[#314] wc: Simplify WC

Do not use write-cache as a read cache: always remove objects from the
WC, not only if an object hasn't been used for some time (LRU cache is
dropped). Use object size (in bytes) as a metric of used space, not an
approximate (and too inaccurate) maximum stored objects number.

Signed-off-by: Pavel Karpy <p.karpy@yadro.com>

pkg/local_object_storage/shard/get.go

@@ -91,7 +91,7 @@ func (s *Shard) Get(ctx context.Context, prm GetPrm) (GetRes, error) {
 		return res.Object, nil
 	}
 
-	wc := func(c writecache.Cache) (*objectSDK.Object, error) {
+	wc := func(c *writecache.Cache) (*objectSDK.Object, error) {
 		return c.Get(ctx, prm.addr)
 	}
 
@@ -109,7 +109,7 @@ func (s *Shard) Get(ctx context.Context, prm GetPrm) (GetRes, error) {
 var emptyStorageID = make([]byte, 0)
 
 // fetchObjectData looks through writeCache and blobStor to find object.
-func (s *Shard) fetchObjectData(ctx context.Context, addr oid.Address, skipMeta bool, cb storFetcher, wc func(w writecache.Cache) (*objectSDK.Object, error)) (*objectSDK.Object, bool, error) {
+func (s *Shard) fetchObjectData(ctx context.Context, addr oid.Address, skipMeta bool, cb storFetcher, wc func(w *writecache.Cache) (*objectSDK.Object, error)) (*objectSDK.Object, bool, error) {
 	var (
 		mErr error
 		mRes meta.ExistsRes

pkg/local_object_storage/shard/mode/mode.go

@@ -57,3 +57,7 @@ func (m Mode) NoMetabase() bool {
 func (m Mode) ReadOnly() bool {
 	return m&ReadOnly != 0
 }
+
+func (m Mode) ReadWrite() bool {
+	return m == 0
+}

pkg/local_object_storage/shard/range.go

@@ -104,7 +104,7 @@ func (s *Shard) GetRange(ctx context.Context, prm RngPrm) (RngRes, error) {
 		return obj, nil
 	}
 
-	wc := func(c writecache.Cache) (*object.Object, error) {
+	wc := func(c *writecache.Cache) (*object.Object, error) {
 		res, err := c.Get(ctx, prm.addr)
 		if err != nil {
 			return nil, err

pkg/local_object_storage/shard/shard.go

@@ -23,7 +23,7 @@ type Shard struct {
 
 	gc *gc
 
-	writeCache writecache.Cache
+	writeCache *writecache.Cache
 
 	blobStor *blobstor.BlobStor
 

pkg/local_object_storage/writecache/delete.go

@@ -2,10 +2,12 @@ package writecache
 
 import (
 	"context"
+	"errors"
 
 	"git.frostfs.info/TrueCloudLab/frostfs-api-go/v2/pkg/tracing"
 	"git.frostfs.info/TrueCloudLab/frostfs-node/pkg/local_object_storage/blobstor/common"
 	storagelog "git.frostfs.info/TrueCloudLab/frostfs-node/pkg/local_object_storage/internal/log"
+	apistatus "git.frostfs.info/TrueCloudLab/frostfs-sdk-go/client/status"
 	oid "git.frostfs.info/TrueCloudLab/frostfs-sdk-go/object/id"
 	"go.etcd.io/bbolt"
 	"go.opentelemetry.io/otel/attribute"
@@ -15,7 +17,7 @@ import (
 // Delete removes object from write-cache.
 //
 // Returns an error of type apistatus.ObjectNotFound if object is missing in write-cache.
-func (c *cache) Delete(ctx context.Context, addr oid.Address) error {
+func (c *Cache) Delete(ctx context.Context, addr oid.Address) error {
 	ctx, span := tracing.StartSpanFromContext(ctx, "writecache.Delete",
 		trace.WithAttributes(
 			attribute.String("address", addr.EncodeToString()),
@@ -31,14 +33,14 @@ func (c *cache) Delete(ctx context.Context, addr oid.Address) error {
 	saddr := addr.EncodeToString()
 
 	// Check disk cache.
-	var has int
+	var valLen int
 	_ = c.db.View(func(tx *bbolt.Tx) error {
 		b := tx.Bucket(defaultBucket)
-		has = len(b.Get([]byte(saddr)))
+		valLen = len(b.Get([]byte(saddr)))
 		return nil
 	})
 
-	if 0 < has {
+	if valLen > 0 {
 		err := c.db.Update(func(tx *bbolt.Tx) error {
 			b := tx.Bucket(defaultBucket)
 			err := b.Delete([]byte(saddr))
@@ -52,18 +54,32 @@ func (c *cache) Delete(ctx context.Context, addr oid.Address) error {
 			storagelog.StorageTypeField(wcStorageType),
 			storagelog.OpField("db DELETE"),
 		)
-		c.objCounters.DecDB()
+		c.objCounters.decDB(valLen)
 		return nil
 	}
 
