VictoriaMetrics/lib/mergeset/table.go

package mergeset

import (
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"sort"
	"strings"
	"sync"
	"sync/atomic"
	"time"
	"unsafe"

	"github.com/VictoriaMetrics/VictoriaMetrics/lib/bytesutil"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/cgroup"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/fasttime"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/fs"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/logger"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/memory"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/storagepacelimiter"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/syncwg"
)

// maxParts is the maximum number of parts in the table.
//
// This number may be reached when the insertion pace outreaches merger pace.
const maxParts = 512

// Default number of parts to merge at once.
//
// This number has been obtained empirically - it gives the lowest possible overhead.
// See appendPartsToMerge tests for details.
const defaultPartsToMerge = 15

// The final number of parts to merge at once.
//
// It must be smaller than defaultPartsToMerge.
// Lower value improves select performance at the cost of increased
// write amplification.
const finalPartsToMerge = 2

// maxPartSize is the maximum part size in bytes.
//
// This number should be limited by the amount of time required to merge parts of this summary size.
// The required time shouldn't exceed a day.
const maxPartSize = 400e9

// maxItemsPerCachedPart is the maximum items per created part by the merge,
// which must be cached in the OS page cache.
//
// Such parts are usually frequently accessed, so it is good to cache their
// contents in OS page cache.
func maxItemsPerCachedPart() uint64 {
	mem := memory.Remaining()
	// Production data shows that each item occupies ~4 bytes in the compressed part.
	// It is expected no more than defaultPartsToMerge/2 parts exist
	// in the OS page cache before they are merged into bigger part.
	// Halft of the remaining RAM must be left for lib/storage parts,
	// so the maxItems is calculated using the below code:
	maxItems := uint64(mem) / (4 * defaultPartsToMerge)
	if maxItems < 1e6 {
		maxItems = 1e6
	}
	return maxItems
}

// The interval for flushing (converting) recent raw items into parts,
// so they become visible to search.
const rawItemsFlushInterval = time.Second

// Table represents mergeset table.
type Table struct {
	// Atomically updated counters must go first in the struct, so they are properly
	// aligned to 8 bytes on 32-bit architectures.
	// See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/212

	activeMerges        uint64
	mergesCount         uint64
	itemsMerged         uint64
	assistedMerges      uint64
	itemsAdded          uint64
	itemsAddedSizeBytes uint64

	mergeIdx uint64

	path string

	flushCallback         func()
	flushCallbackWorkerWG sync.WaitGroup
	needFlushCallbackCall uint32

	prepareBlock PrepareBlockCallback
	isReadOnly   *uint32

	partsLock sync.Mutex
	parts     []*partWrapper

	// rawItems contains recently added items that haven't been converted to parts yet.
	//
	// rawItems aren't used in search for performance reasons
	rawItems rawItemsShards

	snapshotLock sync.RWMutex

	flockF *os.File

	stopCh chan struct{}

	wg sync.WaitGroup

	// Use syncwg instead of sync, since Add/Wait may be called from concurrent goroutines.
	rawItemsPendingFlushesWG syncwg.WaitGroup
}

type rawItemsShards struct {
	shardIdx uint32

	// shards reduce lock contention when adding rows on multi-CPU systems.
	shards []rawItemsShard
}

// The number of shards for rawItems per table.
//
// Higher number of shards reduces CPU contention and increases the max bandwidth on multi-core systems.
var rawItemsShardsPerTable = func() int {
	cpus := cgroup.AvailableCPUs()
	multiplier := cpus
	if multiplier > 16 {
		multiplier = 16
	}
	return (cpus*multiplier + 1) / 2
}()

const maxBlocksPerShard = 256

func (riss *rawItemsShards) init() {
	riss.shards = make([]rawItemsShard, rawItemsShardsPerTable)
}

func (riss *rawItemsShards) addItems(tb *Table, items [][]byte) error {
	n := atomic.AddUint32(&riss.shardIdx, 1)
	shards := riss.shards
	idx := n % uint32(len(shards))
	shard := &shards[idx]
	return shard.addItems(tb, items)
}

func (riss *rawItemsShards) Len() int {
	n := 0
	for i := range riss.shards {
		n += riss.shards[i].Len()
	}
	return n
}

type rawItemsShardNopad struct {
	// Put lastFlushTime to the top in order to avoid unaligned memory access on 32-bit architectures
	lastFlushTime uint64

	mu  sync.Mutex
	ibs []*inmemoryBlock
}

type rawItemsShard struct {
	rawItemsShardNopad

	// The padding prevents false sharing on widespread platforms with
	// 128 mod (cache line size) = 0 .
	_ [128 - unsafe.Sizeof(rawItemsShardNopad{})%128]byte
}

func (ris *rawItemsShard) Len() int {
	ris.mu.Lock()
	n := 0
	for _, ib := range ris.ibs {
		n += len(ib.items)
	}
	ris.mu.Unlock()
	return n
}

func (ris *rawItemsShard) addItems(tb *Table, items [][]byte) error {
	var err error
	var blocksToFlush []*inmemoryBlock

	ris.mu.Lock()
	ibs := ris.ibs
	if len(ibs) == 0 {
		ib := getInmemoryBlock()
		ibs = append(ibs, ib)
		ris.ibs = ibs
	}
	ib := ibs[len(ibs)-1]
	for _, item := range items {
		if !ib.Add(item) {
			ib = getInmemoryBlock()
			if !ib.Add(item) {
				putInmemoryBlock(ib)
				err = fmt.Errorf("cannot insert an item %q into an empty inmemoryBlock; it looks like the item is too large? len(item)=%d", item, len(item))
				break
			}
			ibs = append(ibs, ib)
			ris.ibs = ibs
		}
	}
	if len(ibs) >= maxBlocksPerShard {
		blocksToFlush = append(blocksToFlush, ibs...)
		for i := range ibs {
			ibs[i] = nil
		}
		ris.ibs = ibs[:0]
		atomic.StoreUint64(&ris.lastFlushTime, fasttime.UnixTimestamp())
	}
	ris.mu.Unlock()

