diff --git a/vendor/go.etcd.io/etcd/mvcc/backend/backend.go b/vendor/go.etcd.io/etcd/mvcc/backend/backend.go
new file mode 100644
index 0000000..bffd749
--- /dev/null
+++ b/vendor/go.etcd.io/etcd/mvcc/backend/backend.go
@@ -0,0 +1,570 @@
+// Copyright 2015 The etcd Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package backend
+
+import (
+	"fmt"
+	"hash/crc32"
+	"io"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"sync"
+	"sync/atomic"
+	"time"
+
+	"github.com/coreos/pkg/capnslog"
+	humanize "github.com/dustin/go-humanize"
+	bolt "go.etcd.io/bbolt"
+	"go.uber.org/zap"
+)
+
+var (
+	defaultBatchLimit    = 10000
+	defaultBatchInterval = 100 * time.Millisecond
+
+	defragLimit = 10000
+
+	// initialMmapSize is the initial size of the mmapped region. Setting this larger than
+	// the potential max db size can prevent writer from blocking reader.
+	// This only works for linux.
+	initialMmapSize = uint64(10 * 1024 * 1024 * 1024)
+
+	plog = capnslog.NewPackageLogger("go.etcd.io/etcd", "mvcc/backend")
+
+	// minSnapshotWarningTimeout is the minimum threshold to trigger a long running snapshot warning.
+	minSnapshotWarningTimeout = 30 * time.Second
+)
+
+type Backend interface {
+	// ReadTx returns a read transaction. It is replaced by ConcurrentReadTx in the main data path, see #10523.
+	ReadTx() ReadTx
+	BatchTx() BatchTx
+	// ConcurrentReadTx returns a non-blocking read transaction.
+	ConcurrentReadTx() ReadTx
+
+	Snapshot() Snapshot
+	Hash(ignores map[IgnoreKey]struct{}) (uint32, error)
+	// Size returns the current size of the backend physically allocated.
+	// The backend can hold DB space that is not utilized at the moment,
+	// since it can conduct pre-allocation or spare unused space for recycling.
+	// Use SizeInUse() instead for the actual DB size.
+	Size() int64
+	// SizeInUse returns the current size of the backend logically in use.
+	// Since the backend can manage free space in a non-byte unit such as
+	// number of pages, the returned value can be not exactly accurate in bytes.
+	SizeInUse() int64
+	// OpenReadTxN returns the number of currently open read transactions in the backend.
+	OpenReadTxN() int64
+	Defrag() error
+	ForceCommit()
+	Close() error
+}
+
+type Snapshot interface {
+	// Size gets the size of the snapshot.
+	Size() int64
+	// WriteTo writes the snapshot into the given writer.
+	WriteTo(w io.Writer) (n int64, err error)
+	// Close closes the snapshot.
+	Close() error
+}
+
+type backend struct {
+	// size and commits are used with atomic operations so they must be
+	// 64-bit aligned, otherwise 32-bit tests will crash
+
+	// size is the number of bytes allocated in the backend
+	size int64
+	// sizeInUse is the number of bytes actually used in the backend
+	sizeInUse int64
+	// commits counts number of commits since start
+	commits int64
+	// openReadTxN is the number of currently open read transactions in the backend
+	openReadTxN int64
+
+	mu sync.RWMutex
+	db *bolt.DB
+
+	batchInterval time.Duration
+	batchLimit    int
+	batchTx       *batchTxBuffered
+
+	readTx *readTx
+
+	stopc chan struct{}
+	donec chan struct{}
+
+	lg *zap.Logger
+}
+
+type BackendConfig struct {
+	// Path is the file path to the backend file.
+	Path string
+	// BatchInterval is the maximum time before flushing the BatchTx.
+	BatchInterval time.Duration
+	// BatchLimit is the maximum puts before flushing the BatchTx.
+	BatchLimit int
+	// BackendFreelistType is the backend boltdb's freelist type.
+	BackendFreelistType bolt.FreelistType
+	// MmapSize is the number of bytes to mmap for the backend.
+	MmapSize uint64
+	// Logger logs backend-side operations.
+	Logger *zap.Logger
+}
+
+func DefaultBackendConfig() BackendConfig {
+	return BackendConfig{
+		BatchInterval: defaultBatchInterval,
+		BatchLimit:    defaultBatchLimit,
+		MmapSize:      initialMmapSize,
+	}
+}
+
+func New(bcfg BackendConfig) Backend {
+	return newBackend(bcfg)
+}
+
+func NewDefaultBackend(path string) Backend {
+	bcfg := DefaultBackendConfig()
+	bcfg.Path = path
+	return newBackend(bcfg)
+}
+
+func newBackend(bcfg BackendConfig) *backend {
+	bopts := &bolt.Options{}
+	if boltOpenOptions != nil {
+		*bopts = *boltOpenOptions
+	}
+	bopts.InitialMmapSize = bcfg.mmapSize()
+	bopts.FreelistType = bcfg.BackendFreelistType
+
+	db, err := bolt.Open(bcfg.Path, 0600, bopts)
+	if err != nil {
+		if bcfg.Logger != nil {
+			bcfg.Logger.Panic("failed to open database", zap.String("path", bcfg.Path), zap.Error(err))
+		} else {
+			plog.Panicf("cannot open database at %s (%v)", bcfg.Path, err)
+		}
+	}
+
+	// In future, may want to make buffering optional for low-concurrency systems
+	// or dynamically swap between buffered/non-buffered depending on workload.
+	b := &backend{
+		db: db,
+
+		batchInterval: bcfg.BatchInterval,
+		batchLimit:    bcfg.BatchLimit,
+
+		readTx: &readTx{
+			buf: txReadBuffer{
+				txBuffer: txBuffer{make(map[string]*bucketBuffer)},
+			},
+			buckets: make(map[string]*bolt.Bucket),
+			txWg:    new(sync.WaitGroup),
+		},
+
+		stopc: make(chan struct{}),
+		donec: make(chan struct{}),
+
+		lg: bcfg.Logger,
+	}
+	b.batchTx = newBatchTxBuffered(b)
+	go b.run()
+	return b
+}
+
+// BatchTx returns the current batch tx in coalescer. The tx can be used for read and
+// write operations. The write result can be retrieved within the same tx immediately.
+// The write result is isolated with other txs until the current one get committed.
