forked from prometheus-junkyard/tsdb
/
wal.go
822 lines (736 loc) · 20.5 KB
/
wal.go
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// Copyright 2017 The Prometheus 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 (
"bufio"
"encoding/binary"
"fmt"
"hash/crc32"
"io"
"math"
"os"
"path/filepath"
"sort"
"strconv"
"sync"
"time"
"github.com/go-kit/kit/log"
"github.com/go-kit/kit/log/level"
"github.com/pkg/errors"
"github.com/prometheus/client_golang/prometheus"
"github.com/prometheus/tsdb/fileutil"
)
const (
defaultSegmentSize = 128 * 1024 * 1024 // 128 MB
pageSize = 32 * 1024 // 32KB
recordHeaderSize = 7
)
// The table gets initialized with sync.Once but may still cause a race
// with any other use of the crc32 package anywhere. Thus we initialize it
// before.
var castagnoliTable = crc32.MakeTable(crc32.Castagnoli)
type page struct {
alloc int
flushed int
buf [pageSize]byte
}
func (p *page) remaining() int {
return pageSize - p.alloc
}
func (p *page) full() bool {
return pageSize-p.alloc < recordHeaderSize
}
// Segment represents a segment file.
type Segment struct {
*os.File
dir string
i int
}
// Index returns the index of the segment.
func (s *Segment) Index() int {
return s.i
}
// Dir returns the directory of the segment.
func (s *Segment) Dir() string {
return s.dir
}
// CorruptionErr is an error that's returned when corruption is encountered.
type CorruptionErr struct {
Segment int
Offset int64
Err error
}
func (e *CorruptionErr) Error() string {
if e.Segment < 0 {
return fmt.Sprintf("corruption after %d bytes: %s", e.Offset, e.Err)
}
return fmt.Sprintf("corruption in segment %d at %d: %s", e.Segment, e.Offset, e.Err)
}
// OpenWriteSegment opens segment k in dir. The returned segment is ready for new appends.
func OpenWriteSegment(dir string, k int) (*Segment, error) {
f, err := os.OpenFile(SegmentName(dir, k), os.O_WRONLY|os.O_APPEND, 0666)
if err != nil {
return nil, err
}
stat, err := f.Stat()
if err != nil {
f.Close()
return nil, err
}
// If the last page is torn, fill it with zeros.
// In case it was torn after all records were written successfully, this
// will just pad the page and everything will be fine.
// If it was torn mid-record, a full read (which the caller should do anyway
// to ensure integrity) will detect it as a corruption by the end.
if d := stat.Size() % pageSize; d != 0 {
if _, err := f.Write(make([]byte, pageSize-d)); err != nil {
f.Close()
return nil, errors.Wrap(err, "zero-pad torn page")
}
}
return &Segment{File: f, i: k, dir: dir}, nil
}
// CreateSegment creates a new segment k in dir.
func CreateSegment(dir string, k int) (*Segment, error) {
f, err := os.OpenFile(SegmentName(dir, k), os.O_WRONLY|os.O_CREATE|os.O_APPEND, 0666)
if err != nil {
return nil, err
}
return &Segment{File: f, i: k, dir: dir}, nil
}
// OpenReadSegment opens the segment with the given filename.
func OpenReadSegment(fn string) (*Segment, error) {
k, err := strconv.Atoi(filepath.Base(fn))
if err != nil {
return nil, errors.New("not a valid filename")
}
f, err := os.Open(fn)
if err != nil {
return nil, err
}
return &Segment{File: f, i: k, dir: filepath.Dir(fn)}, nil
}
// WAL is a write ahead log that stores records in segment files.
// It must be read from start to end once before logging new data.
// If an error occurs during read, the repair procedure must be called
// before it's safe to do further writes.
//
// Segments are written to in pages of 32KB, with records possibly split
// across page boundaries.
// Records are never split across segments to allow full segments to be
// safely truncated. It also ensures that torn writes never corrupt records
// beyond the most recent segment.
type WAL struct {
dir string
logger log.Logger
segmentSize int
mtx sync.RWMutex
segment *Segment // active segment
donePages int // pages written to the segment
page *page // active page
stopc chan chan struct{}
actorc chan func()
fsyncDuration prometheus.Summary
pageFlushes prometheus.Counter
pageCompletions prometheus.Counter
}
// New returns a new WAL over the given directory.
func New(logger log.Logger, reg prometheus.Registerer, dir string) (*WAL, error) {
return NewSize(logger, reg, dir, defaultSegmentSize)
}
// NewSize returns a new WAL over the given directory.