	tb.mergeRawItemsBlocks(blocksToFlush, false)
	return err
}

type partWrapper struct {
	p *part

	mp *inmemoryPart

	refCount uint64

	isInMerge bool
}

func (pw *partWrapper) incRef() {
	atomic.AddUint64(&pw.refCount, 1)
}

func (pw *partWrapper) decRef() {
	n := atomic.AddUint64(&pw.refCount, ^uint64(0))
	if int64(n) < 0 {
		logger.Panicf("BUG: pw.refCount must be bigger than 0; got %d", int64(n))
	}
	if n > 0 {
		return
	}

	if pw.mp != nil {
		// Do not return pw.mp to pool via putInmemoryPart(),
		// since pw.mp size may be too big compared to other entries stored in the pool.
		// This may result in increased memory usage because of high fragmentation.
		pw.mp = nil
	}
	pw.p.MustClose()
	pw.p = nil
}

// OpenTable opens a table on the given path.
//
// Optional flushCallback is called every time new data batch is flushed
// to the underlying storage and becomes visible to search.
//
// Optional prepareBlock is called during merge before flushing the prepared block
// to persistent storage.
//
// The table is created if it doesn't exist yet.
func OpenTable(path string, flushCallback func(), prepareBlock PrepareBlockCallback, isReadOnly *uint32) (*Table, error) {
	path = filepath.Clean(path)
	logger.Infof("opening table %q...", path)
	startTime := time.Now()

	// Create a directory for the table if it doesn't exist yet.
	if err := fs.MkdirAllIfNotExist(path); err != nil {
		return nil, fmt.Errorf("cannot create directory %q: %w", path, err)
	}

	// Protect from concurrent opens.
	flockF, err := fs.CreateFlockFile(path)
	if err != nil {
		return nil, err
	}

	// Open table parts.
	pws, err := openParts(path)
	if err != nil {
		return nil, fmt.Errorf("cannot open table parts at %q: %w", path, err)
	}

	tb := &Table{
		path:          path,
		flushCallback: flushCallback,
		prepareBlock:  prepareBlock,
		isReadOnly:    isReadOnly,
		parts:         pws,
		mergeIdx:      uint64(time.Now().UnixNano()),
		flockF:        flockF,
		stopCh:        make(chan struct{}),
	}
	tb.rawItems.init()
	tb.startPartMergers()
	tb.startRawItemsFlusher()

	var m TableMetrics
	tb.UpdateMetrics(&m)
	logger.Infof("table %q has been opened in %.3f seconds; partsCount: %d; blocksCount: %d, itemsCount: %d; sizeBytes: %d",
		path, time.Since(startTime).Seconds(), m.PartsCount, m.BlocksCount, m.ItemsCount, m.SizeBytes)

	if flushCallback != nil {
		tb.flushCallbackWorkerWG.Add(1)
		go func() {
			// call flushCallback once per 10 seconds in order to improve the effectiveness of caches,
			// which are reset by the flushCallback.
			tc := time.NewTicker(10 * time.Second)
			for {
				select {
				case <-tb.stopCh:
					tb.flushCallback()
					tb.flushCallbackWorkerWG.Done()
					return
				case <-tc.C:
					if atomic.CompareAndSwapUint32(&tb.needFlushCallbackCall, 1, 0) {
						tb.flushCallback()
					}
				}
			}
		}()
	}

	return tb, nil
}

// MustClose closes the table.
func (tb *Table) MustClose() {
	close(tb.stopCh)

	logger.Infof("waiting for background workers to stop on %q...", tb.path)
	startTime := time.Now()
	tb.wg.Wait()
	logger.Infof("background workers stopped in %.3f seconds on %q", time.Since(startTime).Seconds(), tb.path)

	logger.Infof("flushing inmemory parts to files on %q...", tb.path)
	startTime = time.Now()

	// Flush raw items the last time before exit.
	tb.flushPendingItems(true)

	// Flush inmemory parts to disk.
	var pws []*partWrapper
	tb.partsLock.Lock()
	for _, pw := range tb.parts {
		if pw.mp == nil {
			continue
		}
		if pw.isInMerge {
			logger.Panicf("BUG: the inmemory part %s mustn't be in merge after stopping parts merger in %q", &pw.mp.ph, tb.path)
		}
		pw.isInMerge = true
		pws = append(pws, pw)
	}
	tb.partsLock.Unlock()

	if err := tb.mergePartsOptimal(pws); err != nil {
		logger.Panicf("FATAL: cannot flush inmemory parts to files in %q: %s", tb.path, err)
	}
	logger.Infof("%d inmemory parts have been flushed to files in %.3f seconds on %q", len(pws), time.Since(startTime).Seconds(), tb.path)

	logger.Infof("waiting for flush callback worker to stop on %q...", tb.path)
	startTime = time.Now()
	tb.flushCallbackWorkerWG.Wait()
	logger.Infof("flush callback worker stopped in %.3f seconds on %q", time.Since(startTime).Seconds(), tb.path)

	// Remove references to parts from the tb, so they may be eventually closed
	// after all the searches are done.
	tb.partsLock.Lock()
	parts := tb.parts
	tb.parts = nil
	tb.partsLock.Unlock()

	for _, pw := range parts {
		pw.decRef()
	}

	// Release flockF
	if err := tb.flockF.Close(); err != nil {
		logger.Panicf("FATAL:cannot close %q: %s", tb.flockF.Name(), err)
	}
}

// Path returns the path to tb on the filesystem.
func (tb *Table) Path() string {
	return tb.path
}