+func (b *backend) BatchTx() BatchTx {
+	return b.batchTx
+}
+
+func (b *backend) ReadTx() ReadTx { return b.readTx }
+
+// ConcurrentReadTx creates and returns a new ReadTx, which:
+// A) creates and keeps a copy of backend.readTx.txReadBuffer,
+// B) references the boltdb read Tx (and its bucket cache) of current batch interval.
+func (b *backend) ConcurrentReadTx() ReadTx {
+	b.readTx.RLock()
+	defer b.readTx.RUnlock()
+	// prevent boltdb read Tx from been rolled back until store read Tx is done. Needs to be called when holding readTx.RLock().
+	b.readTx.txWg.Add(1)
+	// TODO: might want to copy the read buffer lazily - create copy when A) end of a write transaction B) end of a batch interval.
+	return &concurrentReadTx{
+		buf:     b.readTx.buf.unsafeCopy(),
+		tx:      b.readTx.tx,
+		txMu:    &b.readTx.txMu,
+		buckets: b.readTx.buckets,
+		txWg:    b.readTx.txWg,
+	}
+}
+
+// ForceCommit forces the current batching tx to commit.
+func (b *backend) ForceCommit() {
+	b.batchTx.Commit()
+}
+
+func (b *backend) Snapshot() Snapshot {
+	b.batchTx.Commit()
+
+	b.mu.RLock()
+	defer b.mu.RUnlock()
+	tx, err := b.db.Begin(false)
+	if err != nil {
+		if b.lg != nil {
+			b.lg.Fatal("failed to begin tx", zap.Error(err))
+		} else {
+			plog.Fatalf("cannot begin tx (%s)", err)
+		}
+	}
+
+	stopc, donec := make(chan struct{}), make(chan struct{})
+	dbBytes := tx.Size()
+	go func() {
+		defer close(donec)
+		// sendRateBytes is based on transferring snapshot data over a 1 gigabit/s connection
+		// assuming a min tcp throughput of 100MB/s.
+		var sendRateBytes int64 = 100 * 1024 * 1014
+		warningTimeout := time.Duration(int64((float64(dbBytes) / float64(sendRateBytes)) * float64(time.Second)))
+		if warningTimeout < minSnapshotWarningTimeout {
+			warningTimeout = minSnapshotWarningTimeout
+		}
+		start := time.Now()
+		ticker := time.NewTicker(warningTimeout)
+		defer ticker.Stop()
+		for {
+			select {
+			case <-ticker.C:
+				if b.lg != nil {
+					b.lg.Warn(
+						"snapshotting taking too long to transfer",
+						zap.Duration("taking", time.Since(start)),
+						zap.Int64("bytes", dbBytes),
+						zap.String("size", humanize.Bytes(uint64(dbBytes))),
+					)
+				} else {
+					plog.Warningf("snapshotting is taking more than %v seconds to finish transferring %v MB [started at %v]", time.Since(start).Seconds(), float64(dbBytes)/float64(1024*1014), start)
+				}
+
+			case <-stopc:
+				snapshotTransferSec.Observe(time.Since(start).Seconds())
+				return
+			}
+		}
+	}()
+
+	return &snapshot{tx, stopc, donec}
+}
+
+type IgnoreKey struct {
+	Bucket string
+	Key    string
+}
+
+func (b *backend) Hash(ignores map[IgnoreKey]struct{}) (uint32, error) {
+	h := crc32.New(crc32.MakeTable(crc32.Castagnoli))
+
+	b.mu.RLock()
+	defer b.mu.RUnlock()
+	err := b.db.View(func(tx *bolt.Tx) error {
+		c := tx.Cursor()
+		for next, _ := c.First(); next != nil; next, _ = c.Next() {
+			b := tx.Bucket(next)
+			if b == nil {
+				return fmt.Errorf("cannot get hash of bucket %s", string(next))
+			}
+			h.Write(next)
+			b.ForEach(func(k, v []byte) error {
+				bk := IgnoreKey{Bucket: string(next), Key: string(k)}
+				if _, ok := ignores[bk]; !ok {
+					h.Write(k)
+					h.Write(v)
+				}
+				return nil
+			})
+		}
+		return nil
+	})
+
+	if err != nil {
+		return 0, err
+	}
+
+	return h.Sum32(), nil
+}
+
+func (b *backend) Size() int64 {
+	return atomic.LoadInt64(&b.size)
+}
+
+func (b *backend) SizeInUse() int64 {
+	return atomic.LoadInt64(&b.sizeInUse)
+}
+
+func (b *backend) run() {
+	defer close(b.donec)
+	t := time.NewTimer(b.batchInterval)
+	defer t.Stop()
+	for {
+		select {
+		case <-t.C:
+		case <-b.stopc:
+			b.batchTx.CommitAndStop()
+			return
+		}
+		if b.batchTx.safePending() != 0 {
+			b.batchTx.Commit()
+		}
+		t.Reset(b.batchInterval)
+	}
+}
+
+func (b *backend) Close() error {
+	close(b.stopc)
+	<-b.donec
+	return b.db.Close()
+}
+
+// Commits returns total number of commits since start
+func (b *backend) Commits() int64 {
+	return atomic.LoadInt64(&b.commits)
+}
+
+func (b *backend) Defrag() error {
+	return b.defrag()
+}
+
+func (b *backend) defrag() error {
+	now := time.Now()
+
+	// TODO: make this non-blocking?
+	// lock batchTx to ensure nobody is using previous tx, and then
+	// close previous ongoing tx.
+	b.batchTx.Lock()
+	defer b.batchTx.Unlock()
+
+	// lock database after lock tx to avoid deadlock.
