diff --git a/vendor/go.etcd.io/etcd/wal/wal.go b/vendor/go.etcd.io/etcd/wal/wal.go
new file mode 100644
index 0000000..e177c8e
--- /dev/null
+++ b/vendor/go.etcd.io/etcd/wal/wal.go
@@ -0,0 +1,912 @@
+// 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 wal
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"hash/crc32"
+	"io"
+	"os"
+	"path/filepath"
+	"sync"
+	"time"
+
+	"go.etcd.io/etcd/pkg/fileutil"
+	"go.etcd.io/etcd/pkg/pbutil"
+	"go.etcd.io/etcd/raft"
+	"go.etcd.io/etcd/raft/raftpb"
+	"go.etcd.io/etcd/wal/walpb"
+
+	"github.com/coreos/pkg/capnslog"
+	"go.uber.org/zap"
+)
+
+const (
+	metadataType int64 = iota + 1
+	entryType
+	stateType
+	crcType
+	snapshotType
+
+	// warnSyncDuration is the amount of time allotted to an fsync before
+	// logging a warning
+	warnSyncDuration = time.Second
+)
+
+var (
+	// SegmentSizeBytes is the preallocated size of each wal segment file.
+	// The actual size might be larger than this. In general, the default
+	// value should be used, but this is defined as an exported variable
+	// so that tests can set a different segment size.
+	SegmentSizeBytes int64 = 64 * 1000 * 1000 // 64MB
+
+	plog = capnslog.NewPackageLogger("go.etcd.io/etcd", "wal")
+
+	ErrMetadataConflict = errors.New("wal: conflicting metadata found")
+	ErrFileNotFound     = errors.New("wal: file not found")
+	ErrCRCMismatch      = errors.New("wal: crc mismatch")
+	ErrSnapshotMismatch = errors.New("wal: snapshot mismatch")
+	ErrSnapshotNotFound = errors.New("wal: snapshot not found")
+	crcTable            = crc32.MakeTable(crc32.Castagnoli)
+)
+
+// WAL is a logical representation of the stable storage.
+// WAL is either in read mode or append mode but not both.
+// A newly created WAL is in append mode, and ready for appending records.
+// A just opened WAL is in read mode, and ready for reading records.
+// The WAL will be ready for appending after reading out all the previous records.
+type WAL struct {
+	lg *zap.Logger
+
+	dir string // the living directory of the underlay files
+
+	// dirFile is a fd for the wal directory for syncing on Rename
+	dirFile *os.File
+
+	metadata []byte           // metadata recorded at the head of each WAL
+	state    raftpb.HardState // hardstate recorded at the head of WAL
+
+	start     walpb.Snapshot // snapshot to start reading
+	decoder   *decoder       // decoder to decode records
+	readClose func() error   // closer for decode reader
+
+	mu      sync.Mutex
+	enti    uint64   // index of the last entry saved to the wal
+	encoder *encoder // encoder to encode records
+
+	locks []*fileutil.LockedFile // the locked files the WAL holds (the name is increasing)
+	fp    *filePipeline
+}
+
+// Create creates a WAL ready for appending records. The given metadata is
+// recorded at the head of each WAL file, and can be retrieved with ReadAll.
