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Commit 3bd41cc9 authored by Krasi Georgiev's avatar Krasi Georgiev Committed by Goutham Veeramachaneni
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Udpate tsdb to 0.4 (#5110) 



* update tsdb to v0.4.0
Signed-off-by: default avatarKrasi Georgiev <kgeorgie@redhat.com>

* remove unused struct field
Signed-off-by: default avatarKrasi Georgiev <kgeorgie@redhat.com>
parent 68e4c211
Hide whitespace changes
Inline Side-by-side
go.mod
View file @ 3bd41cc9
......@@ -89,7 +89,7 @@ require (
github.com/prometheus/client_golang v0.9.1
github.com/prometheus/client_model v0.0.0-20180712105110-5c3871d89910
github.com/prometheus/common v0.0.0-20181119215939-b36ad289a3ea
github.com/prometheus/tsdb v0.3.2-0.20181219094047-6d489a1004dc
github.com/prometheus/tsdb v0.4.0
github.com/rcrowley/go-metrics v0.0.0-20181016184325-3113b8401b8a // indirect
github.com/rlmcpherson/s3gof3r v0.5.0 // indirect
github.com/rubyist/circuitbreaker v2.2.1+incompatible // indirect
......
storage/tsdb/tsdb.go
View file @ 3bd41cc9
......@@ -108,9 +108,6 @@ type adapter struct {
// Options of the DB storage.
type Options struct {
// The interval at which the write ahead log is flushed to disc.
WALFlushInterval time.Duration
// The timestamp range of head blocks after which they get persisted.
// It's the minimum duration of any persisted block.
MinBlockDuration model.Duration
......@@ -185,7 +182,6 @@ func Open(path string, l log.Logger, r prometheus.Registerer, opts *Options) (*t
}
db, err := tsdb.Open(path, l, r, &tsdb.Options{
WALFlushInterval: 10 * time.Second,
WALSegmentSize: int(opts.WALSegmentSize),
RetentionDuration: uint64(time.Duration(opts.Retention).Seconds() * 1000),
BlockRanges: rngs,
......
vendor/github.com/prometheus/tsdb/.travis.yml
View file @ 3bd41cc9
......@@ -15,10 +15,10 @@ go_import_path: github.com/prometheus/tsdb
before_install:
- if [[ "$TRAVIS_OS_NAME" == "windows" ]]; then choco install make; fi
install:
- go get -v -t ./...
- make deps
script:
# `staticcheck` target is omitted due to linting errors
- if [[ "$TRAVIS_OS_NAME" == "windows" ]]; then make test; else make check_license style unused test; fi
- if [[ "$TRAVIS_OS_NAME" == "windows" ]]; then make test; else make; fi
vendor/github.com/prometheus/tsdb/CHANGELOG.md
View file @ 3bd41cc9
## master / unreleased
## 0.4.0
- [CHANGE] New `WALSegmentSize` option to override the `DefaultOptions.WALSegmentSize`. Added to allow using smaller wal files. For example using tmpfs on a RPI to minimise the SD card wear out from the constant WAL writes. As part of this change the `DefaultOptions.WALSegmentSize` constant was also exposed.
- [CHANGE] Empty blocks are not written during compaction [#374](https://github.com/prometheus/tsdb/pull/374)
- [FEATURE] Size base retention through `Options.MaxBytes`. As part of this change:
- added new metrics - `prometheus_tsdb_storage_blocks_bytes_total`, `prometheus_tsdb_size_retentions_total`, `prometheus_tsdb_time_retentions_total`
- new public interface `SizeReader: Size() int64`
- `OpenBlock` signature changed to take a logger.
- [REMOVED] `PrefixMatcher` is considered unused so was removed.
- [CLEANUP] `Options.WALFlushInterval` is removed as it wasn't used anywhere.
- [FEATURE] Add new `LiveReader` to WAL pacakge. Added to allow live tailing of a WAL segment, used by Prometheus Remote Write after refactor. The main difference between the new reader and the existing `Reader` is that for `LiveReader` a call to `Next()` that returns false does not mean that there will never be more data to read.
## 0.3.1
- [BUGFIX] Fixed most windows test and some actual bugs for unclosed file readers.
- [BUGFIX] Fixed most windows test and some actual bugs for unclosed file readers.