+	b.mu.Lock()
+	defer b.mu.Unlock()
+
+	// block concurrent read requests while resetting tx
+	b.readTx.Lock()
+	defer b.readTx.Unlock()
+
+	b.batchTx.unsafeCommit(true)
+
+	b.batchTx.tx = nil
+
+	tmpdb, err := bolt.Open(b.db.Path()+".tmp", 0600, boltOpenOptions)
+	if err != nil {
+		return err
+	}
+
+	dbp := b.db.Path()
+	tdbp := tmpdb.Path()
+	size1, sizeInUse1 := b.Size(), b.SizeInUse()
+	if b.lg != nil {
+		b.lg.Info(
+			"defragmenting",
+			zap.String("path", dbp),
+			zap.Int64("current-db-size-bytes", size1),
+			zap.String("current-db-size", humanize.Bytes(uint64(size1))),
+			zap.Int64("current-db-size-in-use-bytes", sizeInUse1),
+			zap.String("current-db-size-in-use", humanize.Bytes(uint64(sizeInUse1))),
+		)
+	}
+
+	err = defragdb(b.db, tmpdb, defragLimit)
+	if err != nil {
+		tmpdb.Close()
+		os.RemoveAll(tmpdb.Path())
+		return err
+	}
+
+	err = b.db.Close()
+	if err != nil {
+		if b.lg != nil {
+			b.lg.Fatal("failed to close database", zap.Error(err))
+		} else {
+			plog.Fatalf("cannot close database (%s)", err)
+		}
+	}
+	err = tmpdb.Close()
+	if err != nil {
+		if b.lg != nil {
+			b.lg.Fatal("failed to close tmp database", zap.Error(err))
+		} else {
+			plog.Fatalf("cannot close database (%s)", err)
+		}
+	}
+	err = os.Rename(tdbp, dbp)
+	if err != nil {
+		if b.lg != nil {
+			b.lg.Fatal("failed to rename tmp database", zap.Error(err))
+		} else {
+			plog.Fatalf("cannot rename database (%s)", err)
+		}
+	}
+
+	b.db, err = bolt.Open(dbp, 0600, boltOpenOptions)
+	if err != nil {
+		if b.lg != nil {
+			b.lg.Fatal("failed to open database", zap.String("path", dbp), zap.Error(err))
+		} else {
+			plog.Panicf("cannot open database at %s (%v)", dbp, err)
+		}
+	}
+	b.batchTx.tx = b.unsafeBegin(true)
+
+	b.readTx.reset()
+	b.readTx.tx = b.unsafeBegin(false)
+
+	size := b.readTx.tx.Size()
+	db := b.readTx.tx.DB()
+	atomic.StoreInt64(&b.size, size)
+	atomic.StoreInt64(&b.sizeInUse, size-(int64(db.Stats().FreePageN)*int64(db.Info().PageSize)))
+
+	took := time.Since(now)
+	defragSec.Observe(took.Seconds())
+
+	size2, sizeInUse2 := b.Size(), b.SizeInUse()
+	if b.lg != nil {
+		b.lg.Info(
+			"defragmented",
+			zap.String("path", dbp),
+			zap.Int64("current-db-size-bytes-diff", size2-size1),
+			zap.Int64("current-db-size-bytes", size2),
+			zap.String("current-db-size", humanize.Bytes(uint64(size2))),
+			zap.Int64("current-db-size-in-use-bytes-diff", sizeInUse2-sizeInUse1),
+			zap.Int64("current-db-size-in-use-bytes", sizeInUse2),
+			zap.String("current-db-size-in-use", humanize.Bytes(uint64(sizeInUse2))),
+			zap.Duration("took", took),
+		)
+	}
+	return nil
+}
+
+func defragdb(odb, tmpdb *bolt.DB, limit int) error {
+	// open a tx on tmpdb for writes
+	tmptx, err := tmpdb.Begin(true)
+	if err != nil {
+		return err
+	}
+
+	// open a tx on old db for read
+	tx, err := odb.Begin(false)
+	if err != nil {
+		return err
+	}
+	defer tx.Rollback()
+
+	c := tx.Cursor()
+
+	count := 0
+	for next, _ := c.First(); next != nil; next, _ = c.Next() {
+		b := tx.Bucket(next)
+		if b == nil {
+			return fmt.Errorf("backend: cannot defrag bucket %s", string(next))
+		}
+
+		tmpb, berr := tmptx.CreateBucketIfNotExists(next)
+		if berr != nil {
+			return berr
+		}
+		tmpb.FillPercent = 0.9 // for seq write in for each
+
+		b.ForEach(func(k, v []byte) error {
+			count++
+			if count > limit {
+				err = tmptx.Commit()
+				if err != nil {
+					return err
+				}
+				tmptx, err = tmpdb.Begin(true)
+				if err != nil {
+					return err
+				}
+				tmpb = tmptx.Bucket(next)
+				tmpb.FillPercent = 0.9 // for seq write in for each
+
+				count = 0
+			}
+			return tmpb.Put(k, v)
+		})
+	}
+
+	return tmptx.Commit()
+}
+
+func (b *backend) begin(write bool) *bolt.Tx {
+	b.mu.RLock()
+	tx := b.unsafeBegin(write)
+	b.mu.RUnlock()
+
+	size := tx.Size()
+	db := tx.DB()
+	stats := db.Stats()
+	atomic.StoreInt64(&b.size, size)
+	atomic.StoreInt64(&b.sizeInUse, size-(int64(stats.FreePageN)*int64(db.Info().PageSize)))
+	atomic.StoreInt64(&b.openReadTxN, int64(stats.OpenTxN))
+
+	return tx
+}
+
+func (b *backend) unsafeBegin(write bool) *bolt.Tx {
+	tx, err := b.db.Begin(write)
+	if err != nil {
+		if b.lg != nil {
+			b.lg.Fatal("failed to begin tx", zap.Error(err))
+		} else {
+			plog.Fatalf("cannot begin tx (%s)", err)
+		}
+	}
+	return tx
+}
+
+func (b *backend) OpenReadTxN() int64 {
+	return atomic.LoadInt64(&b.openReadTxN)
+}
+
+// NewTmpBackend creates a backend implementation for testing.
+func NewTmpBackend(batchInterval time.Duration, batchLimit int) (*backend, string) {
+	dir, err := ioutil.TempDir(os.TempDir(), "etcd_backend_test")
+	if err != nil {
+		panic(err)
+	}
+	tmpPath := filepath.Join(dir, "database")
+	bcfg := DefaultBackendConfig()
+	bcfg.Path, bcfg.BatchInterval, bcfg.BatchLimit = tmpPath, batchInterval, batchLimit
+	return newBackend(bcfg), tmpPath
+}
+
+func NewDefaultTmpBackend() (*backend, string) {
+	return NewTmpBackend(defaultBatchInterval, defaultBatchLimit)
+}
+
+type snapshot struct {
+	*bolt.Tx
+	stopc chan struct{}
+	donec chan struct{}
+}
+
+func (s *snapshot) Close() error {
+	close(s.stopc)
+	<-s.donec
+	return s.Tx.Rollback()
+}
diff --git a/vendor/go.etcd.io/etcd/mvcc/backend/batch_tx.go b/vendor/go.etcd.io/etcd/mvcc/backend/batch_tx.go
new file mode 100644
index 0000000..d5c8a88
--- /dev/null
+++ b/vendor/go.etcd.io/etcd/mvcc/backend/batch_tx.go
@@ -0,0 +1,339 @@
+// Copyright 2015 The etcd Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package backend
+
+import (
+	"bytes"
+	"math"
+	"sync"
+	"sync/atomic"
+	"time"
+
+	bolt "go.etcd.io/bbolt"
+	"go.uber.org/zap"
+)
+
+type BatchTx interface {
+	ReadTx
+	UnsafeCreateBucket(name []byte)
+	UnsafePut(bucketName []byte, key []byte, value []byte)
+	UnsafeSeqPut(bucketName []byte, key []byte, value []byte)
+	UnsafeDelete(bucketName []byte, key []byte)
+	// Commit commits a previous tx and begins a new writable one.