// New segments are created with the specified size.
func NewSize(logger log.Logger, reg prometheus.Registerer, dir string, segmentSize int) (*WAL, error) {
if segmentSize%pageSize != 0 {
return nil, errors.New("invalid segment size")
}
if err := os.MkdirAll(dir, 0777); err != nil {
return nil, errors.Wrap(err, "create dir")
}
if logger == nil {
logger = log.NewNopLogger()
}
w := &WAL{
dir: dir,
logger: logger,
segmentSize: segmentSize,
page: &page{},
actorc: make(chan func(), 100),
stopc: make(chan chan struct{}),
}
w.fsyncDuration = prometheus.NewSummary(prometheus.SummaryOpts{
Name: "prometheus_tsdb_wal_fsync_duration_seconds",
Help: "Duration of WAL fsync.",
})
w.pageFlushes = prometheus.NewCounter(prometheus.CounterOpts{
Name: "prometheus_tsdb_wal_page_flushes_total",
Help: "Total number of page flushes.",
})
w.pageCompletions = prometheus.NewCounter(prometheus.CounterOpts{
Name: "prometheus_tsdb_wal_completed_pages_total",
Help: "Total number of completed pages.",
})
if reg != nil {
reg.MustRegister(w.fsyncDuration, w.pageFlushes, w.pageCompletions)
}
_, j, err := w.Segments()
if err != nil {
return nil, errors.Wrap(err, "get segment range")
}
// Fresh dir, no segments yet.
if j == -1 {
if w.segment, err = CreateSegment(w.dir, 0); err != nil {
return nil, err
}
} else {
if w.segment, err = OpenWriteSegment(w.dir, j); err != nil {
return nil, err
}
// Correctly initialize donePages.
stat, err := w.segment.Stat()
if err != nil {
return nil, err
}
w.donePages = int(stat.Size() / pageSize)
}
go w.run()
return w, nil
}
// Dir returns the directory of the WAL.
func (w *WAL) Dir() string {
return w.dir
}
func (w *WAL) run() {
Loop:
for {
select {
case f := <-w.actorc:
f()
case donec := <-w.stopc:
close(w.actorc)
defer close(donec)
break Loop
}
}
// Drain and process any remaining functions.
for f := range w.actorc {
f()
}
}
// Repair attempts to repair the WAL based on the error.
// It discards all data after the corruption.
func (w *WAL) Repair(err error) error {
// We could probably have a mode that only discards torn records right around
// the corruption to preserve as data much as possible.
// But that's not generally applicable if the records have any kind of causality.
// Maybe as an extra mode in the future if mid-WAL corruptions become
// a frequent concern.
cerr, ok := err.(*CorruptionErr)
if !ok {
return errors.New("cannot handle error")
}
if cerr.Segment < 0 {
return errors.New("corruption error does not specify position")
}
level.Warn(w.logger).Log("msg", "starting corruption repair",
"segment", cerr.Segment, "offset", cerr.Offset)
// All segments behind the corruption can no longer be used.
segs, err := listSegments(w.dir)
if err != nil {
return errors.Wrap(err, "list segments")
}
level.Warn(w.logger).Log("msg", "deleting all segments behind corruption")
for _, s := range segs {
if s.n <= cerr.Segment {
continue
}
if err := os.Remove(filepath.Join(w.dir, s.s)); err != nil {
return errors.Wrap(err, "delete segment")
}
}
// Regardless of the corruption offset, no record reaches into the previous segment.