-	_, err := c.fsTree.Delete(ctx, common.DeletePrm{Address: addr})
+	// While getting an object looks overheadly, it allows to
+	// get its size correctly without any additional memory/disk/CPU
+	// usage on the WC's side _for every object_. `Delete` is not
+	// expected to be called right after an object is put to the
+	// Write-cache often, and for non-existing objects (persisted
+	// to the main storage and dropped from the WC's storage) it
+	// is `os.Stat` vs `os.Remove` calls after all.
+	res, err := c.fsTree.Get(ctx, common.GetPrm{Address: addr})
+	if errors.As(err, new(apistatus.ObjectNotFound)) {
+		return nil
+	} else if err != nil {
+		return err
+	}
+
+	_, err = c.fsTree.Delete(ctx, common.DeletePrm{Address: addr})
 	if err == nil {
 		storagelog.Write(c.log,
 			storagelog.AddressField(saddr),
 			storagelog.StorageTypeField(wcStorageType),
 			storagelog.OpField("fstree DELETE"),
 		)
-		c.objCounters.DecFS()
+		c.objCounters.decFS(len(res.RawData))
 	}
 
 	return err

pkg/local_object_storage/writecache/flush.go

@@ -11,7 +11,9 @@ import (
 	objectCore "git.frostfs.info/TrueCloudLab/frostfs-node/pkg/core/object"
 	"git.frostfs.info/TrueCloudLab/frostfs-node/pkg/local_object_storage/blobstor"
 	"git.frostfs.info/TrueCloudLab/frostfs-node/pkg/local_object_storage/blobstor/common"
+	storagelog "git.frostfs.info/TrueCloudLab/frostfs-node/pkg/local_object_storage/internal/log"
 	meta "git.frostfs.info/TrueCloudLab/frostfs-node/pkg/local_object_storage/metabase"
+	apistatus "git.frostfs.info/TrueCloudLab/frostfs-sdk-go/client/status"
 	"git.frostfs.info/TrueCloudLab/frostfs-sdk-go/object"
 	oid "git.frostfs.info/TrueCloudLab/frostfs-sdk-go/object/id"
 	"github.com/mr-tron/base58"
@@ -33,11 +35,16 @@ const (
 	defaultFlushInterval = time.Second
 )
 
+type objWithData struct {
+	obj  *object.Object
+	data []byte
+}
+
 // runFlushLoop starts background workers which periodically flush objects to the blobstor.
-func (c *cache) runFlushLoop() {
+func (c *Cache) runFlushLoop() {
 	for i := 0; i < c.workersCount; i++ {
 		c.wg.Add(1)
-		go c.flushWorker(i)
+		go c.smallObjectsFlusher()
 	}
 
 	c.wg.Add(1)
@@ -56,34 +63,27 @@ func (c *cache) runFlushLoop() {
 		for {
 			select {
 			case <-tt.C:
-				c.flushDB()
+				c.flushSmallObjects()
 				tt.Reset(defaultFlushInterval)
-			case <-c.closeCh:
+			case <-c.workersChan:
 				return
 			}
 		}
 	}()
 }
 
-func (c *cache) flushDB() {
+func (c *Cache) flushSmallObjects() {
 	var lastKey []byte
 	var m []objectInfo
 	for {
 		select {
-		case <-c.closeCh:
+		case <-c.workersChan:
 			return
 		default:
 		}
 
 		m = m[:0]
 
-		c.modeMtx.RLock()
-		if c.readOnly() || !c.initialized.Load() {
-			c.modeMtx.RUnlock()
-			time.Sleep(time.Second)
-			continue
-		}
-
 		// We put objects in batches of fixed size to not interfere with main put cycle a lot.
 		_ = c.db.View(func(tx *bbolt.Tx) error {
 			b := tx.Bucket(defaultBucket)
@@ -117,38 +117,32 @@ func (c *cache) flushDB() {
 
 		var count int
 		for i := range m {
-			if c.flushed.Contains(m[i].addr) {
-				continue
-			}
-
 			obj := object.New()
-			if err := obj.Unmarshal(m[i].data); err != nil {
+			data := m[i].data
+
+			if err := obj.Unmarshal(data); err != nil {
 				continue
 			}
 
 			count++
 			select {
-			case c.flushCh <- obj:
-			case <-c.closeCh:
-				c.modeMtx.RUnlock()
+			case c.smallFlushCh <- objWithData{obj: obj, data: data}:
+			case <-c.workersChan:
 				return
 			}
 		}
 
 		if count == 0 {
-			c.modeMtx.RUnlock()
 			break
 		}
 
-		c.modeMtx.RUnlock()
-
 		c.log.Debug(logs.WritecacheTriedToFlushItemsFromWritecache,
 			zap.Int("count", count),
 			zap.String("start", base58.Encode(lastKey)))
 	}
 }
 
