volume: avoid file.stat(), file.seek() if possible during writes

weed/storage/backend/disk_file.go

@@ -12,6 +12,8 @@ var (
 type DiskFile struct {
 	File         *os.File
 	fullFilePath string
+	fileSize     int64
+	modTime      time.Time
 }
 
 func NewDiskFile(f *os.File) *DiskFile {
@@ -26,11 +28,24 @@ func (df *DiskFile) ReadAt(p []byte, off int64) (n int, err error) {
 }
 
 func (df *DiskFile) WriteAt(p []byte, off int64) (n int, err error) {
-	return df.File.WriteAt(p, off)
+	n, err = df.File.WriteAt(p, off)
+	if err == nil {
+		waterMark := off + int64(n)
+		if waterMark > df.fileSize {
+			df.fileSize = waterMark
+			df.modTime = time.Now()
+		}
+	}
+	return
 }
 
 func (df *DiskFile) Truncate(off int64) error {
-	return df.File.Truncate(off)
+	err := df.File.Truncate(off)
+	if err == nil {
+		df.fileSize = off
+		df.modTime = time.Now()
+	}
+	return err
 }
 
 func (df *DiskFile) Close() error {
@@ -38,6 +53,9 @@ func (df *DiskFile) Close() error {
 }
 
 func (df *DiskFile) GetStat() (datSize int64, modTime time.Time, err error) {
+	if df.fileSize != 0 {
+		return df.fileSize, df.modTime, nil
+	}
 	stat, e := df.File.Stat()
 	if e == nil {
 		return stat.Size(), stat.ModTime(), nil
@@ -50,5 +68,6 @@ func (df *DiskFile) Name() string {
 }
 
 func (df *DiskFile) Sync() error {
+	df.fileSize = 0
 	return df.File.Sync()
 }