## 0.3.0
- [CHANGE] `LastCheckpoint()` used to return just the segment name and now it returns the full relative path.
......@@ -11,3 +21,4 @@
- [CHANGE] `Head.Init()` is changed to `Head.Init(minValidTime int64)`
- [CHANGE] `SymbolTable()` renamed to `SymbolTableSize()` to make the name consistent with the `Block{ symbolTableSize uint64 }` field.
- [CHANGE] `wal.Reader{}` now exposes `Segment()` for the current segment being read and `Offset()` for the current offset.
-[FEATURE] tsdbutil analyze subcomand to find churn, high cardinality, etc.
vendor/github.com/prometheus/tsdb/Makefile
View file @ 3bd41cc9
......@@ -18,11 +18,15 @@ TSDB_BENCHMARK_NUM_METRICS ?= 1000
TSDB_BENCHMARK_DATASET ?= "$(TSDB_PROJECT_DIR)/testdata/20kseries.json"
TSDB_BENCHMARK_OUTPUT_DIR ?= "$(TSDB_CLI_DIR)/benchout"
STATICCHECK_IGNORE =
include Makefile.common
.PHONY: deps
deps:
@echo ">> getting dependencies"
GO111MODULE=$(GO111MODULE) $(GO) get $(GOOPTS) -t ./...
build:
@$(GO) build -o $(TSDB_BIN) $(TSDB_CLI_DIR)
GO111MODULE=$(GO111MODULE) $(GO) build -o $(TSDB_BIN) $(TSDB_CLI_DIR)
bench: build
@echo ">> running benchmark, writing result to $(TSDB_BENCHMARK_OUTPUT_DIR)"
......
vendor/github.com/prometheus/tsdb/block.go
View file @ 3bd41cc9
......@@ -21,6 +21,8 @@ import (
"path/filepath"
"sync"
"github.com/go-kit/kit/log"
"github.com/go-kit/kit/log/level"
"github.com/oklog/ulid"
"github.com/pkg/errors"
"github.com/prometheus/tsdb/chunkenc"
......@@ -140,6 +142,12 @@ type Appendable interface {
Appender() Appender
}
// SizeReader returns the size of the object in bytes.
type SizeReader interface {
// Size returns the size in bytes.
Size() int64
}
// BlockMeta provides meta information about a block.
type BlockMeta struct {
// Unique identifier for the block and its contents. Changes on compaction.
......@@ -166,6 +174,7 @@ type BlockStats struct {
NumSeries uint64 `json:"numSeries,omitempty"`
NumChunks uint64 `json:"numChunks,omitempty"`
NumTombstones uint64 `json:"numTombstones,omitempty"`
NumBytes int64 `json:"numBytes,omitempty"`
}
// BlockDesc describes a block by ULID and time range.
......@@ -182,6 +191,9 @@ type BlockMetaCompaction struct {
Level int `json:"level"`
// ULIDs of all source head blocks that went into the block.
Sources []ulid.ULID `json:"sources,omitempty"`
// Indicates that during compaction it resulted in a block without any samples
// so it should be deleted on the next reload.
Deletable bool `json:"deletable,omitempty"`
// Short descriptions of the direct blocks that were used to create
// this block.
Parents []BlockDesc `json:"parents,omitempty"`
......@@ -257,7 +269,10 @@ type Block struct {
// OpenBlock opens the block in the directory. It can be passed a chunk pool, which is used
// to instantiate chunk structs.
func OpenBlock(dir string, pool chunkenc.Pool) (*Block, error) {
func OpenBlock(logger log.Logger, dir string, pool chunkenc.Pool) (*Block, error) {
if logger == nil {
logger = log.NewNopLogger()
}
meta, err := readMetaFile(dir)
if err != nil {
return nil, err
......@@ -272,11 +287,20 @@ func OpenBlock(dir string, pool chunkenc.Pool) (*Block, error) {
return nil, err
}
tr, err := readTombstones(dir)
tr, tsr, err := readTombstones(dir)
if err != nil {
return nil, err
}
// TODO refactor to set this at block creation time as
// that would be the logical place for a block size to be calculated.