+func Create(lg *zap.Logger, dirpath string, metadata []byte) (*WAL, error) {
+	if Exist(dirpath) {
+		return nil, os.ErrExist
+	}
+
+	// keep temporary wal directory so WAL initialization appears atomic
+	tmpdirpath := filepath.Clean(dirpath) + ".tmp"
+	if fileutil.Exist(tmpdirpath) {
+		if err := os.RemoveAll(tmpdirpath); err != nil {
+			return nil, err
+		}
+	}
+	if err := fileutil.CreateDirAll(tmpdirpath); err != nil {
+		if lg != nil {
+			lg.Warn(
+				"failed to create a temporary WAL directory",
+				zap.String("tmp-dir-path", tmpdirpath),
+				zap.String("dir-path", dirpath),
+				zap.Error(err),
+			)
+		}
+		return nil, err
+	}
+
+	p := filepath.Join(tmpdirpath, walName(0, 0))
+	f, err := fileutil.LockFile(p, os.O_WRONLY|os.O_CREATE, fileutil.PrivateFileMode)
+	if err != nil {
+		if lg != nil {
+			lg.Warn(
+				"failed to flock an initial WAL file",
+				zap.String("path", p),
+				zap.Error(err),
+			)
+		}
+		return nil, err
+	}
+	if _, err = f.Seek(0, io.SeekEnd); err != nil {
+		if lg != nil {
+			lg.Warn(
+				"failed to seek an initial WAL file",
+				zap.String("path", p),
+				zap.Error(err),
+			)
+		}
+		return nil, err
+	}
+	if err = fileutil.Preallocate(f.File, SegmentSizeBytes, true); err != nil {
+		if lg != nil {
+			lg.Warn(
+				"failed to preallocate an initial WAL file",
+				zap.String("path", p),
+				zap.Int64("segment-bytes", SegmentSizeBytes),
+				zap.Error(err),
+			)
+		}
+		return nil, err
+	}
+
+	w := &WAL{
+		lg:       lg,
+		dir:      dirpath,
+		metadata: metadata,
+	}
+	w.encoder, err = newFileEncoder(f.File, 0)
+	if err != nil {
+		return nil, err
+	}
+	w.locks = append(w.locks, f)
+	if err = w.saveCrc(0); err != nil {
+		return nil, err
+	}
+	if err = w.encoder.encode(&walpb.Record{Type: metadataType, Data: metadata}); err != nil {
+		return nil, err
+	}
+	if err = w.SaveSnapshot(walpb.Snapshot{}); err != nil {
+		return nil, err
+	}
+
+	if w, err = w.renameWAL(tmpdirpath); err != nil {
+		if lg != nil {
+			lg.Warn(
+				"failed to rename the temporary WAL directory",
+				zap.String("tmp-dir-path", tmpdirpath),
+				zap.String("dir-path", w.dir),
+				zap.Error(err),
+			)
+		}
+		return nil, err
+	}
+
+	var perr error
+	defer func() {
+		if perr != nil {
+			w.cleanupWAL(lg)
+		}
+	}()
+
+	// directory was renamed; sync parent dir to persist rename
+	pdir, perr := fileutil.OpenDir(filepath.Dir(w.dir))
+	if perr != nil {
+		if lg != nil {
+			lg.Warn(
+				"failed to open the parent data directory",
+				zap.String("parent-dir-path", filepath.Dir(w.dir)),
+				zap.String("dir-path", w.dir),
+				zap.Error(perr),
+			)
+		}
+		return nil, perr
+	}
+	if perr = fileutil.Fsync(pdir); perr != nil {
+		if lg != nil {
+			lg.Warn(
+				"failed to fsync the parent data directory file",
+				zap.String("parent-dir-path", filepath.Dir(w.dir)),
+				zap.String("dir-path", w.dir),
+				zap.Error(perr),
+			)
+		}
+		return nil, perr
+	}
+	if perr = pdir.Close(); perr != nil {
+		if lg != nil {
+			lg.Warn(
+				"failed to close the parent data directory file",
+				zap.String("parent-dir-path", filepath.Dir(w.dir)),
+				zap.String("dir-path", w.dir),
+				zap.Error(perr),
+			)
+		}
+		return nil, perr
+	}
+
+	return w, nil
+}
+
+func (w *WAL) cleanupWAL(lg *zap.Logger) {
+	var err error
+	if err = w.Close(); err != nil {
+		if lg != nil {
+			lg.Panic("failed to close WAL during cleanup", zap.Error(err))
+		} else {
+			plog.Panicf("failed to close WAL during cleanup: %v", err)
+		}
+	}
+	brokenDirName := fmt.Sprintf("%s.broken.%v", w.dir, time.Now().Format("20060102.150405.999999"))
+	if err = os.Rename(w.dir, brokenDirName); err != nil {
+		if lg != nil {
+			lg.Panic(
+				"failed to rename WAL during cleanup",
+				zap.Error(err),
+				zap.String("source-path", w.dir),
+				zap.String("rename-path", brokenDirName),
+			)
+		} else {
+			plog.Panicf("failed to rename WAL during cleanup: %v", err)
+		}
+	}
+}
+
+func (w *WAL) renameWAL(tmpdirpath string) (*WAL, error) {
+	if err := os.RemoveAll(w.dir); err != nil {
+		return nil, err
+	}
+	// On non-Windows platforms, hold the lock while renaming. Releasing
+	// the lock and trying to reacquire it quickly can be flaky because
+	// it's possible the process will fork to spawn a process while this is
+	// happening. The fds are set up as close-on-exec by the Go runtime,
+	// but there is a window between the fork and the exec where another
+	// process holds the lock.