+	Commit()
+	// CommitAndStop commits the previous tx and does not create a new one.
+	CommitAndStop()
+}
+
+type batchTx struct {
+	sync.Mutex
+	tx      *bolt.Tx
+	backend *backend
+
+	pending int
+}
+
+func (t *batchTx) Lock() {
+	t.Mutex.Lock()
+}
+
+func (t *batchTx) Unlock() {
+	if t.pending >= t.backend.batchLimit {
+		t.commit(false)
+	}
+	t.Mutex.Unlock()
+}
+
+// BatchTx interface embeds ReadTx interface. But RLock() and RUnlock() do not
+// have appropriate semantics in BatchTx interface. Therefore should not be called.
+// TODO: might want to decouple ReadTx and BatchTx
+
+func (t *batchTx) RLock() {
+	panic("unexpected RLock")
+}
+
+func (t *batchTx) RUnlock() {
+	panic("unexpected RUnlock")
+}
+
+func (t *batchTx) UnsafeCreateBucket(name []byte) {
+	_, err := t.tx.CreateBucket(name)
+	if err != nil && err != bolt.ErrBucketExists {
+		if t.backend.lg != nil {
+			t.backend.lg.Fatal(
+				"failed to create a bucket",
+				zap.String("bucket-name", string(name)),
+				zap.Error(err),
+			)
+		} else {
+			plog.Fatalf("cannot create bucket %s (%v)", name, err)
+		}
+	}
+	t.pending++
+}
+
+// UnsafePut must be called holding the lock on the tx.
+func (t *batchTx) UnsafePut(bucketName []byte, key []byte, value []byte) {
+	t.unsafePut(bucketName, key, value, false)
+}
+
+// UnsafeSeqPut must be called holding the lock on the tx.
+func (t *batchTx) UnsafeSeqPut(bucketName []byte, key []byte, value []byte) {
+	t.unsafePut(bucketName, key, value, true)
+}
+
+func (t *batchTx) unsafePut(bucketName []byte, key []byte, value []byte, seq bool) {
+	bucket := t.tx.Bucket(bucketName)
+	if bucket == nil {
+		if t.backend.lg != nil {
+			t.backend.lg.Fatal(
+				"failed to find a bucket",
+				zap.String("bucket-name", string(bucketName)),
+			)
+		} else {
+			plog.Fatalf("bucket %s does not exist", bucketName)
+		}
+	}
+	if seq {
+		// it is useful to increase fill percent when the workloads are mostly append-only.
+		// this can delay the page split and reduce space usage.
+		bucket.FillPercent = 0.9
+	}
+	if err := bucket.Put(key, value); err != nil {
+		if t.backend.lg != nil {
+			t.backend.lg.Fatal(
+				"failed to write to a bucket",
+				zap.String("bucket-name", string(bucketName)),
+				zap.Error(err),
+			)
+		} else {
+			plog.Fatalf("cannot put key into bucket (%v)", err)
+		}
+	}
+	t.pending++
+}
+
+// UnsafeRange must be called holding the lock on the tx.
+func (t *batchTx) UnsafeRange(bucketName, key, endKey []byte, limit int64) ([][]byte, [][]byte) {
+	bucket := t.tx.Bucket(bucketName)
+	if bucket == nil {
+		if t.backend.lg != nil {
+			t.backend.lg.Fatal(
+				"failed to find a bucket",
+				zap.String("bucket-name", string(bucketName)),
+			)
+		} else {
+			plog.Fatalf("bucket %s does not exist", bucketName)
+		}
+	}
+	return unsafeRange(bucket.Cursor(), key, endKey, limit)
+}
+
+func unsafeRange(c *bolt.Cursor, key, endKey []byte, limit int64) (keys [][]byte, vs [][]byte) {
+	if limit <= 0 {
+		limit = math.MaxInt64
+	}
+	var isMatch func(b []byte) bool
+	if len(endKey) > 0 {
+		isMatch = func(b []byte) bool { return bytes.Compare(b, endKey) < 0 }
+	} else {
+		isMatch = func(b []byte) bool { return bytes.Equal(b, key) }
+		limit = 1
+	}
+
+	for ck, cv := c.Seek(key); ck != nil && isMatch(ck); ck, cv = c.Next() {
+		vs = append(vs, cv)
+		keys = append(keys, ck)
+		if limit == int64(len(keys)) {
+			break
+		}
+	}
+	return keys, vs
+}
+
+// UnsafeDelete must be called holding the lock on the tx.
+func (t *batchTx) UnsafeDelete(bucketName []byte, key []byte) {
+	bucket := t.tx.Bucket(bucketName)
+	if bucket == nil {
+		if t.backend.lg != nil {
+			t.backend.lg.Fatal(
+				"failed to find a bucket",
+				zap.String("bucket-name", string(bucketName)),
+			)
+		} else {
+			plog.Fatalf("bucket %s does not exist", bucketName)
+		}
+	}
+	err := bucket.Delete(key)
+	if err != nil {
+		if t.backend.lg != nil {
+			t.backend.lg.Fatal(
+				"failed to delete a key",
+				zap.String("bucket-name", string(bucketName)),
+				zap.Error(err),
+			)
+		} else {
+			plog.Fatalf("cannot delete key from bucket (%v)", err)
+		}
+	}
+	t.pending++
+}
+
+// UnsafeForEach must be called holding the lock on the tx.
+func (t *batchTx) UnsafeForEach(bucketName []byte, visitor func(k, v []byte) error) error {
+	return unsafeForEach(t.tx, bucketName, visitor)
+}
+
+func unsafeForEach(tx *bolt.Tx, bucket []byte, visitor func(k, v []byte) error) error {
+	if b := tx.Bucket(bucket); b != nil {
+		return b.ForEach(visitor)
+	}
+	return nil
+}
+
+// Commit commits a previous tx and begins a new writable one.