bs := blockSize(cr, ir, tsr)
meta.Stats.NumBytes = bs
err = writeMetaFile(dir, meta)
if err != nil {
level.Warn(logger).Log("msg", "couldn't write the meta file for the block size", "block", dir, "err", err)
}
pb := &Block{
dir: dir,
meta: *meta,
......@@ -288,6 +312,16 @@ func OpenBlock(dir string, pool chunkenc.Pool) (*Block, error) {
return pb, nil
}
func blockSize(rr ...SizeReader) int64 {
var total int64
for _, r := range rr {
if r != nil {
total += r.Size()
}
}
return total
}
// Close closes the on-disk block. It blocks as long as there are readers reading from the block.
func (pb *Block) Close() error {
pb.mtx.Lock()
......@@ -315,6 +349,9 @@ func (pb *Block) Dir() string { return pb.dir }
// Meta returns meta information about the block.
func (pb *Block) Meta() BlockMeta { return pb.meta }
// Size returns the number of bytes that the block takes up.
func (pb *Block) Size() int64 { return pb.meta.Stats.NumBytes }
// ErrClosing is returned when a block is in the process of being closed.
var ErrClosing = errors.New("block is closing")
......
vendor/github.com/prometheus/tsdb/checkpoint.go
View file @ 3bd41cc9
......@@ -128,7 +128,7 @@ func Checkpoint(w *wal.WAL, from, to int, keep func(id uint64) bool, mint int64)
defer sgmReader.Close()
}
cpdir := filepath.Join(w.Dir(), fmt.Sprintf("checkpoint.%06d", to))
cpdir := filepath.Join(w.Dir(), fmt.Sprintf(checkpointPrefix+"%06d", to))
cpdirtmp := cpdir + ".tmp"
if err := os.MkdirAll(cpdirtmp, 0777); err != nil {
......@@ -139,6 +139,12 @@ func Checkpoint(w *wal.WAL, from, to int, keep func(id uint64) bool, mint int64)
return nil, errors.Wrap(err, "open checkpoint")
}
// Ensures that an early return caused by an error doesn't leave any tmp files.
defer func() {
cp.Close()
os.RemoveAll(cpdirtmp)
}()
r := wal.NewReader(sgmReader)
var (
......
vendor/github.com/prometheus/tsdb/chunks/chunks.go
View file @ 3bd41cc9
......@@ -205,6 +205,7 @@ func (w *Writer) WriteChunks(chks ...Meta) error {
for _, c := range chks {
maxLen += binary.MaxVarintLen32 + 1 // The number of bytes in the chunk and its encoding.
maxLen += int64(len(c.Chunk.Bytes()))
maxLen += 4 // The 4 bytes of crc32
}
newsz := w.n + maxLen
......@@ -284,17 +285,15 @@ func (b realByteSlice) Sub(start, end int) ByteSlice {
// Reader implements a SeriesReader for a serialized byte stream
// of series data.
type Reader struct {
// The underlying bytes holding the encoded series data.
bs []ByteSlice
// Closers for resources behind the byte slices.
cs []io.Closer
bs []ByteSlice // The underlying bytes holding the encoded series data.
cs []io.Closer // Closers for resources behind the byte slices.
size int64 // The total size of bytes in the reader.
pool chunkenc.Pool
}
func newReader(bs []ByteSlice, cs []io.Closer, pool chunkenc.Pool) (*Reader, error) {
cr := Reader{pool: pool, bs: bs, cs: cs}
var totalSize int64
for i, b := range cr.bs {
if b.Len() < 4 {
......@@ -304,7 +303,9 @@ func newReader(bs []ByteSlice, cs []io.Closer, pool chunkenc.Pool) (*Reader, err
if m := binary.BigEndian.Uint32(b.Range(0, 4)); m != MagicChunks {
return nil, errors.Errorf("invalid magic number %x", m)
}
totalSize += int64(b.Len())
}
cr.size = totalSize
return &cr, nil
}
......@@ -327,9 +328,10 @@ func NewDirReader(dir string, pool chunkenc.Pool) (*Reader, error) {
pool = chunkenc.NewPool()
}
var bs []ByteSlice
var cs []io.Closer
var (
bs []ByteSlice
cs []io.Closer
)
for _, fn := range files {
f, err := fileutil.OpenMmapFile(fn)
if err != nil {
......@@ -345,6 +347,11 @@ func (s *Reader) Close() error {
return closeAll(s.cs...)