+	if err := os.Rename(tmpdirpath, w.dir); err != nil {
+		if _, ok := err.(*os.LinkError); ok {
+			return w.renameWALUnlock(tmpdirpath)
+		}
+		return nil, err
+	}
+	w.fp = newFilePipeline(w.lg, w.dir, SegmentSizeBytes)
+	df, err := fileutil.OpenDir(w.dir)
+	w.dirFile = df
+	return w, err
+}
+
+func (w *WAL) renameWALUnlock(tmpdirpath string) (*WAL, error) {
+	// rename of directory with locked files doesn't work on windows/cifs;
+	// close the WAL to release the locks so the directory can be renamed.
+	if w.lg != nil {
+		w.lg.Info(
+			"closing WAL to release flock and retry directory renaming",
+			zap.String("from", tmpdirpath),
+			zap.String("to", w.dir),
+		)
+	} else {
+		plog.Infof("releasing file lock to rename %q to %q", tmpdirpath, w.dir)
+	}
+	w.Close()
+
+	if err := os.Rename(tmpdirpath, w.dir); err != nil {
+		return nil, err
+	}
+
+	// reopen and relock
+	newWAL, oerr := Open(w.lg, w.dir, walpb.Snapshot{})
+	if oerr != nil {
+		return nil, oerr
+	}
+	if _, _, _, err := newWAL.ReadAll(); err != nil {
+		newWAL.Close()
+		return nil, err
+	}
+	return newWAL, nil
+}
+
+// Open opens the WAL at the given snap.
+// The snap SHOULD have been previously saved to the WAL, or the following
+// ReadAll will fail.
+// The returned WAL is ready to read and the first record will be the one after
+// the given snap. The WAL cannot be appended to before reading out all of its
+// previous records.
+func Open(lg *zap.Logger, dirpath string, snap walpb.Snapshot) (*WAL, error) {
+	w, err := openAtIndex(lg, dirpath, snap, true)
+	if err != nil {
+		return nil, err
+	}
+	if w.dirFile, err = fileutil.OpenDir(w.dir); err != nil {
+		return nil, err
+	}
+	return w, nil
+}
+
+// OpenForRead only opens the wal files for read.
+// Write on a read only wal panics.
+func OpenForRead(lg *zap.Logger, dirpath string, snap walpb.Snapshot) (*WAL, error) {
+	return openAtIndex(lg, dirpath, snap, false)
+}
+
+func openAtIndex(lg *zap.Logger, dirpath string, snap walpb.Snapshot, write bool) (*WAL, error) {
+	names, nameIndex, err := selectWALFiles(lg, dirpath, snap)
+	if err != nil {
+		return nil, err
+	}
+
+	rs, ls, closer, err := openWALFiles(lg, dirpath, names, nameIndex, write)
+	if err != nil {
+		return nil, err
+	}
+
+	// create a WAL ready for reading
+	w := &WAL{
+		lg:        lg,
+		dir:       dirpath,
+		start:     snap,
+		decoder:   newDecoder(rs...),
+		readClose: closer,
+		locks:     ls,
+	}
+
+	if write {
+		// write reuses the file descriptors from read; don't close so
+		// WAL can append without dropping the file lock
+		w.readClose = nil
+		if _, _, err := parseWALName(filepath.Base(w.tail().Name())); err != nil {
+			closer()
+			return nil, err
+		}
+		w.fp = newFilePipeline(lg, w.dir, SegmentSizeBytes)
+	}
+
+	return w, nil
+}
+
+func selectWALFiles(lg *zap.Logger, dirpath string, snap walpb.Snapshot) ([]string, int, error) {
+	names, err := readWALNames(lg, dirpath)
+	if err != nil {
+		return nil, -1, err
+	}
+
+	nameIndex, ok := searchIndex(lg, names, snap.Index)
+	if !ok || !isValidSeq(lg, names[nameIndex:]) {
+		err = ErrFileNotFound
+		return nil, -1, err
+	}
+
+	return names, nameIndex, nil
+}
+
+func openWALFiles(lg *zap.Logger, dirpath string, names []string, nameIndex int, write bool) ([]io.Reader, []*fileutil.LockedFile, func() error, error) {
+	rcs := make([]io.ReadCloser, 0)
+	rs := make([]io.Reader, 0)
+	ls := make([]*fileutil.LockedFile, 0)
+	for _, name := range names[nameIndex:] {
+		p := filepath.Join(dirpath, name)
+		if write {
+			l, err := fileutil.TryLockFile(p, os.O_RDWR, fileutil.PrivateFileMode)
+			if err != nil {
+				closeAll(rcs...)