+func (t *batchTx) Commit() {
+	t.Lock()
+	t.commit(false)
+	t.Unlock()
+}
+
+// CommitAndStop commits the previous tx and does not create a new one.
+func (t *batchTx) CommitAndStop() {
+	t.Lock()
+	t.commit(true)
+	t.Unlock()
+}
+
+func (t *batchTx) safePending() int {
+	t.Mutex.Lock()
+	defer t.Mutex.Unlock()
+	return t.pending
+}
+
+func (t *batchTx) commit(stop bool) {
+	// commit the last tx
+	if t.tx != nil {
+		if t.pending == 0 && !stop {
+			return
+		}
+
+		start := time.Now()
+
+		// gofail: var beforeCommit struct{}
+		err := t.tx.Commit()
+		// gofail: var afterCommit struct{}
+
+		rebalanceSec.Observe(t.tx.Stats().RebalanceTime.Seconds())
+		spillSec.Observe(t.tx.Stats().SpillTime.Seconds())
+		writeSec.Observe(t.tx.Stats().WriteTime.Seconds())
+		commitSec.Observe(time.Since(start).Seconds())
+		atomic.AddInt64(&t.backend.commits, 1)
+
+		t.pending = 0
+		if err != nil {
+			if t.backend.lg != nil {
+				t.backend.lg.Fatal("failed to commit tx", zap.Error(err))
+			} else {
+				plog.Fatalf("cannot commit tx (%s)", err)
+			}
+		}
+	}
+	if !stop {
+		t.tx = t.backend.begin(true)
+	}
+}
+
+type batchTxBuffered struct {
+	batchTx
+	buf txWriteBuffer
+}
+
+func newBatchTxBuffered(backend *backend) *batchTxBuffered {
+	tx := &batchTxBuffered{
+		batchTx: batchTx{backend: backend},
+		buf: txWriteBuffer{
+			txBuffer: txBuffer{make(map[string]*bucketBuffer)},
+			seq:      true,
+		},
+	}
+	tx.Commit()
+	return tx
+}
+
+func (t *batchTxBuffered) Unlock() {
+	if t.pending != 0 {
+		t.backend.readTx.Lock() // blocks txReadBuffer for writing.
+		t.buf.writeback(&t.backend.readTx.buf)
+		t.backend.readTx.Unlock()
+		if t.pending >= t.backend.batchLimit {
+			t.commit(false)
+		}
+	}
+	t.batchTx.Unlock()
+}
+
+func (t *batchTxBuffered) Commit() {
+	t.Lock()
+	t.commit(false)
+	t.Unlock()
+}
+
+func (t *batchTxBuffered) CommitAndStop() {
+	t.Lock()
+	t.commit(true)
+	t.Unlock()
+}
+
+func (t *batchTxBuffered) commit(stop bool) {
+	// all read txs must be closed to acquire boltdb commit rwlock
+	t.backend.readTx.Lock()
+	t.unsafeCommit(stop)
+	t.backend.readTx.Unlock()
+}
+
+func (t *batchTxBuffered) unsafeCommit(stop bool) {
+	if t.backend.readTx.tx != nil {
+		// wait all store read transactions using the current boltdb tx to finish,
+		// then close the boltdb tx
+		go func(tx *bolt.Tx, wg *sync.WaitGroup) {
+			wg.Wait()
+			if err := tx.Rollback(); err != nil {
+				if t.backend.lg != nil {
+					t.backend.lg.Fatal("failed to rollback tx", zap.Error(err))
+				} else {
+					plog.Fatalf("cannot rollback tx (%s)", err)
+				}
+			}
+		}(t.backend.readTx.tx, t.backend.readTx.txWg)
+		t.backend.readTx.reset()
+	}
+
+	t.batchTx.commit(stop)
+
+	if !stop {
+		t.backend.readTx.tx = t.backend.begin(false)
+	}
+}
+
+func (t *batchTxBuffered) UnsafePut(bucketName []byte, key []byte, value []byte) {
+	t.batchTx.UnsafePut(bucketName, key, value)
+	t.buf.put(bucketName, key, value)
+}
+
+func (t *batchTxBuffered) UnsafeSeqPut(bucketName []byte, key []byte, value []byte) {
+	t.batchTx.UnsafeSeqPut(bucketName, key, value)
+	t.buf.putSeq(bucketName, key, value)
+}
diff --git a/vendor/go.etcd.io/etcd/mvcc/backend/config_default.go b/vendor/go.etcd.io/etcd/mvcc/backend/config_default.go
new file mode 100644
index 0000000..f15f030
--- /dev/null
+++ b/vendor/go.etcd.io/etcd/mvcc/backend/config_default.go
@@ -0,0 +1,23 @@
+// Copyright 2016 The etcd Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// +build !linux,!windows
+
+package backend
+
+import bolt "go.etcd.io/bbolt"
+
+var boltOpenOptions *bolt.Options
+
+func (bcfg *BackendConfig) mmapSize() int { return int(bcfg.MmapSize) }
diff --git a/vendor/go.etcd.io/etcd/mvcc/backend/config_linux.go b/vendor/go.etcd.io/etcd/mvcc/backend/config_linux.go
new file mode 100644
index 0000000..f712671
--- /dev/null
+++ b/vendor/go.etcd.io/etcd/mvcc/backend/config_linux.go
@@ -0,0 +1,34 @@
+// Copyright 2015 The etcd Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package backend
+
+import (
+	"syscall"
+
+	bolt "go.etcd.io/bbolt"
+)
+
+// syscall.MAP_POPULATE on linux 2.6.23+ does sequential read-ahead
+// which can speed up entire-database read with boltdb. We want to
+// enable MAP_POPULATE for faster key-value store recovery in storage
+// package. If your kernel version is lower than 2.6.23
+// (https://github.com/torvalds/linux/releases/tag/v2.6.23), mmap might
+// silently ignore this flag. Please update your kernel to prevent this.
