etcdctl/ctlv2/command/backup_command.go
func saveDB(destDB, srcDB string, idx uint64, v3 bool) {
	// open src db to safely copy db state
	if v3 {
		var src *bolt.DB
		ch := make(chan *bolt.DB, 1)
		go func() {
			db, err := bolt.Open(srcDB, 0444, &bolt.Options{ReadOnly: true})
			if err != nil {
				log.Fatal(err)
			}
			ch <- db
		}()
		select {
		case src = <-ch:
		case <-time.After(time.Second):
			log.Println("waiting to acquire lock on", srcDB)
			src = <-ch
		}
		defer src.Close()

		tx, err := src.Begin(false)
		if err != nil {
			log.Fatal(err)
		}

		// copy srcDB to destDB
		dest, err := os.Create(destDB)
		if err != nil {
			log.Fatal(err)
		}
		if _, err := tx.WriteTo(dest); err != nil {
			log.Fatal(err)
		}
		dest.Close()
		if err := tx.Rollback(); err != nil {
			log.Fatal(err)
		}
	}

	db, err := bolt.Open(destDB, 0644, &bolt.Options{})
	if err != nil {
		log.Fatal(err)
	}
	tx, err := db.Begin(true)
	if err != nil {
		log.Fatal(err)
	}

	// remove membership information; should be clobbered by --force-new-cluster
	for _, bucket := range []string{"members", "members_removed", "cluster"} {
		tx.DeleteBucket([]byte(bucket))
	}

	// update consistent index to match hard state
	if !v3 {
		idxBytes := make([]byte, 8)
		binary.BigEndian.PutUint64(idxBytes, idx)
		b, err := tx.CreateBucketIfNotExists([]byte("meta"))
		if err != nil {
			log.Fatal(err)
		}
		b.Put([]byte("consistent_index"), idxBytes)
	}

	if err := tx.Commit(); err != nil {
		log.Fatal(err)
	}
	if err := db.Close(); err != nil {
		log.Fatal(err)
	}
}
func saveDB(destDB, srcDB string, idx uint64, v3 bool) {
	// open src db to safely copy db state
	if v3 {
		var src *bolt.DB
		ch := make(chan *bolt.DB, 1)
		go func() {
			db, err := bolt.Open(srcDB, 0444, &bolt.Options{ReadOnly: true})
			if err != nil {
				log.Fatal(err)
			}
			ch <- db
		}()
		select {
		case src = <-ch:
		case <-time.After(time.Second):
			log.Println("waiting to acquire lock on", srcDB)
			src = <-ch
		}
		defer src.Close()

		tx, err := src.Begin(false)
		if err != nil {
			log.Fatal(err)
		}

		// copy srcDB to destDB
		dest, err := os.Create(destDB)
		if err != nil {
			log.Fatal(err)
		}
		if _, err := tx.WriteTo(dest); err != nil {
			log.Fatal(err)
		}
		dest.Close()
		if err := tx.Rollback(); err != nil {
			log.Fatal(err)
		}
	}

	db, err := bolt.Open(destDB, 0644, &bolt.Options{})
	if err != nil {
		log.Fatal(err)
	}
	tx, err := db.Begin(true)
	if err != nil {
		log.Fatal(err)
	}

	// remove membership information; should be clobbered by --force-new-cluster
	for _, bucket := range []string{"members", "members_removed", "cluster"} {
		tx.DeleteBucket([]byte(bucket))
	}

	// update consistent index to match hard state
	if !v3 {
		idxBytes := make([]byte, 8)
		binary.BigEndian.PutUint64(idxBytes, idx)
		b, err := tx.CreateBucketIfNotExists([]byte("meta"))
		if err != nil {
			log.Fatal(err)
		}
		b.Put([]byte("consistent_index"), idxBytes)
	}

	if err := tx.Commit(); err != nil {
		log.Fatal(err)
	}
	if err := db.Close(); err != nil {
		log.Fatal(err)
	}
}
// saveDB copies the v3 backend and strips cluster information.