// TableMetrics contains essential metrics for the Table.
type TableMetrics struct {
	ActiveMerges        uint64
	MergesCount         uint64
	ItemsMerged         uint64
	AssistedMerges      uint64
	ItemsAdded          uint64
	ItemsAddedSizeBytes uint64

	PendingItems uint64

	PartsCount uint64

	BlocksCount uint64
	ItemsCount  uint64
	SizeBytes   uint64

	DataBlocksCacheSize         uint64
	DataBlocksCacheSizeBytes    uint64
	DataBlocksCacheSizeMaxBytes uint64
	DataBlocksCacheRequests     uint64
	DataBlocksCacheMisses       uint64

	IndexBlocksCacheSize         uint64
	IndexBlocksCacheSizeBytes    uint64
	IndexBlocksCacheSizeMaxBytes uint64
	IndexBlocksCacheRequests     uint64
	IndexBlocksCacheMisses       uint64

	PartsRefCount uint64
}

// UpdateMetrics updates m with metrics from tb.
func (tb *Table) UpdateMetrics(m *TableMetrics) {
	m.ActiveMerges += atomic.LoadUint64(&tb.activeMerges)
	m.MergesCount += atomic.LoadUint64(&tb.mergesCount)
	m.ItemsMerged += atomic.LoadUint64(&tb.itemsMerged)
	m.AssistedMerges += atomic.LoadUint64(&tb.assistedMerges)
	m.ItemsAdded += atomic.LoadUint64(&tb.itemsAdded)
	m.ItemsAddedSizeBytes += atomic.LoadUint64(&tb.itemsAddedSizeBytes)

	m.PendingItems += uint64(tb.rawItems.Len())

	tb.partsLock.Lock()
	m.PartsCount += uint64(len(tb.parts))
	for _, pw := range tb.parts {
		p := pw.p

		m.BlocksCount += p.ph.blocksCount
		m.ItemsCount += p.ph.itemsCount
		m.SizeBytes += p.size

		m.PartsRefCount += atomic.LoadUint64(&pw.refCount)
	}
	tb.partsLock.Unlock()

	m.DataBlocksCacheSize = uint64(ibCache.Len())
	m.DataBlocksCacheSizeBytes = uint64(ibCache.SizeBytes())
	m.DataBlocksCacheSizeMaxBytes = uint64(ibCache.SizeMaxBytes())
	m.DataBlocksCacheRequests = ibCache.Requests()
	m.DataBlocksCacheMisses = ibCache.Misses()

	m.IndexBlocksCacheSize = uint64(idxbCache.Len())
	m.IndexBlocksCacheSizeBytes = uint64(idxbCache.SizeBytes())
	m.IndexBlocksCacheSizeMaxBytes = uint64(idxbCache.SizeMaxBytes())
	m.IndexBlocksCacheRequests = idxbCache.Requests()
	m.IndexBlocksCacheMisses = idxbCache.Misses()
}

// AddItems adds the given items to the tb.
func (tb *Table) AddItems(items [][]byte) error {
	if err := tb.rawItems.addItems(tb, items); err != nil {
		return fmt.Errorf("cannot insert data into %q: %w", tb.path, err)
	}
	atomic.AddUint64(&tb.itemsAdded, uint64(len(items)))
	n := 0
	for _, item := range items {
		n += len(item)
	}
	atomic.AddUint64(&tb.itemsAddedSizeBytes, uint64(n))
	return nil
}

// getParts appends parts snapshot to dst and returns it.
//
// The appended parts must be released with putParts.
func (tb *Table) getParts(dst []*partWrapper) []*partWrapper {
	tb.partsLock.Lock()
	for _, pw := range tb.parts {
		pw.incRef()
	}
	dst = append(dst, tb.parts...)
	tb.partsLock.Unlock()

	return dst
}

// putParts releases the given pws obtained via getParts.
func (tb *Table) putParts(pws []*partWrapper) {
	for _, pw := range pws {
		pw.decRef()
	}
}

func (tb *Table) startRawItemsFlusher() {
	tb.wg.Add(1)
	go func() {
		tb.rawItemsFlusher()
		tb.wg.Done()
	}()
}

func (tb *Table) rawItemsFlusher() {
	ticker := time.NewTicker(rawItemsFlushInterval)
	defer ticker.Stop()
	for {
		select {
		case <-tb.stopCh:
			return
		case <-ticker.C:
			tb.flushPendingItems(false)
		}
	}
}

func (tb *Table) mergePartsOptimal(pws []*partWrapper) error {
	for len(pws) > defaultPartsToMerge {
		pwsChunk := pws[:defaultPartsToMerge]
		pws = pws[defaultPartsToMerge:]
		if err := tb.mergeParts(pwsChunk, nil, false); err != nil {
			tb.releasePartsToMerge(pws)
			return fmt.Errorf("cannot merge %d parts: %w", defaultPartsToMerge, err)
		}
	}
	if len(pws) == 0 {
		return nil
	}
	if err := tb.mergeParts(pws, nil, false); err != nil {
		return fmt.Errorf("cannot merge %d parts: %w", len(pws), err)
	}
	return nil
}

// DebugFlush flushes all the added items to the storage,
// so they become visible to search.
//
// This function is only for debugging and testing.
func (tb *Table) DebugFlush() {
	tb.flushPendingItems(true)