+var boltOpenOptions = &bolt.Options{
+	MmapFlags:      syscall.MAP_POPULATE,
+	NoFreelistSync: true,
+}
+
+func (bcfg *BackendConfig) mmapSize() int { return int(bcfg.MmapSize) }
diff --git a/vendor/go.etcd.io/etcd/mvcc/backend/config_windows.go b/vendor/go.etcd.io/etcd/mvcc/backend/config_windows.go
new file mode 100644
index 0000000..c650059
--- /dev/null
+++ b/vendor/go.etcd.io/etcd/mvcc/backend/config_windows.go
@@ -0,0 +1,26 @@
+// Copyright 2017 The etcd Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// +build windows
+
+package backend
+
+import bolt "go.etcd.io/bbolt"
+
+var boltOpenOptions *bolt.Options = nil
+
+// setting mmap size != 0 on windows will allocate the entire
+// mmap size for the file, instead of growing it. So, force 0.
+
+func (bcfg *BackendConfig) mmapSize() int { return 0 }
diff --git a/vendor/go.etcd.io/etcd/mvcc/backend/doc.go b/vendor/go.etcd.io/etcd/mvcc/backend/doc.go
new file mode 100644
index 0000000..9cc42fa
--- /dev/null
+++ b/vendor/go.etcd.io/etcd/mvcc/backend/doc.go
@@ -0,0 +1,16 @@
+// Copyright 2015 The etcd Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package backend defines a standard interface for etcd's backend MVCC storage.
+package backend
diff --git a/vendor/go.etcd.io/etcd/mvcc/backend/metrics.go b/vendor/go.etcd.io/etcd/mvcc/backend/metrics.go
new file mode 100644
index 0000000..d9641af
--- /dev/null
+++ b/vendor/go.etcd.io/etcd/mvcc/backend/metrics.go
@@ -0,0 +1,95 @@
+// Copyright 2016 The etcd Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package backend
+
+import "github.com/prometheus/client_golang/prometheus"
+
+var (
+	commitSec = prometheus.NewHistogram(prometheus.HistogramOpts{
+		Namespace: "etcd",
+		Subsystem: "disk",
+		Name:      "backend_commit_duration_seconds",
+		Help:      "The latency distributions of commit called by backend.",
+
+		// lowest bucket start of upper bound 0.001 sec (1 ms) with factor 2
+		// highest bucket start of 0.001 sec * 2^13 == 8.192 sec
+		Buckets: prometheus.ExponentialBuckets(0.001, 2, 14),
+	})
+
+	rebalanceSec = prometheus.NewHistogram(prometheus.HistogramOpts{
+		Namespace: "etcd_debugging",
+		Subsystem: "disk",
+		Name:      "backend_commit_rebalance_duration_seconds",
+		Help:      "The latency distributions of commit.rebalance called by bboltdb backend.",
+
+		// lowest bucket start of upper bound 0.001 sec (1 ms) with factor 2
+		// highest bucket start of 0.001 sec * 2^13 == 8.192 sec
+		Buckets: prometheus.ExponentialBuckets(0.001, 2, 14),
+	})
+
+	spillSec = prometheus.NewHistogram(prometheus.HistogramOpts{
+		Namespace: "etcd_debugging",
+		Subsystem: "disk",
+		Name:      "backend_commit_spill_duration_seconds",
+		Help:      "The latency distributions of commit.spill called by bboltdb backend.",
+
+		// lowest bucket start of upper bound 0.001 sec (1 ms) with factor 2
+		// highest bucket start of 0.001 sec * 2^13 == 8.192 sec
+		Buckets: prometheus.ExponentialBuckets(0.001, 2, 14),
+	})
+
+	writeSec = prometheus.NewHistogram(prometheus.HistogramOpts{
+		Namespace: "etcd_debugging",
+		Subsystem: "disk",
+		Name:      "backend_commit_write_duration_seconds",
+		Help:      "The latency distributions of commit.write called by bboltdb backend.",
+
+		// lowest bucket start of upper bound 0.001 sec (1 ms) with factor 2
+		// highest bucket start of 0.001 sec * 2^13 == 8.192 sec
+		Buckets: prometheus.ExponentialBuckets(0.001, 2, 14),
+	})
+
+	defragSec = prometheus.NewHistogram(prometheus.HistogramOpts{
+		Namespace: "etcd",
+		Subsystem: "disk",
+		Name:      "backend_defrag_duration_seconds",
+		Help:      "The latency distribution of backend defragmentation.",
+
+		// 100 MB usually takes 1 sec, so start with 10 MB of 100 ms
+		// lowest bucket start of upper bound 0.1 sec (100 ms) with factor 2
+		// highest bucket start of 0.1 sec * 2^12 == 409.6 sec
+		Buckets: prometheus.ExponentialBuckets(.1, 2, 13),
+	})
+
+	snapshotTransferSec = prometheus.NewHistogram(prometheus.HistogramOpts{
+		Namespace: "etcd",
+		Subsystem: "disk",
+		Name:      "backend_snapshot_duration_seconds",
+		Help:      "The latency distribution of backend snapshots.",
+
+		// lowest bucket start of upper bound 0.01 sec (10 ms) with factor 2
+		// highest bucket start of 0.01 sec * 2^16 == 655.36 sec
+		Buckets: prometheus.ExponentialBuckets(.01, 2, 17),
+	})
+)
+
+func init() {
+	prometheus.MustRegister(commitSec)
+	prometheus.MustRegister(rebalanceSec)
+	prometheus.MustRegister(spillSec)
+	prometheus.MustRegister(writeSec)
+	prometheus.MustRegister(defragSec)
+	prometheus.MustRegister(snapshotTransferSec)
+}
diff --git a/vendor/go.etcd.io/etcd/mvcc/backend/read_tx.go b/vendor/go.etcd.io/etcd/mvcc/backend/read_tx.go
new file mode 100644
index 0000000..91fe72e
--- /dev/null
+++ b/vendor/go.etcd.io/etcd/mvcc/backend/read_tx.go
@@ -0,0 +1,210 @@
+// Copyright 2017 The etcd Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package backend
+
+import (
+	"bytes"
+	"math"
+	"sync"
+
+	bolt "go.etcd.io/bbolt"
+)
+
+// safeRangeBucket is a hack to avoid inadvertently reading duplicate keys;
+// overwrites on a bucket should only fetch with limit=1, but safeRangeBucket
+// is known to never overwrite any key so range is safe.
+var safeRangeBucket = []byte("key")
+
+type ReadTx interface {
+	Lock()
+	Unlock()
+	RLock()
+	RUnlock()
+
+	UnsafeRange(bucketName []byte, key, endKey []byte, limit int64) (keys [][]byte, vals [][]byte)
+	UnsafeForEach(bucketName []byte, visitor func(k, v []byte) error) error
+}
+
+type readTx struct {
+	// mu protects accesses to the txReadBuffer
+	mu  sync.RWMutex
+	buf txReadBuffer
+
+	// TODO: group and encapsulate {txMu, tx, buckets, txWg}, as they share the same lifecycle.