+				return nil, nil, nil, err
+			}
+			ls = append(ls, l)
+			rcs = append(rcs, l)
+		} else {
+			rf, err := os.OpenFile(p, os.O_RDONLY, fileutil.PrivateFileMode)
+			if err != nil {
+				closeAll(rcs...)
+				return nil, nil, nil, err
+			}
+			ls = append(ls, nil)
+			rcs = append(rcs, rf)
+		}
+		rs = append(rs, rcs[len(rcs)-1])
+	}
+
+	closer := func() error { return closeAll(rcs...) }
+
+	return rs, ls, closer, nil
+}
+
+// ReadAll reads out records of the current WAL.
+// If opened in write mode, it must read out all records until EOF. Or an error
+// will be returned.
+// If opened in read mode, it will try to read all records if possible.
+// If it cannot read out the expected snap, it will return ErrSnapshotNotFound.
+// If loaded snap doesn't match with the expected one, it will return
+// all the records and error ErrSnapshotMismatch.
+// TODO: detect not-last-snap error.
+// TODO: maybe loose the checking of match.
+// After ReadAll, the WAL will be ready for appending new records.
+func (w *WAL) ReadAll() (metadata []byte, state raftpb.HardState, ents []raftpb.Entry, err error) {
+	w.mu.Lock()
+	defer w.mu.Unlock()
+
+	rec := &walpb.Record{}
+	decoder := w.decoder
+
+	var match bool
+	for err = decoder.decode(rec); err == nil; err = decoder.decode(rec) {
+		switch rec.Type {
+		case entryType:
+			e := mustUnmarshalEntry(rec.Data)
+			if e.Index > w.start.Index {
+				ents = append(ents[:e.Index-w.start.Index-1], e)
+			}
+			w.enti = e.Index
+
+		case stateType:
+			state = mustUnmarshalState(rec.Data)
+
+		case metadataType:
+			if metadata != nil && !bytes.Equal(metadata, rec.Data) {
+				state.Reset()
+				return nil, state, nil, ErrMetadataConflict
+			}
+			metadata = rec.Data
+
+		case crcType:
+			crc := decoder.crc.Sum32()
+			// current crc of decoder must match the crc of the record.
+			// do no need to match 0 crc, since the decoder is a new one at this case.
+			if crc != 0 && rec.Validate(crc) != nil {
+				state.Reset()
+				return nil, state, nil, ErrCRCMismatch
+			}
+			decoder.updateCRC(rec.Crc)
+
+		case snapshotType:
+			var snap walpb.Snapshot
+			pbutil.MustUnmarshal(&snap, rec.Data)
+			if snap.Index == w.start.Index {
+				if snap.Term != w.start.Term {
+					state.Reset()
+					return nil, state, nil, ErrSnapshotMismatch
+				}
+				match = true
+			}
+
+		default:
+			state.Reset()
+			return nil, state, nil, fmt.Errorf("unexpected block type %d", rec.Type)
+		}
+	}
+
+	switch w.tail() {
+	case nil:
+		// We do not have to read out all entries in read mode.
+		// The last record maybe a partial written one, so
+		// ErrunexpectedEOF might be returned.
+		if err != io.EOF && err != io.ErrUnexpectedEOF {
+			state.Reset()
+			return nil, state, nil, err
+		}
+	default:
+		// We must read all of the entries if WAL is opened in write mode.
