etcdserver/api/snap/db.go
func (s *Snapshotter) SaveDBFrom(r io.Reader, id uint64) (int64, error) {
	start := time.Now()

	f, err := ioutil.TempFile(s.dir, "tmp")
	if err != nil {
		return 0, err
	}
	var n int64
	n, err = io.Copy(f, r)
	if err == nil {
		fsyncStart := time.Now()
		err = fileutil.Fsync(f)
		snapDBFsyncSec.Observe(time.Since(fsyncStart).Seconds())
	}
	f.Close()
	if err != nil {
		os.Remove(f.Name())
		return n, err
	}
	fn := s.dbFilePath(id)
	if fileutil.Exist(fn) {
		os.Remove(f.Name())
		return n, nil
	}
	err = os.Rename(f.Name(), fn)
	if err != nil {
		os.Remove(f.Name())
		return n, err
	}

	if s.lg != nil {
		s.lg.Info(
			"saved database snapshot to disk",
			zap.String("path", fn),
			zap.Int64("bytes", n),
			zap.String("size", humanize.Bytes(uint64(n))),
		)
	} else {
		plog.Infof("saved database snapshot to disk [total bytes: %d]", n)
	}

	snapDBSaveSec.Observe(time.Since(start).Seconds())
	return n, nil
}
func (s *Snapshotter) SaveDBFrom(r io.Reader, id uint64) (int64, error) {
	start := time.Now()

	f, err := ioutil.TempFile(s.dir, "tmp")
	if err != nil {
		return 0, err
	}
	var n int64
	n, err = io.Copy(f, r)
	if err == nil {
		fsyncStart := time.Now()
		err = fileutil.Fsync(f)
		snapDBFsyncSec.Observe(time.Since(fsyncStart).Seconds())
	}
	f.Close()
	if err != nil {
		os.Remove(f.Name())
		return n, err
	}
	fn := s.dbFilePath(id)
	if fileutil.Exist(fn) {
		os.Remove(f.Name())
		return n, nil
	}
	err = os.Rename(f.Name(), fn)
	if err != nil {
		os.Remove(f.Name())
		return n, err
	}

	if s.lg != nil {
		s.lg.Info(
			"saved database snapshot to disk",
			zap.String("path", fn),
			zap.Int64("bytes", n),
			zap.String("size", humanize.Bytes(uint64(n))),
		)
	} else {
		plog.Infof("saved database snapshot to disk [total bytes: %d]", n)
	}

	snapDBSaveSec.Observe(time.Since(start).Seconds())
	return n, nil
}
// SaveDBFrom saves snapshot of the database from the given reader. It
// guarantees the save operation is atomic.