+	// txMu protects accesses to buckets and tx on Range requests.
+	txMu    sync.RWMutex
+	tx      *bolt.Tx
+	buckets map[string]*bolt.Bucket
+	// txWg protects tx from being rolled back at the end of a batch interval until all reads using this tx are done.
+	txWg *sync.WaitGroup
+}
+
+func (rt *readTx) Lock()    { rt.mu.Lock() }
+func (rt *readTx) Unlock()  { rt.mu.Unlock() }
+func (rt *readTx) RLock()   { rt.mu.RLock() }
+func (rt *readTx) RUnlock() { rt.mu.RUnlock() }
+
+func (rt *readTx) UnsafeRange(bucketName, key, endKey []byte, limit int64) ([][]byte, [][]byte) {
+	if endKey == nil {
+		// forbid duplicates for single keys
+		limit = 1
+	}
+	if limit <= 0 {
+		limit = math.MaxInt64
+	}
+	if limit > 1 && !bytes.Equal(bucketName, safeRangeBucket) {
+		panic("do not use unsafeRange on non-keys bucket")
+	}
+	keys, vals := rt.buf.Range(bucketName, key, endKey, limit)
+	if int64(len(keys)) == limit {
+		return keys, vals
+	}
+
+	// find/cache bucket
+	bn := string(bucketName)
+	rt.txMu.RLock()
+	bucket, ok := rt.buckets[bn]
+	rt.txMu.RUnlock()
+	if !ok {
+		rt.txMu.Lock()
+		bucket = rt.tx.Bucket(bucketName)
+		rt.buckets[bn] = bucket
+		rt.txMu.Unlock()
+	}
+
+	// ignore missing bucket since may have been created in this batch
+	if bucket == nil {
+		return keys, vals
+	}
+	rt.txMu.Lock()
+	c := bucket.Cursor()
+	rt.txMu.Unlock()
+
+	k2, v2 := unsafeRange(c, key, endKey, limit-int64(len(keys)))
+	return append(k2, keys...), append(v2, vals...)
+}
+
+func (rt *readTx) UnsafeForEach(bucketName []byte, visitor func(k, v []byte) error) error {
+	dups := make(map[string]struct{})
+	getDups := func(k, v []byte) error {
+		dups[string(k)] = struct{}{}
+		return nil
+	}
+	visitNoDup := func(k, v []byte) error {
+		if _, ok := dups[string(k)]; ok {
+			return nil
+		}
+		return visitor(k, v)
+	}
+	if err := rt.buf.ForEach(bucketName, getDups); err != nil {
+		return err
+	}
+	rt.txMu.Lock()
+	err := unsafeForEach(rt.tx, bucketName, visitNoDup)
+	rt.txMu.Unlock()
+	if err != nil {
+		return err
+	}
+	return rt.buf.ForEach(bucketName, visitor)
+}
+
+func (rt *readTx) reset() {
+	rt.buf.reset()
+	rt.buckets = make(map[string]*bolt.Bucket)
+	rt.tx = nil
+	rt.txWg = new(sync.WaitGroup)
+}
+
+// TODO: create a base type for readTx and concurrentReadTx to avoid duplicated function implementation?
+type concurrentReadTx struct {
+	buf     txReadBuffer
+	txMu    *sync.RWMutex
+	tx      *bolt.Tx
+	buckets map[string]*bolt.Bucket
+	txWg    *sync.WaitGroup
+}
+
+func (rt *concurrentReadTx) Lock()   {}
+func (rt *concurrentReadTx) Unlock() {}
+
+// RLock is no-op. concurrentReadTx does not need to be locked after it is created.
+func (rt *concurrentReadTx) RLock() {}
+
+// RUnlock signals the end of concurrentReadTx.
+func (rt *concurrentReadTx) RUnlock() { rt.txWg.Done() }
+
+func (rt *concurrentReadTx) UnsafeForEach(bucketName []byte, visitor func(k, v []byte) error) error {
+	dups := make(map[string]struct{})
+	getDups := func(k, v []byte) error {
+		dups[string(k)] = struct{}{}
+		return nil
+	}
+	visitNoDup := func(k, v []byte) error {
+		if _, ok := dups[string(k)]; ok {
+			return nil
+		}
+		return visitor(k, v)
+	}
+	if err := rt.buf.ForEach(bucketName, getDups); err != nil {
+		return err
+	}
+	rt.txMu.Lock()
+	err := unsafeForEach(rt.tx, bucketName, visitNoDup)
+	rt.txMu.Unlock()
+	if err != nil {
+		return err
+	}
+	return rt.buf.ForEach(bucketName, visitor)
+}
+
+func (rt *concurrentReadTx) UnsafeRange(bucketName, key, endKey []byte, limit int64) ([][]byte, [][]byte) {
+	if endKey == nil {
+		// forbid duplicates for single keys
+		limit = 1
+	}
+	if limit <= 0 {
+		limit = math.MaxInt64
+	}
+	if limit > 1 && !bytes.Equal(bucketName, safeRangeBucket) {
+		panic("do not use unsafeRange on non-keys bucket")
+	}
+	keys, vals := rt.buf.Range(bucketName, key, endKey, limit)
+	if int64(len(keys)) == limit {
+		return keys, vals
+	}
+
+	// find/cache bucket
+	bn := string(bucketName)
+	rt.txMu.RLock()
+	bucket, ok := rt.buckets[bn]
+	rt.txMu.RUnlock()
+	if !ok {
+		rt.txMu.Lock()
+		bucket = rt.tx.Bucket(bucketName)
+		rt.buckets[bn] = bucket
+		rt.txMu.Unlock()
+	}
+
+	// ignore missing bucket since may have been created in this batch
+	if bucket == nil {
+		return keys, vals
+	}
+	rt.txMu.Lock()
+	c := bucket.Cursor()
+	rt.txMu.Unlock()
+
+	k2, v2 := unsafeRange(c, key, endKey, limit-int64(len(keys)))
+	return append(k2, keys...), append(v2, vals...)
+}
diff --git a/vendor/go.etcd.io/etcd/mvcc/backend/tx_buffer.go b/vendor/go.etcd.io/etcd/mvcc/backend/tx_buffer.go
new file mode 100644
index 0000000..d734638
--- /dev/null
+++ b/vendor/go.etcd.io/etcd/mvcc/backend/tx_buffer.go
@@ -0,0 +1,203 @@
+// Copyright 2017 The etcd Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package backend
+
+import (
+	"bytes"
+	"sort"
+)
+
+// txBuffer handles functionality shared between txWriteBuffer and txReadBuffer.