	// Wait for background flushers to finish.
	tb.rawItemsPendingFlushesWG.Wait()
}

func (tb *Table) flushPendingItems(isFinal bool) {
	tb.rawItems.flush(tb, isFinal)
}

func (riss *rawItemsShards) flush(tb *Table, isFinal bool) {
	tb.rawItemsPendingFlushesWG.Add(1)
	defer tb.rawItemsPendingFlushesWG.Done()

	var blocksToFlush []*inmemoryBlock
	for i := range riss.shards {
		blocksToFlush = riss.shards[i].appendBlocksToFlush(blocksToFlush, tb, isFinal)
	}
	tb.mergeRawItemsBlocks(blocksToFlush, isFinal)
}

func (ris *rawItemsShard) appendBlocksToFlush(dst []*inmemoryBlock, tb *Table, isFinal bool) []*inmemoryBlock {
	currentTime := fasttime.UnixTimestamp()
	flushSeconds := int64(rawItemsFlushInterval.Seconds())
	if flushSeconds <= 0 {
		flushSeconds = 1
	}
	lastFlushTime := atomic.LoadUint64(&ris.lastFlushTime)
	if !isFinal && currentTime <= lastFlushTime+uint64(flushSeconds) {
		// Fast path - nothing to flush
		return dst
	}
	// Slow path - move ris.ibs to dst
	ris.mu.Lock()
	ibs := ris.ibs
	dst = append(dst, ibs...)
	for i := range ibs {
		ibs[i] = nil
	}
	ris.ibs = ibs[:0]
	atomic.StoreUint64(&ris.lastFlushTime, currentTime)
	ris.mu.Unlock()
	return dst
}

func (tb *Table) mergeRawItemsBlocks(ibs []*inmemoryBlock, isFinal bool) {
	if len(ibs) == 0 {
		return
	}

	pws := make([]*partWrapper, 0, (len(ibs)+defaultPartsToMerge-1)/defaultPartsToMerge)
	var pwsLock sync.Mutex
	var wg sync.WaitGroup
	for len(ibs) > 0 {
		n := defaultPartsToMerge
		if n > len(ibs) {
			n = len(ibs)
		}
		wg.Add(1)
		go func(ibsPart []*inmemoryBlock) {
			defer wg.Done()
			pw := tb.mergeInmemoryBlocks(ibsPart)
			if pw == nil {
				return
			}
			pw.isInMerge = true
			pwsLock.Lock()
			pws = append(pws, pw)
			pwsLock.Unlock()
		}(ibs[:n])
		ibs = ibs[n:]
	}
	wg.Wait()
	if len(pws) > 0 {
		if err := tb.mergeParts(pws, nil, true); err != nil {
			logger.Panicf("FATAL: cannot merge raw parts: %s", err)
		}
		if tb.flushCallback != nil {
			if isFinal {
				tb.flushCallback()
			} else {
				atomic.CompareAndSwapUint32(&tb.needFlushCallbackCall, 0, 1)
			}
		}
	}

	for {
		tb.partsLock.Lock()
		ok := len(tb.parts) <= maxParts
		tb.partsLock.Unlock()
		if ok {
			return
		}

		// The added part exceeds maxParts count. Assist with merging other parts.
		//
		// Prioritize assisted merges over searches.
		storagepacelimiter.Search.Inc()
		err := tb.mergeExistingParts(false)
		storagepacelimiter.Search.Dec()
		if err == nil {
			atomic.AddUint64(&tb.assistedMerges, 1)
			continue
		}
		if errors.Is(err, errNothingToMerge) || errors.Is(err, errForciblyStopped) || errors.Is(err, errReadOnlyMode) {
			return
		}
		logger.Panicf("FATAL: cannot merge small parts: %s", err)
	}
}

func (tb *Table) mergeInmemoryBlocks(ibs []*inmemoryBlock) *partWrapper {
	atomic.AddUint64(&tb.mergesCount, 1)
	atomic.AddUint64(&tb.activeMerges, 1)
	defer atomic.AddUint64(&tb.activeMerges, ^uint64(0))

	outItemsCount := uint64(0)
	for _, ib := range ibs {
		outItemsCount += uint64(ib.Len())
	}

	// Prepare blockStreamReaders for source blocks.
	bsrs := make([]*blockStreamReader, 0, len(ibs))
	for _, ib := range ibs {
		if len(ib.items) == 0 {
			continue
		}
		bsr := getBlockStreamReader()
		bsr.InitFromInmemoryBlock(ib)
		putInmemoryBlock(ib)
		bsrs = append(bsrs, bsr)
	}
	if len(bsrs) == 0 {
		return nil
	}
	if len(bsrs) == 1 {
		// Nothing to merge. Just return a single inmemory part.
		mp := &inmemoryPart{}
		mp.Init(&bsrs[0].Block)
		p := mp.NewPart()
		return &partWrapper{
			p:        p,
			mp:       mp,
			refCount: 1,
		}
	}

	// Prepare blockStreamWriter for destination part.
	compressLevel := getCompressLevel(outItemsCount)
	bsw := getBlockStreamWriter()
	mpDst := &inmemoryPart{}
	bsw.InitFromInmemoryPart(mpDst, compressLevel)

	// Merge parts.
	// The merge shouldn't be interrupted by stopCh,
	// since it may be final after stopCh is closed.
	err := mergeBlockStreams(&mpDst.ph, bsw, bsrs, tb.prepareBlock, nil, &tb.itemsMerged)
	if err != nil {
		logger.Panicf("FATAL: cannot merge inmemoryBlocks: %s", err)
	}
	putBlockStreamWriter(bsw)
	for _, bsr := range bsrs {
		putBlockStreamReader(bsr)
	}

	p := mpDst.NewPart()
	return &partWrapper{
		p:        p,
		mp:       mpDst,
		refCount: 1,
	}
}

func (tb *Table) startPartMergers() {
	for i := 0; i < mergeWorkersCount; i++ {
		tb.wg.Add(1)
		go func() {
			if err := tb.partMerger(); err != nil {
				logger.Panicf("FATAL: unrecoverable error when merging parts in %q: %s", tb.path, err)
			}
			tb.wg.Done()
		}()
	}
}

func (tb *Table) canBackgroundMerge() bool {
	return atomic.LoadUint32(tb.isReadOnly) == 0
}