// So we can safely repair the WAL by removing the segment and re-inserting all
// its records up to the corruption.
level.Warn(w.logger).Log("msg", "rewrite corrupted segment")
fn := SegmentName(w.dir, cerr.Segment)
tmpfn := fn + ".repair"
if err := fileutil.Rename(fn, tmpfn); err != nil {
return err
}
// Create a clean segment and make it the active one.
s, err := CreateSegment(w.dir, cerr.Segment)
if err != nil {
return err
}
w.segment = s
f, err := os.Open(tmpfn)
if err != nil {
return errors.Wrap(err, "open segment")
}
defer f.Close()
r := NewReader(bufio.NewReader(f))
for r.Next() {
if err := w.Log(r.Record()); err != nil {
return errors.Wrap(err, "insert record")
}
}
// We expect an error here, so nothing to handle.
if err := os.Remove(tmpfn); err != nil {
return errors.Wrap(err, "delete corrupted segment")
}
return nil
}
// SegmentName builds a segment name for the directory.
func SegmentName(dir string, i int) string {
return filepath.Join(dir, fmt.Sprintf("%08d", i))
}
// nextSegment creates the next segment and closes the previous one.
func (w *WAL) nextSegment() error {
// Only flush the current page if it actually holds data.
if w.page.alloc > 0 {
if err := w.flushPage(true); err != nil {
return err
}
}
next, err := CreateSegment(w.dir, w.segment.Index()+1)
if err != nil {
return errors.Wrap(err, "create new segment file")
}
prev := w.segment
w.segment = next
w.donePages = 0
// Don't block further writes by fsyncing the last segment.
w.actorc <- func() {
if err := w.fsync(prev); err != nil {
level.Error(w.logger).Log("msg", "sync previous segment", "err", err)
}
if err := prev.Close(); err != nil {
level.Error(w.logger).Log("msg", "close previous segment", "err", err)
}
}
return nil
}
// flushPage writes the new contents of the page to disk. If no more records will fit into
// the page, the remaining bytes will be set to zero and a new page will be started.
// If clear is true, this is enforced regardless of how many bytes are left in the page.
func (w *WAL) flushPage(clear bool) error {
w.pageFlushes.Inc()
p := w.page
clear = clear || p.full()
// No more data will fit into the page. Enqueue and clear it.
if clear {
p.alloc = pageSize // write till end of page
w.pageCompletions.Inc()
}
n, err := w.segment.Write(p.buf[p.flushed:p.alloc])
if err != nil {
return err
}
p.flushed += n
// We flushed an entire page, prepare a new one.
if clear {
for i := range p.buf {
p.buf[i] = 0
}
p.alloc = 0
p.flushed = 0
w.donePages++
}
return nil
}
type recType uint8
const (
recPageTerm recType = 0 // Rest of page is empty.
recFull recType = 1 // Full record.
recFirst recType = 2 // First fragment of a record.
recMiddle recType = 3 // Middle fragments of a record.
recLast recType = 4 // Final fragment of a record.
)
func (t recType) String() string {
switch t {
case recPageTerm:
return "zero"
case recFull:
return "full"
case recFirst:
return "first"
case recMiddle:
return "middle"
case recLast:
return "last"
default:
return "<invalid>"
}
}
func (w *WAL) pagesPerSegment() int {
return w.segmentSize / pageSize
}
// Log writes the records into the log.
// Multiple records can be passed at once to reduce writes and increase throughput.
func (w *WAL) Log(recs ...[]byte) error {
w.mtx.Lock()
defer w.mtx.Unlock()
// Callers could just implement their own list record format but adding
// a bit of extra logic here frees them from that overhead.
for i, r := range recs {
if err := w.log(r, i == len(recs)-1); err != nil {
return err
}
}
return nil
}
// log writes rec to the log and forces a flush of the current page if its
// the final record of a batch.
func (w *WAL) log(rec []byte, final bool) error {
// If the record is too big to fit within pages in the current
// segment, terminate the active segment and advance to the next one.
// This ensures that records do not cross segment boundaries.
left := w.page.remaining() - recordHeaderSize // Active pages.
left += (pageSize - recordHeaderSize) * (w.pagesPerSegment() - w.donePages - 1) // Free pages.
if len(rec) > left {
if err := w.nextSegment(); err != nil {
return err
}
}
// Populate as many pages as necessary to fit the record.