}
// Size returns the size of the chunks.
func (s *Reader) Size() int64 {
return s.size
}
func (s *Reader) Chunk(ref uint64) (chunkenc.Chunk, error) {
var (
seq = int(ref >> 32)
......
vendor/github.com/prometheus/tsdb/compact.go
View file @ 3bd41cc9
......@@ -55,12 +55,17 @@ type Compactor interface {
Plan(dir string) ([]string, error)
// Write persists a Block into a directory.
// No Block is written when resulting Block has 0 samples, and returns empty ulid.ULID{}.
Write(dest string, b BlockReader, mint, maxt int64, parent *BlockMeta) (ulid.ULID, error)
// Compact runs compaction against the provided directories. Must
// only be called concurrently with results of Plan().
// Can optionally pass a list of already open blocks,
// to avoid having to reopen them.
// When resulting Block has 0 samples
// * No block is written.
// * The source dirs are marked Deletable.
// * Returns empty ulid.ULID{}.
Compact(dest string, dirs []string, open []*Block) (ulid.ULID, error)
}
......@@ -186,13 +191,12 @@ func (c *LeveledCompactor) plan(dms []dirMeta) ([]string, error) {
return res, nil
}
// Compact any blocks that have >5% tombstones.
// Compact any blocks with big enough time range that have >5% tombstones.
for i := len(dms) - 1; i >= 0; i-- {
meta := dms[i].meta
if meta.MaxTime-meta.MinTime < c.ranges[len(c.ranges)/2] {
break
}
if float64(meta.Stats.NumTombstones)/float64(meta.Stats.NumSeries+1) > 0.05 {
return []string{dms[i].dir}, nil
}
......@@ -347,7 +351,7 @@ func (c *LeveledCompactor) Compact(dest string, dirs []string, open []*Block) (u
if b == nil {
var err error
b, err = OpenBlock(d, c.chunkPool)
b, err = OpenBlock(c.logger, d, c.chunkPool)
if err != nil {
return uid, err
}
......@@ -366,15 +370,34 @@ func (c *LeveledCompactor) Compact(dest string, dirs []string, open []*Block) (u
meta := compactBlockMetas(uid, metas...)
err = c.write(dest, meta, blocks...)
if err == nil {
level.Info(c.logger).Log(
"msg", "compact blocks",
"count", len(blocks),
"mint", meta.MinTime,
"maxt", meta.MaxTime,
"ulid", meta.ULID,
"sources", fmt.Sprintf("%v", uids),
"duration", time.Since(start),
)
if meta.Stats.NumSamples == 0 {
for _, b := range bs {
b.meta.Compaction.Deletable = true
if err = writeMetaFile(b.dir, &b.meta); err != nil {
level.Error(c.logger).Log(
"msg", "Failed to write 'Deletable' to meta file after compaction",
"ulid", b.meta.ULID,
)
}
}
uid = ulid.ULID{}
level.Info(c.logger).Log(
"msg", "compact blocks resulted in empty block",
"count", len(blocks),
"sources", fmt.Sprintf("%v", uids),
"duration", time.Since(start),
)
} else {
level.Info(c.logger).Log(
"msg", "compact blocks",
"count", len(blocks),
"mint", meta.MinTime,
"maxt", meta.MaxTime,
"ulid", meta.ULID,
"sources", fmt.Sprintf("%v", uids),
"duration", time.Since(start),
)
}
return uid, nil
}
......@@ -413,6 +436,10 @@ func (c *LeveledCompactor) Write(dest string, b BlockReader, mint, maxt int64, p
return uid, err
}
if meta.Stats.NumSamples == 0 {
return ulid.ULID{}, nil
}
level.Info(c.logger).Log("msg", "write block", "mint", meta.MinTime, "maxt", meta.MaxTime, "ulid", meta.ULID)
return uid, nil
}
......@@ -490,11 +517,6 @@ func (c *LeveledCompactor) write(dest string, meta *BlockMeta, blocks ...BlockRe
if err := c.populateBlock(blocks, meta, indexw, chunkw); err != nil {
return errors.Wrap(err, "write compaction")
}
if err = writeMetaFile(tmp, meta); err != nil {
return errors.Wrap(err, "write merged meta")
}
// We are explicitly closing them here to check for error even
// though these are covered under defer. This is because in Windows,
// you cannot delete these unless they are closed and the defer is to
......@@ -506,6 +528,18 @@ func (c *LeveledCompactor) write(dest string, meta *BlockMeta, blocks ...BlockRe
return errors.Wrap(err, "close index writer")
}
// Populated block is empty, so cleanup and exit.