var errReadOnlyMode = fmt.Errorf("storage is in readonly mode")

func (tb *Table) mergeExistingParts(isFinal bool) error {
	if !tb.canBackgroundMerge() {
		// Do not perform background merge in read-only mode
		// in order to prevent from disk space shortage.
		// See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/2603
		return errReadOnlyMode
	}
	n := fs.MustGetFreeSpace(tb.path)
	// Divide free space by the max number of concurrent merges.
	maxOutBytes := n / uint64(mergeWorkersCount)
	if maxOutBytes > maxPartSize {
		maxOutBytes = maxPartSize
	}

	tb.partsLock.Lock()
	pws := getPartsToMerge(tb.parts, maxOutBytes, isFinal)
	tb.partsLock.Unlock()

	return tb.mergeParts(pws, tb.stopCh, false)
}

const (
	minMergeSleepTime = time.Millisecond
	maxMergeSleepTime = time.Second
)

func (tb *Table) partMerger() error {
	sleepTime := minMergeSleepTime
	var lastMergeTime uint64
	isFinal := false
	t := time.NewTimer(sleepTime)
	for {
		err := tb.mergeExistingParts(isFinal)
		if err == nil {
			// Try merging additional parts.
			sleepTime = minMergeSleepTime
			lastMergeTime = fasttime.UnixTimestamp()
			isFinal = false
			continue
		}
		if errors.Is(err, errForciblyStopped) {
			// The merger has been stopped.
			return nil
		}
		if !errors.Is(err, errNothingToMerge) && !errors.Is(err, errReadOnlyMode) {
			return err
		}
		if fasttime.UnixTimestamp()-lastMergeTime > 30 {
			// We have free time for merging into bigger parts.
			// This should improve select performance.
			lastMergeTime = fasttime.UnixTimestamp()
			isFinal = true
			continue
		}

		// Nothing to merge. Sleep for a while and try again.
		sleepTime *= 2
		if sleepTime > maxMergeSleepTime {
			sleepTime = maxMergeSleepTime
		}
		select {
		case <-tb.stopCh:
			return nil
		case <-t.C:
			t.Reset(sleepTime)
		}
	}
}

var errNothingToMerge = fmt.Errorf("nothing to merge")

func (tb *Table) releasePartsToMerge(pws []*partWrapper) {
	tb.partsLock.Lock()
	for _, pw := range pws {
		if !pw.isInMerge {
			logger.Panicf("BUG: missing isInMerge flag on the part %q", pw.p.path)
		}
		pw.isInMerge = false
	}
	tb.partsLock.Unlock()
}

// mergeParts merges pws.
//
// Merging is immediately stopped if stopCh is closed.
//
// All the parts inside pws must have isInMerge field set to true.
func (tb *Table) mergeParts(pws []*partWrapper, stopCh <-chan struct{}, isOuterParts bool) error {
	if len(pws) == 0 {
		// Nothing to merge.
		return errNothingToMerge
	}
	defer tb.releasePartsToMerge(pws)

	atomic.AddUint64(&tb.mergesCount, 1)
	atomic.AddUint64(&tb.activeMerges, 1)
	defer atomic.AddUint64(&tb.activeMerges, ^uint64(0))

	startTime := time.Now()

	// Prepare blockStreamReaders for source parts.
	bsrs := make([]*blockStreamReader, 0, len(pws))
	defer func() {
		for _, bsr := range bsrs {
			putBlockStreamReader(bsr)
		}
	}()
	for _, pw := range pws {
		bsr := getBlockStreamReader()
		if pw.mp != nil {
			if !isOuterParts {
				logger.Panicf("BUG: inmemory part must be always outer")
			}
			bsr.InitFromInmemoryPart(pw.mp)
		} else {
			if err := bsr.InitFromFilePart(pw.p.path); err != nil {
				return fmt.Errorf("cannot open source part for merging: %w", err)
			}
		}
		bsrs = append(bsrs, bsr)
	}

	outItemsCount := uint64(0)
	outBlocksCount := uint64(0)
	for _, pw := range pws {
		outItemsCount += pw.p.ph.itemsCount
		outBlocksCount += pw.p.ph.blocksCount
	}
	nocache := true
	if outItemsCount < maxItemsPerCachedPart() {
		// Cache small (i.e. recent) output parts in OS file cache,
		// since there is high chance they will be read soon.
		nocache = false
	}

	// Prepare blockStreamWriter for destination part.
	mergeIdx := tb.nextMergeIdx()
	tmpPartPath := fmt.Sprintf("%s/tmp/%016X", tb.path, mergeIdx)
	bsw := getBlockStreamWriter()
	compressLevel := getCompressLevel(outItemsCount)
	if err := bsw.InitFromFilePart(tmpPartPath, nocache, compressLevel); err != nil {
		return fmt.Errorf("cannot create destination part %q: %w", tmpPartPath, err)
	}

	// Merge parts into a temporary location.
	var ph partHeader
	err := mergeBlockStreams(&ph, bsw, bsrs, tb.prepareBlock, stopCh, &tb.itemsMerged)
	putBlockStreamWriter(bsw)
	if err != nil {
		return fmt.Errorf("error when merging parts to %q: %w", tmpPartPath, err)
	}
	if err := ph.WriteMetadata(tmpPartPath); err != nil {
		return fmt.Errorf("cannot write metadata to destination part %q: %w", tmpPartPath, err)
	}

	// Close bsrs (aka source parts).
	for _, bsr := range bsrs {
		putBlockStreamReader(bsr)
	}
	bsrs = nil