+		if err != io.EOF {
+			state.Reset()
+			return nil, state, nil, err
+		}
+		// decodeRecord() will return io.EOF if it detects a zero record,
+		// but this zero record may be followed by non-zero records from
+		// a torn write. Overwriting some of these non-zero records, but
+		// not all, will cause CRC errors on WAL open. Since the records
+		// were never fully synced to disk in the first place, it's safe
+		// to zero them out to avoid any CRC errors from new writes.
+		if _, err = w.tail().Seek(w.decoder.lastOffset(), io.SeekStart); err != nil {
+			return nil, state, nil, err
+		}
+		if err = fileutil.ZeroToEnd(w.tail().File); err != nil {
+			return nil, state, nil, err
+		}
+	}
+
+	err = nil
+	if !match {
+		err = ErrSnapshotNotFound
+	}
+
+	// close decoder, disable reading
+	if w.readClose != nil {
+		w.readClose()
+		w.readClose = nil
+	}
+	w.start = walpb.Snapshot{}
+
+	w.metadata = metadata
+
+	if w.tail() != nil {
+		// create encoder (chain crc with the decoder), enable appending
+		w.encoder, err = newFileEncoder(w.tail().File, w.decoder.lastCRC())
+		if err != nil {
+			return
+		}
+	}
+	w.decoder = nil
+
+	return metadata, state, ents, err
+}
+
+// Verify reads through the given WAL and verifies that it is not corrupted.
+// It creates a new decoder to read through the records of the given WAL.
+// It does not conflict with any open WAL, but it is recommended not to
+// call this function after opening the WAL for writing.
+// If it cannot read out the expected snap, it will return ErrSnapshotNotFound.
+// If the loaded snap doesn't match with the expected one, it will
+// return error ErrSnapshotMismatch.
+func Verify(lg *zap.Logger, walDir string, snap walpb.Snapshot) error {
+	var metadata []byte
+	var err error
+	var match bool
+
+	rec := &walpb.Record{}
+
+	names, nameIndex, err := selectWALFiles(lg, walDir, snap)
+	if err != nil {
+		return err
+	}
+
+	// open wal files in read mode, so that there is no conflict
+	// when the same WAL is opened elsewhere in write mode
+	rs, _, closer, err := openWALFiles(lg, walDir, names, nameIndex, false)
+	if err != nil {
+		return err
+	}
+	defer func() {
+		if closer != nil {
+			closer()
+		}
+	}()
+
+	// create a new decoder from the readers on the WAL files
+	decoder := newDecoder(rs...)
+
+	for err = decoder.decode(rec); err == nil; err = decoder.decode(rec) {
+		switch rec.Type {
+		case metadataType:
+			if metadata != nil && !bytes.Equal(metadata, rec.Data) {
+				return ErrMetadataConflict
+			}
+			metadata = rec.Data
+		case crcType:
+			crc := decoder.crc.Sum32()
+			// Current crc of decoder must match the crc of the record.
+			// We need not match 0 crc, since the decoder is a new one at this point.
+			if crc != 0 && rec.Validate(crc) != nil {
+				return ErrCRCMismatch
+			}
+			decoder.updateCRC(rec.Crc)
+		case snapshotType:
+			var loadedSnap walpb.Snapshot
+			pbutil.MustUnmarshal(&loadedSnap, rec.Data)
+			if loadedSnap.Index == snap.Index {
+				if loadedSnap.Term != snap.Term {
+					return ErrSnapshotMismatch
+				}
+				match = true
+			}
+		// We ignore all entry and state type records as these
+		// are not necessary for validating the WAL contents
+		case entryType:
+		case stateType:
+		default:
+			return fmt.Errorf("unexpected block type %d", rec.Type)
+		}
+	}
+
+	// We do not have to read out all the WAL entries
+	// as the decoder is opened in read mode.
+	if err != io.EOF && err != io.ErrUnexpectedEOF {
+		return err
+	}
+
+	if !match {
+		return ErrSnapshotNotFound
+	}
+
+	return nil
+}
+
+// cut closes current file written and creates a new one ready to append.
+// cut first creates a temp wal file and writes necessary headers into it.
+// Then cut atomically rename temp wal file to a wal file.