+type txBuffer struct {
+	buckets map[string]*bucketBuffer
+}
+
+func (txb *txBuffer) reset() {
+	for k, v := range txb.buckets {
+		if v.used == 0 {
+			// demote
+			delete(txb.buckets, k)
+		}
+		v.used = 0
+	}
+}
+
+// txWriteBuffer buffers writes of pending updates that have not yet committed.
+type txWriteBuffer struct {
+	txBuffer
+	seq bool
+}
+
+func (txw *txWriteBuffer) put(bucket, k, v []byte) {
+	txw.seq = false
+	txw.putSeq(bucket, k, v)
+}
+
+func (txw *txWriteBuffer) putSeq(bucket, k, v []byte) {
+	b, ok := txw.buckets[string(bucket)]
+	if !ok {
+		b = newBucketBuffer()
+		txw.buckets[string(bucket)] = b
+	}
+	b.add(k, v)
+}
+
+func (txw *txWriteBuffer) writeback(txr *txReadBuffer) {
+	for k, wb := range txw.buckets {
+		rb, ok := txr.buckets[k]
+		if !ok {
+			delete(txw.buckets, k)
+			txr.buckets[k] = wb
+			continue
+		}
+		if !txw.seq && wb.used > 1 {
+			// assume no duplicate keys
+			sort.Sort(wb)
+		}
+		rb.merge(wb)
+	}
+	txw.reset()
+}
+
+// txReadBuffer accesses buffered updates.
+type txReadBuffer struct{ txBuffer }
+
+func (txr *txReadBuffer) Range(bucketName, key, endKey []byte, limit int64) ([][]byte, [][]byte) {
+	if b := txr.buckets[string(bucketName)]; b != nil {
+		return b.Range(key, endKey, limit)
+	}
+	return nil, nil
+}
+
+func (txr *txReadBuffer) ForEach(bucketName []byte, visitor func(k, v []byte) error) error {
+	if b := txr.buckets[string(bucketName)]; b != nil {
+		return b.ForEach(visitor)
+	}
+	return nil
+}
+
+// unsafeCopy returns a copy of txReadBuffer, caller should acquire backend.readTx.RLock()
+func (txr *txReadBuffer) unsafeCopy() txReadBuffer {
+	txrCopy := txReadBuffer{
+		txBuffer: txBuffer{
+			buckets: make(map[string]*bucketBuffer, len(txr.txBuffer.buckets)),
+		},
+	}
+	for bucketName, bucket := range txr.txBuffer.buckets {
+		txrCopy.txBuffer.buckets[bucketName] = bucket.Copy()
+	}
+	return txrCopy
+}
+
+type kv struct {
+	key []byte
+	val []byte
+}
+
+// bucketBuffer buffers key-value pairs that are pending commit.
+type bucketBuffer struct {
+	buf []kv
+	// used tracks number of elements in use so buf can be reused without reallocation.
+	used int
+}
+
+func newBucketBuffer() *bucketBuffer {
+	return &bucketBuffer{buf: make([]kv, 512), used: 0}
+}
+
+func (bb *bucketBuffer) Range(key, endKey []byte, limit int64) (keys [][]byte, vals [][]byte) {
+	f := func(i int) bool { return bytes.Compare(bb.buf[i].key, key) >= 0 }
+	idx := sort.Search(bb.used, f)
+	if idx < 0 {
+		return nil, nil
+	}
+	if len(endKey) == 0 {
+		if bytes.Equal(key, bb.buf[idx].key) {
+			keys = append(keys, bb.buf[idx].key)
+			vals = append(vals, bb.buf[idx].val)
+		}
+		return keys, vals
+	}
+	if bytes.Compare(endKey, bb.buf[idx].key) <= 0 {
+		return nil, nil
+	}
+	for i := idx; i < bb.used && int64(len(keys)) < limit; i++ {
+		if bytes.Compare(endKey, bb.buf[i].key) <= 0 {
+			break
+		}
+		keys = append(keys, bb.buf[i].key)
+		vals = append(vals, bb.buf[i].val)
+	}
+	return keys, vals
+}
+
+func (bb *bucketBuffer) ForEach(visitor func(k, v []byte) error) error {
+	for i := 0; i < bb.used; i++ {
+		if err := visitor(bb.buf[i].key, bb.buf[i].val); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func (bb *bucketBuffer) add(k, v []byte) {
+	bb.buf[bb.used].key, bb.buf[bb.used].val = k, v
+	bb.used++
+	if bb.used == len(bb.buf) {
+		buf := make([]kv, (3*len(bb.buf))/2)
+		copy(buf, bb.buf)
+		bb.buf = buf
+	}
+}
+
+// merge merges data from bb into bbsrc.
+func (bb *bucketBuffer) merge(bbsrc *bucketBuffer) {
+	for i := 0; i < bbsrc.used; i++ {
+		bb.add(bbsrc.buf[i].key, bbsrc.buf[i].val)
+	}
+	if bb.used == bbsrc.used {
+		return
+	}
+	if bytes.Compare(bb.buf[(bb.used-bbsrc.used)-1].key, bbsrc.buf[0].key) < 0 {
+		return
+	}
+
+	sort.Stable(bb)
+
+	// remove duplicates, using only newest update
+	widx := 0
+	for ridx := 1; ridx < bb.used; ridx++ {
+		if !bytes.Equal(bb.buf[ridx].key, bb.buf[widx].key) {
+			widx++
+		}
+		bb.buf[widx] = bb.buf[ridx]
+	}
+	bb.used = widx + 1
+}
+
+func (bb *bucketBuffer) Len() int { return bb.used }
+func (bb *bucketBuffer) Less(i, j int) bool {
+	return bytes.Compare(bb.buf[i].key, bb.buf[j].key) < 0
+}
+func (bb *bucketBuffer) Swap(i, j int) { bb.buf[i], bb.buf[j] = bb.buf[j], bb.buf[i] }
+
+func (bb *bucketBuffer) Copy() *bucketBuffer {
+	bbCopy := bucketBuffer{
+		buf:  make([]kv, len(bb.buf)),
+		used: bb.used,
+	}
+	copy(bbCopy.buf, bb.buf)
+	return &bbCopy
+}