	// Create a transaction for atomic deleting old parts and moving
	// new part to its destination place.
	var bb bytesutil.ByteBuffer
	for _, pw := range pws {
		if pw.mp == nil {
			fmt.Fprintf(&bb, "%s\n", pw.p.path)
		}
	}
	dstPartPath := ph.Path(tb.path, mergeIdx)
	fmt.Fprintf(&bb, "%s -> %s\n", tmpPartPath, dstPartPath)
	txnPath := fmt.Sprintf("%s/txn/%016X", tb.path, mergeIdx)
	if err := fs.WriteFileAtomically(txnPath, bb.B, false); err != nil {
		return fmt.Errorf("cannot create transaction file %q: %w", txnPath, err)
	}

	// Run the created transaction.
	if err := runTransaction(&tb.snapshotLock, tb.path, txnPath); err != nil {
		return fmt.Errorf("cannot execute transaction %q: %w", txnPath, err)
	}

	// Open the merged part.
	newP, err := openFilePart(dstPartPath)
	if err != nil {
		return fmt.Errorf("cannot open merged part %q: %w", dstPartPath, err)
	}
	newPSize := newP.size
	newPW := &partWrapper{
		p:        newP,
		refCount: 1,
	}

	// Atomically remove old parts and add new part.
	m := make(map[*partWrapper]bool, len(pws))
	for _, pw := range pws {
		m[pw] = true
	}
	if len(m) != len(pws) {
		logger.Panicf("BUG: %d duplicate parts found in the merge of %d parts", len(pws)-len(m), len(pws))
	}
	removedParts := 0
	tb.partsLock.Lock()
	tb.parts, removedParts = removeParts(tb.parts, m)
	tb.parts = append(tb.parts, newPW)
	tb.partsLock.Unlock()
	if removedParts != len(m) {
		if !isOuterParts {
			logger.Panicf("BUG: unexpected number of parts removed; got %d; want %d", removedParts, len(m))
		}
		if removedParts != 0 {
			logger.Panicf("BUG: removed non-zero outer parts: %d", removedParts)
		}
	}

	// Remove partition references from old parts.
	for _, pw := range pws {
		pw.decRef()
	}

	d := time.Since(startTime)
	if d > 30*time.Second {
		logger.Infof("merged %d items across %d blocks in %.3f seconds at %d items/sec to %q; sizeBytes: %d",
			outItemsCount, outBlocksCount, d.Seconds(), int(float64(outItemsCount)/d.Seconds()), dstPartPath, newPSize)
	}

	return nil
}

func getCompressLevel(itemsCount uint64) int {
	if itemsCount <= 1<<16 {
		// -5 is the minimum supported compression for zstd.
		// See https://github.com/facebook/zstd/releases/tag/v1.3.4
		return -5
	}
	if itemsCount <= 1<<17 {
		return -4
	}
	if itemsCount <= 1<<18 {
		return -3
	}
	if itemsCount <= 1<<19 {
		return -2
	}
	if itemsCount <= 1<<20 {
		return -1
	}
	if itemsCount <= 1<<22 {
		return 1
	}
	if itemsCount <= 1<<25 {
		return 2
	}
	if itemsCount <= 1<<28 {
		return 3
	}
	return 4
}

func (tb *Table) nextMergeIdx() uint64 {
	return atomic.AddUint64(&tb.mergeIdx, 1)
}

var mergeWorkersCount = cgroup.AvailableCPUs()

func openParts(path string) ([]*partWrapper, error) {
	// The path can be missing after restoring from backup, so create it if needed.
	if err := fs.MkdirAllIfNotExist(path); err != nil {
		return nil, err
	}
	fs.MustRemoveTemporaryDirs(path)
	d, err := os.Open(path)
	if err != nil {
		return nil, fmt.Errorf("cannot open difrectory: %w", err)
	}
	defer fs.MustClose(d)

	// Run remaining transactions and cleanup /txn and /tmp directories.
	// Snapshots cannot be created yet, so use fakeSnapshotLock.
	var fakeSnapshotLock sync.RWMutex
	if err := runTransactions(&fakeSnapshotLock, path); err != nil {
		return nil, fmt.Errorf("cannot run transactions: %w", err)
	}

	txnDir := path + "/txn"
	fs.MustRemoveDirAtomic(txnDir)
	if err := fs.MkdirAllFailIfExist(txnDir); err != nil {
		return nil, fmt.Errorf("cannot create %q: %w", txnDir, err)
	}

	tmpDir := path + "/tmp"
	fs.MustRemoveDirAtomic(tmpDir)
	if err := fs.MkdirAllFailIfExist(tmpDir); err != nil {
		return nil, fmt.Errorf("cannot create %q: %w", tmpDir, err)
	}

	fs.MustSyncPath(path)

	// Open parts.
	fis, err := d.Readdir(-1)
	if err != nil {
		return nil, fmt.Errorf("cannot read directory: %w", err)
	}
	var pws []*partWrapper
	for _, fi := range fis {
		if !fs.IsDirOrSymlink(fi) {
			// Skip non-directories.
			continue
		}
		fn := fi.Name()
		if isSpecialDir(fn) {
			// Skip special dirs.
			continue
		}
		partPath := path + "/" + fn
		if fs.IsEmptyDir(partPath) {
			// Remove empty directory, which can be left after unclean shutdown on NFS.
			// See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/1142
			fs.MustRemoveDirAtomic(partPath)
			continue
		}
		p, err := openFilePart(partPath)
		if err != nil {
			mustCloseParts(pws)
			return nil, fmt.Errorf("cannot open part %q: %w", partPath, err)
		}
		pw := &partWrapper{
			p:        p,
			refCount: 1,
		}
		pws = append(pws, pw)
	}

	return pws, nil
}

func mustCloseParts(pws []*partWrapper) {
	for _, pw := range pws {
		if pw.refCount != 1 {
			logger.Panicf("BUG: unexpected refCount when closing part %q: %d; want 1", pw.p.path, pw.refCount)
		}
		pw.p.MustClose()
	}
}