// Be careful to always do one pass to ensure we write zero-length records.
for i := 0; i == 0 || len(rec) > 0; i++ {
p := w.page
// Find how much of the record we can fit into the page.
var (
l = min(len(rec), (pageSize-p.alloc)-recordHeaderSize)
part = rec[:l]
buf = p.buf[p.alloc:]
typ recType
)
switch {
case i == 0 && len(part) == len(rec):
typ = recFull
case len(part) == len(rec):
typ = recLast
case i == 0:
typ = recFirst
default:
typ = recMiddle
}
buf[0] = byte(typ)
crc := crc32.Checksum(part, castagnoliTable)
binary.BigEndian.PutUint16(buf[1:], uint16(len(part)))
binary.BigEndian.PutUint32(buf[3:], crc)
copy(buf[recordHeaderSize:], part)
p.alloc += len(part) + recordHeaderSize
// If we wrote a full record, we can fit more records of the batch
// into the page before flushing it.
if final || typ != recFull || w.page.full() {
if err := w.flushPage(false); err != nil {
return err
}
}
rec = rec[l:]
}
return nil
}
// Segments returns the range [m, n] of currently existing segments.
// If no segments are found, m and n are -1.
func (w *WAL) Segments() (m, n int, err error) {
refs, err := listSegments(w.dir)
if err != nil {
return 0, 0, err
}
if len(refs) == 0 {
return -1, -1, nil
}
return refs[0].n, refs[len(refs)-1].n, nil
}
// Truncate drops all segments before i.
func (w *WAL) Truncate(i int) error {
refs, err := listSegments(w.dir)
if err != nil {
return err
}
for _, r := range refs {
if r.n >= i {
break
}
if err := os.Remove(filepath.Join(w.dir, r.s)); err != nil {
return err
}
}
return nil
}
func (w *WAL) fsync(f *Segment) error {
start := time.Now()
err := fileutil.Fsync(f.File)
w.fsyncDuration.Observe(time.Since(start).Seconds())
return err
}
// Close flushes all writes and closes active segment.
func (w *WAL) Close() (err error) {
w.mtx.Lock()
defer w.mtx.Unlock()
// Flush the last page and zero out all its remaining size.
// We must not flush an empty page as it would falsely signal
// the segment is done if we start writing to it again after opening.
if w.page.alloc > 0 {
if err := w.flushPage(true); err != nil {
return err
}
}
donec := make(chan struct{})
w.stopc <- donec
<-donec
if err = w.fsync(w.segment); err != nil {
level.Error(w.logger).Log("msg", "sync previous segment", "err", err)
}
if err := w.segment.Close(); err != nil {
level.Error(w.logger).Log("msg", "close previous segment", "err", err)
}
return nil
}
type segmentRef struct {
s string
n int
}
func listSegments(dir string) (refs []segmentRef, err error) {
files, err := fileutil.ReadDir(dir)
if err != nil {
return nil, err
}
var last int
for _, fn := range files {
k, err := strconv.Atoi(fn)
if err != nil {
continue
}
if len(refs) > 0 && k > last+1 {
return nil, errors.New("segments are not sequential")
}
refs = append(refs, segmentRef{s: fn, n: k})
last = k
}
sort.Slice(refs, func(i, j int) bool {
return refs[i].n < refs[j].n
})
return refs, nil
}
// NewSegmentsReader returns a new reader over all segments in the directory.
func NewSegmentsReader(dir string) (io.ReadCloser, error) {
return NewSegmentsRangeReader(dir, 0, math.MaxInt32)
}
// NewSegmentsRangeReader returns a new reader over the given WAL segment range.
// If m or n are -1, the range is open on the respective end.
func NewSegmentsRangeReader(dir string, m, n int) (io.ReadCloser, error) {
refs, err := listSegments(dir)
if err != nil {
return nil, err
}
var segs []*Segment
for _, r := range refs {
if m >= 0 && r.n < m {
continue
}
if n >= 0 && r.n > n {
break
}
s, err := OpenReadSegment(filepath.Join(dir, r.s))
if err != nil {
return nil, err
}
segs = append(segs, s)
}
return newSegmentBufReader(segs...), nil
}
// segmentBufReader is a buffered reader that reads in multiples of pages.
// The main purpose is that we are able to track segment and offset for
// corruption reporting.
type segmentBufReader struct {
buf *bufio.Reader
segs []*Segment
cur int
off int
more bool
}
func newSegmentBufReader(segs ...*Segment) *segmentBufReader {
return &segmentBufReader{
buf: bufio.NewReaderSize(nil, 16*pageSize),
segs: segs,
cur: -1,
}
}
func (r *segmentBufReader) Close() (err error) {
for _, s := range r.segs {
if e := s.Close(); e != nil {
err = e
}
}
return err
}
func (r *segmentBufReader) Read(b []byte) (n int, err error) {
if !r.more {
if r.cur+1 >= len(r.segs) {
return 0, io.EOF
}
r.cur++
r.off = 0
r.more = true
r.buf.Reset(r.segs[r.cur])
}
n, err = r.buf.Read(b)
r.off += n
if err != io.EOF {
return n, err
}
// Just return what we read so far, but don't signal EOF.
