaddingSize, 0); err != nil {
+			return false
+		}
+		oldNeedle := new(Needle)
+		oldNeedle.Read(v.dataFile, nv.Size, v.Version())
+		if len(oldNeedle.Data) == len(n.Data) && oldNeedle.Checksum == n.Checksum {
+			length := len(n.Data)
+			for i := 0; i < length; i++ {
+				if n.Data[i] != oldNeedle.Data[i] {
+					return false
+				}
+			}
+			n.Size = oldNeedle.Size
+			return true
+		}
+	}
+	return false
+}
 func (v *Volume) write(n *Needle) (size uint32, err error) {
 	if v.readOnly {
 		err = fmt.Errorf("%s is read-only", v.dataFile)
@@ -165,6 +186,10 @@ func (v *Volume) write(n *Needle) (size uint32, err error) {
 	}
 	v.accessLock.Lock()
 	defer v.accessLock.Unlock()
+	if v.isFileUnchanged(n) {
+		size = n.Size
+		return
+	}
 	var offset int64
 	if offset, err = v.dataFile.Seek(0, 2); err != nil {
 		return