// CreateSnapshotAt creates tb snapshot in the given dstDir.
//
// Snapshot is created using linux hard links, so it is usually created
// very quickly.
func (tb *Table) CreateSnapshotAt(dstDir string) error {
	logger.Infof("creating Table snapshot of %q...", tb.path)
	startTime := time.Now()

	var err error
	srcDir := tb.path
	srcDir, err = filepath.Abs(srcDir)
	if err != nil {
		return fmt.Errorf("cannot obtain absolute dir for %q: %w", srcDir, err)
	}
	dstDir, err = filepath.Abs(dstDir)
	if err != nil {
		return fmt.Errorf("cannot obtain absolute dir for %q: %w", dstDir, err)
	}
	if strings.HasPrefix(dstDir, srcDir+"/") {
		return fmt.Errorf("cannot create snapshot %q inside the data dir %q", dstDir, srcDir)
	}

	// Flush inmemory items to disk.
	tb.flushPendingItems(true)

	// The snapshot must be created under the lock in order to prevent from
	// concurrent modifications via runTransaction.
	tb.snapshotLock.Lock()
	defer tb.snapshotLock.Unlock()

	if err := fs.MkdirAllFailIfExist(dstDir); err != nil {
		return fmt.Errorf("cannot create snapshot dir %q: %w", dstDir, err)
	}

	d, err := os.Open(srcDir)
	if err != nil {
		return fmt.Errorf("cannot open difrectory: %w", err)
	}
	defer fs.MustClose(d)

	fis, err := d.Readdir(-1)
	if err != nil {
		return fmt.Errorf("cannot read directory: %w", err)
	}
	for _, fi := range fis {
		fn := fi.Name()
		if !fs.IsDirOrSymlink(fi) {
			// Skip non-directories.
			continue
		}
		if isSpecialDir(fn) {
			// Skip special dirs.
			continue
		}
		srcPartPath := srcDir + "/" + fn
		dstPartPath := dstDir + "/" + fn
		if err := fs.HardLinkFiles(srcPartPath, dstPartPath); err != nil {
			return fmt.Errorf("cannot create hard links from %q to %q: %w", srcPartPath, dstPartPath, err)
		}
	}

	fs.MustSyncPath(dstDir)
	parentDir := filepath.Dir(dstDir)
	fs.MustSyncPath(parentDir)

	logger.Infof("created Table snapshot of %q at %q in %.3f seconds", srcDir, dstDir, time.Since(startTime).Seconds())
	return nil
}

func runTransactions(txnLock *sync.RWMutex, path string) error {
	// Wait until all the previous pending transaction deletions are finished.
	pendingTxnDeletionsWG.Wait()

	// Make sure all the current transaction deletions are finished before exiting.
	defer pendingTxnDeletionsWG.Wait()

	txnDir := path + "/txn"
	d, err := os.Open(txnDir)
	if err != nil {
		if os.IsNotExist(err) {
			return nil
		}
		return fmt.Errorf("cannot open %q: %w", txnDir, err)
	}
	defer fs.MustClose(d)

	fis, err := d.Readdir(-1)
	if err != nil {
		return fmt.Errorf("cannot read directory %q: %w", d.Name(), err)
	}

	// Sort transaction files by id, since transactions must be ordered.
	sort.Slice(fis, func(i, j int) bool {
		return fis[i].Name() < fis[j].Name()
	})

	for _, fi := range fis {
		fn := fi.Name()
		if fs.IsTemporaryFileName(fn) {
			// Skip temporary files, which could be left after unclean shutdown.
			continue
		}
		txnPath := txnDir + "/" + fn
		if err := runTransaction(txnLock, path, txnPath); err != nil {
			return fmt.Errorf("cannot run transaction from %q: %w", txnPath, err)
		}
	}
	return nil
}

func runTransaction(txnLock *sync.RWMutex, pathPrefix, txnPath string) error {
	// The transaction must run under read lock in order to provide
	// consistent snapshots with Table.CreateSnapshot().
	txnLock.RLock()
	defer txnLock.RUnlock()

	data, err := os.ReadFile(txnPath)
	if err != nil {
		return fmt.Errorf("cannot read transaction file: %w", err)
	}
	if len(data) > 0 && data[len(data)-1] == '\n' {
		data = data[:len(data)-1]
	}
	paths := strings.Split(string(data), "\n")

	if len(paths) == 0 {
		return fmt.Errorf("empty transaction")
	}
	rmPaths := paths[:len(paths)-1]
	mvPaths := strings.Split(paths[len(paths)-1], " -> ")
	if len(mvPaths) != 2 {
		return fmt.Errorf("invalid last line in the transaction file: got %q; must contain `srcPath -> dstPath`", paths[len(paths)-1])
	}

	// Remove old paths. It is OK if certain paths don't exist.
	for _, path := range rmPaths {
		path, err := validatePath(pathPrefix, path)
		if err != nil {
			return fmt.Errorf("invalid path to remove: %w", err)
		}
		fs.MustRemoveDirAtomic(path)
	}

	// Move the new part to new directory.
	srcPath := mvPaths[0]
	dstPath := mvPaths[1]
	srcPath, err = validatePath(pathPrefix, srcPath)
	if err != nil {
		return fmt.Errorf("invalid source path to rename: %w", err)
	}
	dstPath, err = validatePath(pathPrefix, dstPath)
	if err != nil {
		return fmt.Errorf("invalid destination path to rename: %w", err)
	}
	if fs.IsPathExist(srcPath) {
		if err := os.Rename(srcPath, dstPath); err != nil {
			return fmt.Errorf("cannot rename %q to %q: %w", srcPath, dstPath, err)
		}
	} else if !fs.IsPathExist(dstPath) {
		// Emit info message for the expected condition after unclean shutdown on NFS disk.
		// The dstPath part may be missing because it could be already merged into bigger part
		// while old source parts for the current txn weren't still deleted due to NFS locks.
		logger.Infof("cannot find both source and destination paths: %q -> %q; this may be the case after unclean shutdown (OOM, `kill -9`, hard reset) on NFS disk",
			srcPath, dstPath)
	}