// Only unset more so we don't invalidate the current segment and
// offset before the next read.
r.more = false
return n, nil
}
// Reader reads WAL records from an io.Reader.
type Reader struct {
rdr io.Reader
err error
rec []byte
buf [pageSize]byte
total int64 // total bytes processed.
}
// NewReader returns a new reader.
func NewReader(r io.Reader) *Reader {
return &Reader{rdr: r}
}
// Next advances the reader to the next records and returns true if it exists.
// It must not be called again after it returned false.
func (r *Reader) Next() bool {
err := r.next()
if errors.Cause(err) == io.EOF {
return false
}
r.err = err
return r.err == nil
}
func (r *Reader) next() (err error) {
// We have to use r.buf since allocating byte arrays here fails escape
// analysis and ends up on the heap, even though it seemingly should not.
hdr := r.buf[:recordHeaderSize]
buf := r.buf[recordHeaderSize:]
r.rec = r.rec[:0]
i := 0
for {
if _, err = io.ReadFull(r.rdr, hdr[:1]); err != nil {
return errors.Wrap(err, "read first header byte")
}
r.total++
typ := recType(hdr[0])
// Gobble up zero bytes.
if typ == recPageTerm {
// We are pedantic and check whether the zeros are actually up
// to a page boundary.
// It's not strictly necessary but may catch sketchy state early.
k := pageSize - (r.total % pageSize)
if k == pageSize {
continue // Initial 0 byte was last page byte.
}
n, err := io.ReadFull(r.rdr, buf[:k])
if err != nil {
return errors.Wrap(err, "read remaining zeros")
}
r.total += int64(n)
for _, c := range buf[:k] {
if c != 0 {
return errors.New("unexpected non-zero byte in padded page")
}
}
continue
}
n, err := io.ReadFull(r.rdr, hdr[1:])
if err != nil {
return errors.Wrap(err, "read remaining header")
}
r.total += int64(n)
var (
length = binary.BigEndian.Uint16(hdr[1:])
crc = binary.BigEndian.Uint32(hdr[3:])
)
if length > pageSize-recordHeaderSize {
return errors.Errorf("invalid record size %d", length)
}
n, err = io.ReadFull(r.rdr, buf[:length])
if err != nil {
return err
}
r.total += int64(n)
if n != int(length) {
return errors.Errorf("invalid size: expected %d, got %d", length, n)
}
if c := crc32.Checksum(buf[:length], castagnoliTable); c != crc {
return errors.Errorf("unexpected checksum %x, expected %x", c, crc)
}
r.rec = append(r.rec, buf[:length]...)
switch typ {
case recFull:
if i != 0 {
return errors.New("unexpected full record")
}
return nil
case recFirst:
if i != 0 {
return errors.New("unexpected first record")
}
case recMiddle:
if i == 0 {
return errors.New("unexpected middle record")
}
case recLast:
if i == 0 {
return errors.New("unexpected last record")
}
return nil
default:
return errors.Errorf("unexpected record type %d", typ)
}
// Only increment i for non-zero records since we use it
// to determine valid content record sequences.
i++
}
}
// Err returns the last encountered error wrapped in a corruption error.
// If the reader does not allow to infer a segment index and offset, a total
// offset in the reader stream will be provided.
func (r *Reader) Err() error {
if r.err == nil {
return nil
}
if b, ok := r.rdr.(*segmentBufReader); ok {
return &CorruptionErr{
Err: r.err,
Segment: b.segs[b.cur].Index(),
Offset: int64(b.off),
}
}
return &CorruptionErr{
Err: r.err,
Segment: -1,
Offset: r.total,
}
}
// Record returns the current record. The returned byte slice is only
// valid until the next call to Next.
func (r *Reader) Record() []byte {
return r.rec
}
func min(i, j int) int {
if i < j {
return i
}
return j
}