+func (w *WAL) cut() error {
+	// close old wal file; truncate to avoid wasting space if an early cut
+	off, serr := w.tail().Seek(0, io.SeekCurrent)
+	if serr != nil {
+		return serr
+	}
+
+	if err := w.tail().Truncate(off); err != nil {
+		return err
+	}
+
+	if err := w.sync(); err != nil {
+		return err
+	}
+
+	fpath := filepath.Join(w.dir, walName(w.seq()+1, w.enti+1))
+
+	// create a temp wal file with name sequence + 1, or truncate the existing one
+	newTail, err := w.fp.Open()
+	if err != nil {
+		return err
+	}
+
+	// update writer and save the previous crc
+	w.locks = append(w.locks, newTail)
+	prevCrc := w.encoder.crc.Sum32()
+	w.encoder, err = newFileEncoder(w.tail().File, prevCrc)
+	if err != nil {
+		return err
+	}
+
+	if err = w.saveCrc(prevCrc); err != nil {
+		return err
+	}
+
+	if err = w.encoder.encode(&walpb.Record{Type: metadataType, Data: w.metadata}); err != nil {
+		return err
+	}
+
+	if err = w.saveState(&w.state); err != nil {
+		return err
+	}
+
+	// atomically move temp wal file to wal file
+	if err = w.sync(); err != nil {
+		return err
+	}
+
+	off, err = w.tail().Seek(0, io.SeekCurrent)
+	if err != nil {
+		return err
+	}
+
+	if err = os.Rename(newTail.Name(), fpath); err != nil {
+		return err
+	}
+	if err = fileutil.Fsync(w.dirFile); err != nil {
+		return err
+	}
+
+	// reopen newTail with its new path so calls to Name() match the wal filename format
+	newTail.Close()
+
+	if newTail, err = fileutil.LockFile(fpath, os.O_WRONLY, fileutil.PrivateFileMode); err != nil {
+		return err
+	}
+	if _, err = newTail.Seek(off, io.SeekStart); err != nil {
+		return err
+	}
+
+	w.locks[len(w.locks)-1] = newTail
+
+	prevCrc = w.encoder.crc.Sum32()
+	w.encoder, err = newFileEncoder(w.tail().File, prevCrc)
+	if err != nil {
+		return err
+	}
+
+	if w.lg != nil {
+		w.lg.Info("created a new WAL segment", zap.String("path", fpath))
+	} else {
+		plog.Infof("segmented wal file %v is created", fpath)
+	}
+	return nil
+}
+
+func (w *WAL) sync() error {
+	if w.encoder != nil {
+		if err := w.encoder.flush(); err != nil {
+			return err
+		}
+	}
+	start := time.Now()
+	err := fileutil.Fdatasync(w.tail().File)
+
+	took := time.Since(start)
+	if took > warnSyncDuration {
+		if w.lg != nil {
+			w.lg.Warn(
+				"slow fdatasync",
+				zap.Duration("took", took),
+				zap.Duration("expected-duration", warnSyncDuration),
+			)
+		} else {
+			plog.Warningf("sync duration of %v, expected less than %v", took, warnSyncDuration)
+		}
+	}
+	walFsyncSec.Observe(took.Seconds())
+
+	return err
+}
+
+// ReleaseLockTo releases the locks, which has smaller index than the given index
+// except the largest one among them.
+// For example, if WAL is holding lock 1,2,3,4,5,6, ReleaseLockTo(4) will release
+// lock 1,2 but keep 3. ReleaseLockTo(5) will release 1,2,3 but keep 4.
+func (w *WAL) ReleaseLockTo(index uint64) error {
+	w.mu.Lock()
+	defer w.mu.Unlock()
+
+	if len(w.locks) == 0 {
+		return nil
+	}
+
+	var smaller int
+	found := false
+	for i, l := range w.locks {
+		_, lockIndex, err := parseWALName(filepath.Base(l.Name()))
+		if err != nil {
+			return err
+		}
+		if lockIndex >= index {
+			smaller = i - 1
+			found = true
+			break
+		}
+	}
+
+	// if no lock index is greater than the release index, we can
+	// release lock up to the last one(excluding).