	// Flush pathPrefix directory metadata to the underying storage.
	fs.MustSyncPath(pathPrefix)

	pendingTxnDeletionsWG.Add(1)
	go func() {
		defer pendingTxnDeletionsWG.Done()
		if err := os.Remove(txnPath); err != nil {
			logger.Errorf("cannot remove transaction file %q: %s", txnPath, err)
		}
	}()

	return nil
}

var pendingTxnDeletionsWG syncwg.WaitGroup

func validatePath(pathPrefix, path string) (string, error) {
	var err error

	pathPrefix, err = filepath.Abs(pathPrefix)
	if err != nil {
		return path, fmt.Errorf("cannot determine absolute path for pathPrefix=%q: %w", pathPrefix, err)
	}

	path, err = filepath.Abs(path)
	if err != nil {
		return path, fmt.Errorf("cannot determine absolute path for %q: %w", path, err)
	}
	if !strings.HasPrefix(path, pathPrefix+"/") {
		return path, fmt.Errorf("invalid path %q; must start with %q", path, pathPrefix+"/")
	}
	return path, nil
}

// getPartsToMerge returns optimal parts to merge from pws.
//
// if isFinal is set, then merge harder.
//
// The summary size of the returned parts must be smaller than the maxOutBytes.
func getPartsToMerge(pws []*partWrapper, maxOutBytes uint64, isFinal bool) []*partWrapper {
	pwsRemaining := make([]*partWrapper, 0, len(pws))
	for _, pw := range pws {
		if !pw.isInMerge {
			pwsRemaining = append(pwsRemaining, pw)
		}
	}
	maxPartsToMerge := defaultPartsToMerge
	var dst []*partWrapper
	if isFinal {
		for len(dst) == 0 && maxPartsToMerge >= finalPartsToMerge {
			dst = appendPartsToMerge(dst[:0], pwsRemaining, maxPartsToMerge, maxOutBytes)
			maxPartsToMerge--
		}
	} else {
		dst = appendPartsToMerge(dst[:0], pwsRemaining, maxPartsToMerge, maxOutBytes)
	}
	for _, pw := range dst {
		if pw.isInMerge {
			logger.Panicf("BUG: partWrapper.isInMerge is already set")
		}
		pw.isInMerge = true
	}
	return dst
}

// minMergeMultiplier is the minimum multiplier for the size of the output part
// compared to the size of the maximum input part for the merge.
//
// Higher value reduces write amplification (disk write IO induced by the merge),
// while increases the number of unmerged parts.
// The 1.7 is good enough for production workloads.
const minMergeMultiplier = 1.7

// appendPartsToMerge finds optimal parts to merge from src, appends
// them to dst and returns the result.
func appendPartsToMerge(dst, src []*partWrapper, maxPartsToMerge int, maxOutBytes uint64) []*partWrapper {
	if len(src) < 2 {
		// There is no need in merging zero or one part :)
		return dst
	}
	if maxPartsToMerge < 2 {
		logger.Panicf("BUG: maxPartsToMerge cannot be smaller than 2; got %d", maxPartsToMerge)
	}

	// Filter out too big parts.
	// This should reduce N for O(n^2) algorithm below.
	maxInPartBytes := uint64(float64(maxOutBytes) / minMergeMultiplier)
	tmp := make([]*partWrapper, 0, len(src))
	for _, pw := range src {
		if pw.p.size > maxInPartBytes {
			continue
		}
		tmp = append(tmp, pw)
	}
	src = tmp

	// Sort src parts by size.
	sort.Slice(src, func(i, j int) bool { return src[i].p.size < src[j].p.size })

	maxSrcParts := maxPartsToMerge
	if maxSrcParts > len(src) {
		maxSrcParts = len(src)
	}
	minSrcParts := (maxSrcParts + 1) / 2
	if minSrcParts < 2 {
		minSrcParts = 2
	}

	// Exhaustive search for parts giving the lowest write amplification when merged.
	var pws []*partWrapper
	maxM := float64(0)
	for i := minSrcParts; i <= maxSrcParts; i++ {
		for j := 0; j <= len(src)-i; j++ {
			a := src[j : j+i]
			if a[0].p.size*uint64(len(a)) < a[len(a)-1].p.size {
				// Do not merge parts with too big difference in size,
				// since this results in unbalanced merges.
				continue
			}
			outBytes := uint64(0)
			for _, pw := range a {
				outBytes += pw.p.size
			}
			if outBytes > maxOutBytes {
				// There is no sense in checking the remaining bigger parts.
				break
			}
			m := float64(outBytes) / float64(a[len(a)-1].p.size)
			if m < maxM {
				continue
			}
			maxM = m
			pws = a
		}
	}

	minM := float64(maxPartsToMerge) / 2
	if minM < minMergeMultiplier {
		minM = minMergeMultiplier
	}
	if maxM < minM {
		// There is no sense in merging parts with too small m,
		// since this leads to high disk write IO.
		return dst
	}
	return append(dst, pws...)
}

func removeParts(pws []*partWrapper, partsToRemove map[*partWrapper]bool) ([]*partWrapper, int) {
	removedParts := 0
	dst := pws[:0]
	for _, pw := range pws {
		if !partsToRemove[pw] {
			dst = append(dst, pw)
			continue
		}
		removedParts++
	}
	return dst, removedParts
}

func isSpecialDir(name string) bool {
	// Snapshots and cache dirs aren't used anymore.
	// Keep them here for backwards compatibility.
	return name == "tmp" || name == "txn" || name == "snapshots" || name == "cache"
}