if meta.Stats.NumSamples == 0 {
if err := os.RemoveAll(tmp); err != nil {
return errors.Wrap(err, "remove tmp folder after empty block failed")
}
return nil
}
if err = writeMetaFile(tmp, meta); err != nil {
return errors.Wrap(err, "write merged meta")
}
// Create an empty tombstones file.
if err := writeTombstoneFile(tmp, newMemTombstones()); err != nil {
return errors.Wrap(err, "write new tombstones file")
......
vendor/github.com/prometheus/tsdb/db.go
View file @ 3bd41cc9
......@@ -44,7 +44,6 @@ import (
// DefaultOptions used for the DB. They are sane for setups using
// millisecond precision timestamps.
var DefaultOptions = &Options{
WALFlushInterval: 5 * time.Second,
WALSegmentSize: wal.DefaultSegmentSize,
RetentionDuration: 15 * 24 * 60 * 60 * 1000, // 15 days in milliseconds
BlockRanges: ExponentialBlockRanges(int64(2*time.Hour)/1e6, 3, 5),
......@@ -53,15 +52,19 @@ var DefaultOptions = &Options{
// Options of the DB storage.
type Options struct {
// The interval at which the write ahead log is flushed to disk.
WALFlushInterval time.Duration
// Segments (wal files) max size
WALSegmentSize int
// Duration of persisted data to keep.
RetentionDuration uint64
// Maximum number of bytes in blocks to be retained.
// 0 or less means disabled.
// NOTE: For proper storage calculations need to consider
// the size of the WAL folder which is not added when calculating
// the current size of the database.
MaxBytes int64
// The sizes of the Blocks.
BlockRanges []int64
......@@ -131,11 +134,12 @@ type dbMetrics struct {
reloads prometheus.Counter
reloadsFailed prometheus.Counter
compactionsTriggered prometheus.Counter
timeRetentionCount prometheus.Counter
compactionsSkipped prometheus.Counter
cutoffs prometheus.Counter
cutoffsFailed prometheus.Counter
startTime prometheus.GaugeFunc
tombCleanTimer prometheus.Histogram
blocksBytes prometheus.Gauge
sizeRetentionCount prometheus.Counter
}
func newDBMetrics(db *DB, r prometheus.Registerer) *dbMetrics {
......@@ -174,18 +178,14 @@ func newDBMetrics(db *DB, r prometheus.Registerer) *dbMetrics {
Name: "prometheus_tsdb_compactions_triggered_total",
Help: "Total number of triggered compactions for the partition.",
})
m.timeRetentionCount = prometheus.NewCounter(prometheus.CounterOpts{
Name: "prometheus_tsdb_time_retentions_total",
Help: "The number of times that blocks were deleted because the maximum time limit was exceeded.",
})
m.compactionsSkipped = prometheus.NewCounter(prometheus.CounterOpts{
Name: "prometheus_tsdb_compactions_skipped_total",
Help: "Total number of skipped compactions due to disabled auto compaction.",
})
m.cutoffs = prometheus.NewCounter(prometheus.CounterOpts{
Name: "prometheus_tsdb_retention_cutoffs_total",
Help: "Number of times the database cut off block data from disk.",
})
m.cutoffsFailed = prometheus.NewCounter(prometheus.CounterOpts{
Name: "prometheus_tsdb_retention_cutoffs_failures_total",
Help: "Number of times the database failed to cut off block data from disk.",
})
m.startTime = prometheus.NewGaugeFunc(prometheus.GaugeOpts{
Name: "prometheus_tsdb_lowest_timestamp",
Help: "Lowest timestamp value stored in the database. The unit is decided by the library consumer.",
......@@ -201,6 +201,14 @@ func newDBMetrics(db *DB, r prometheus.Registerer) *dbMetrics {
Name: "prometheus_tsdb_tombstone_cleanup_seconds",
Help: "The time taken to recompact blocks to remove tombstones.",
})
m.blocksBytes = prometheus.NewGauge(prometheus.GaugeOpts{
Name: "prometheus_tsdb_storage_blocks_bytes_total",
Help: "The number of bytes that are currently used for local storage by all blocks.",
})
m.sizeRetentionCount = prometheus.NewCounter(prometheus.CounterOpts{
Name: "prometheus_tsdb_size_retentions_total",
Help: "The number of times that blocks were deleted because the maximum number of bytes was exceeded.",
})
if r != nil {
r.MustRegister(
......@@ -208,11 +216,12 @@ func newDBMetrics(db *DB, r prometheus.Registerer) *dbMetrics {
m.symbolTableSize,
m.reloads,
m.reloadsFailed,
m.cutoffs,
m.cutoffsFailed,
m.timeRetentionCount,
m.compactionsTriggered,
m.startTime,
m.tombCleanTimer,
m.blocksBytes,
m.sizeRetentionCount,
)
}
return m
......@@ -344,25 +353,6 @@ func (db *DB) run() {
}
}
func (db *DB) beyondRetention(meta *BlockMeta) bool {
if db.opts.RetentionDuration == 0 {
return false
}
db.mtx.RLock()
blocks := db.blocks[:]
db.mtx.RUnlock()
if len(blocks) == 0 {
return false
}
last := blocks[len(db.blocks)-1]
mint := last.Meta().MaxTime - int64(db.opts.RetentionDuration)
return meta.MaxTime < mint
}
// Appender opens a new appender against the database.