+	if !found {
+		smaller = len(w.locks) - 1
+	}
+
+	if smaller <= 0 {
+		return nil
+	}
+
+	for i := 0; i < smaller; i++ {
+		if w.locks[i] == nil {
+			continue
+		}
+		w.locks[i].Close()
+	}
+	w.locks = w.locks[smaller:]
+
+	return nil
+}
+
+// Close closes the current WAL file and directory.
+func (w *WAL) Close() error {
+	w.mu.Lock()
+	defer w.mu.Unlock()
+
+	if w.fp != nil {
+		w.fp.Close()
+		w.fp = nil
+	}
+
+	if w.tail() != nil {
+		if err := w.sync(); err != nil {
+			return err
+		}
+	}
+	for _, l := range w.locks {
+		if l == nil {
+			continue
+		}
+		if err := l.Close(); err != nil {
+			if w.lg != nil {
+				w.lg.Warn("failed to close WAL", zap.Error(err))
+			} else {
+				plog.Errorf("failed to unlock during closing wal: %s", err)
+			}
+		}
+	}
+
+	return w.dirFile.Close()
+}
+
+func (w *WAL) saveEntry(e *raftpb.Entry) error {
+	// TODO: add MustMarshalTo to reduce one allocation.
+	b := pbutil.MustMarshal(e)
+	rec := &walpb.Record{Type: entryType, Data: b}
+	if err := w.encoder.encode(rec); err != nil {
+		return err
+	}
+	w.enti = e.Index
+	return nil
+}
+
+func (w *WAL) saveState(s *raftpb.HardState) error {
+	if raft.IsEmptyHardState(*s) {
+		return nil
+	}
+	w.state = *s
+	b := pbutil.MustMarshal(s)
+	rec := &walpb.Record{Type: stateType, Data: b}
+	return w.encoder.encode(rec)
+}
+
+func (w *WAL) Save(st raftpb.HardState, ents []raftpb.Entry) error {
+	w.mu.Lock()
+	defer w.mu.Unlock()
+
+	// short cut, do not call sync
+	if raft.IsEmptyHardState(st) && len(ents) == 0 {
+		return nil
+	}
+
+	mustSync := raft.MustSync(st, w.state, len(ents))
+
+	// TODO(xiangli): no more reference operator
+	for i := range ents {
+		if err := w.saveEntry(&ents[i]); err != nil {
+			return err
+		}
+	}
+	if err := w.saveState(&st); err != nil {
+		return err
+	}
+
+	curOff, err := w.tail().Seek(0, io.SeekCurrent)
+	if err != nil {
+		return err
+	}
+	if curOff < SegmentSizeBytes {
+		if mustSync {
+			return w.sync()
+		}
+		return nil
+	}
+
+	return w.cut()
+}
+
+func (w *WAL) SaveSnapshot(e walpb.Snapshot) error {
+	b := pbutil.MustMarshal(&e)
+
+	w.mu.Lock()
+	defer w.mu.Unlock()
+
+	rec := &walpb.Record{Type: snapshotType, Data: b}
+	if err := w.encoder.encode(rec); err != nil {
+		return err
+	}
+	// update enti only when snapshot is ahead of last index
+	if w.enti < e.Index {
+		w.enti = e.Index
+	}
+	return w.sync()
+}
+
+func (w *WAL) saveCrc(prevCrc uint32) error {
+	return w.encoder.encode(&walpb.Record{Type: crcType, Crc: prevCrc})
+}
+
+func (w *WAL) tail() *fileutil.LockedFile {
+	if len(w.locks) > 0 {
+		return w.locks[len(w.locks)-1]
+	}
+	return nil
+}
+
+func (w *WAL) seq() uint64 {
+	t := w.tail()
+	if t == nil {
+		return 0
+	}
+	seq, _, err := parseWALName(filepath.Base(t.Name()))
+	if err != nil {
+		if w.lg != nil {
+			w.lg.Fatal("failed to parse WAL name", zap.String("name", t.Name()), zap.Error(err))
+		} else {
+			plog.Fatalf("bad wal name %s (%v)", t.Name(), err)
+		}
+	}
+	return seq
+}
+
+func closeAll(rcs ...io.ReadCloser) error {
+	for _, f := range rcs {
+		if err := f.Close(); err != nil {
+			return err
+		}
+	}
+	return nil
+}