func (db *DB) Appender() Appender {
return dbAppender{db: db, Appender: db.head.Appender()}
......@@ -427,7 +417,8 @@ func (db *DB) compact() (err error) {
// from the block interval here.
maxt: maxt - 1,
}
if _, err = db.compactor.Write(db.dir, head, mint, maxt, nil); err != nil {
uid, err := db.compactor.Write(db.dir, head, mint, maxt, nil)
if err != nil {
return errors.Wrap(err, "persist head block")
}
......@@ -436,6 +427,14 @@ func (db *DB) compact() (err error) {
if err := db.reload(); err != nil {
return errors.Wrap(err, "reload blocks")
}
if (uid == ulid.ULID{}) {
// Compaction resulted in an empty block.
// Head truncating during db.reload() depends on the persisted blocks and
// in this case no new block will be persisted so manually truncate the head.
if err = db.head.Truncate(maxt); err != nil {
return errors.Wrap(err, "head truncate failed (in compact)")
}
}
runtime.GC()
}
......@@ -478,8 +477,7 @@ func (db *DB) getBlock(id ulid.ULID) (*Block, bool) {
return nil, false
}
// reload on-disk blocks and trigger head truncation if new blocks appeared. It takes
// a list of block directories which should be deleted during reload.
// reload blocks and trigger head truncation if new blocks appeared.
// Blocks that are obsolete due to replacement or retention will be deleted.
func (db *DB) reload() (err error) {
defer func() {
......@@ -489,110 +487,191 @@ func (db *DB) reload() (err error) {
db.metrics.reloads.Inc()
}()
dirs, err := blockDirs(db.dir)
loadable, corrupted, err := db.openBlocks()
if err != nil {
return errors.Wrap(err, "find blocks")
}
// We delete old blocks that have been superseded by new ones by gathering all parents
// from existing blocks. Those parents all have newer replacements and can be safely deleted
// after we loaded the other blocks.
// This makes us resilient against the process crashing towards the end of a compaction.
// Creation of a new block and deletion of its parents cannot happen atomically. By creating
// blocks with their parents, we can pick up the deletion where it left off during a crash.
var (
blocks []*Block
corrupted = map[ulid.ULID]error{}
opened = map[ulid.ULID]struct{}{}
deleteable = map[ulid.ULID]struct{}{}
)
for _, dir := range dirs {
meta, err := readMetaFile(dir)
if err != nil {
// The block was potentially in the middle of being deleted during a crash.
// Skip it since we may delete it properly further down again.
level.Warn(db.logger).Log("msg", "read meta information", "err", err, "dir", dir)
return err
}
ulid, err2 := ulid.Parse(filepath.Base(dir))
if err2 != nil {
level.Error(db.logger).Log("msg", "not a block dir", "dir", dir)
continue
}
corrupted[ulid] = err
continue
deletable := db.deletableBlocks(loadable)
// Corrupted blocks that have been replaced by parents can be safely ignored and deleted.
// This makes it resilient against the process crashing towards the end of a compaction.
// Creation of a new block and deletion of its parents cannot happen atomically.
// By creating blocks with their parents, we can pick up the deletion where it left off during a crash.