-func (c *cache) flushBigObjects(ctx context.Context) {
+func (c *Cache) flushBigObjects(ctx context.Context) {
 	tick := time.NewTicker(defaultFlushInterval * 10)
 	for {
 		select {
@@ -157,21 +151,18 @@ func (c *cache) flushBigObjects(ctx context.Context) {
 			if c.readOnly() {
 				c.modeMtx.RUnlock()
 				break
-			} else if !c.initialized.Load() {
-				c.modeMtx.RUnlock()
-				continue
 			}
 
 			_ = c.flushFSTree(ctx, true)
 
 			c.modeMtx.RUnlock()
-		case <-c.closeCh:
+		case <-c.workersChan:
 			return
 		}
 	}
 }
 
-func (c *cache) reportFlushError(msg string, addr string, err error) {
+func (c *Cache) reportFlushError(msg string, addr string, err error) {
 	if c.reportError != nil {
 		c.reportError(msg, err)
 	} else {
@@ -181,14 +172,18 @@ func (c *cache) reportFlushError(msg string, addr string, err error) {
 	}
 }
 
-func (c *cache) flushFSTree(ctx context.Context, ignoreErrors bool) error {
+func (c *Cache) flushFSTree(ctx context.Context, ignoreErrors bool) error {
 	var prm common.IteratePrm
 	prm.IgnoreErrors = ignoreErrors
 	prm.LazyHandler = func(addr oid.Address, f func() ([]byte, error)) error {
 		sAddr := addr.EncodeToString()
 
-		if _, ok := c.store.flushed.Peek(sAddr); ok {
-			return nil
+		select {
+		case <-c.workersChan:
+			return stopIter
+		case <-ctx.Done():
+			return ctx.Err()
+		default:
 		}
 
 		data, err := f()
@@ -210,7 +205,7 @@ func (c *cache) flushFSTree(ctx context.Context, ignoreErrors bool) error {
 			return err
 		}
 
-		err = c.flushObject(ctx, &obj, data)
+		err = c.flushObject(ctx, objWithData{obj: &obj, data: data})
 		if err != nil {
 			if ignoreErrors {
 				return nil
@@ -218,38 +213,56 @@ func (c *cache) flushFSTree(ctx context.Context, ignoreErrors bool) error {
 			return err
 		}
 
-		// mark object as flushed
-		c.flushed.Add(sAddr, false)
+		err = c.dropBigObject(ctx, addr, len(data))
+		if err != nil {
+			c.reportFlushError("can't drop an object from FSTree", sAddr, err)
+			if ignoreErrors {
+				return nil
+			}
+			return err
+		}
 
 		return nil
 	}
 
 	_, err := c.fsTree.Iterate(prm)
+	if errors.Is(err, stopIter) {
+		return nil
+	}
+
 	return err
 }
 
-// flushWorker writes objects to the main storage.
-func (c *cache) flushWorker(_ int) {
+// smallObjectsFlusher writes small objects to the main storage.
+func (c *Cache) smallObjectsFlusher() {
 	defer c.wg.Done()
 
-	var obj *object.Object
+	var objAndData objWithData
 	for {
 		// Give priority to direct put.
 		select {
-		case obj = <-c.flushCh:
-		case <-c.closeCh:
+		case objAndData = <-c.smallFlushCh:
+		case <-c.workersChan:
 			return
 		}
 
-		err := c.flushObject(context.TODO(), obj, nil)
+		err := c.flushObject(context.TODO(), objAndData)
 		if err == nil {
-			c.flushed.Add(objectCore.AddressOf(obj).EncodeToString(), true)
+			addr := objectCore.AddressOf(objAndData.obj)
+
+			err = c.dropSmallObject(context.TODO(), addr)
+			if err != nil {
+				c.reportFlushError("can't drop object from write-cache",
+					addr.EncodeToString(), err)
+			}
 		}
 	}
 }
 
 // flushObject is used to write object directly to the main storage.
-func (c *cache) flushObject(ctx context.Context, obj *object.Object, data []byte) error {
+func (c *Cache) flushObject(ctx context.Context, objAndData objWithData) error {
+	obj := objAndData.obj
+	data := objAndData.data
 	addr := objectCore.AddressOf(obj)
 
 	var prm common.PutPrm
@@ -272,6 +285,11 @@ func (c *cache) flushObject(ctx context.Context, obj *object.Object, data []byte
 
 	_, err = c.metabase.UpdateStorageID(updPrm)
 	if err != nil {
+		if errors.As(err, new(apistatus.ObjectNotFound)) || errors.As(err, new(apistatus.ObjectAlreadyRemoved)) {
+			// object info is outdated in the WC
+			return nil
+		}
+
 		c.reportFlushError("can't update object storage ID",
 			addr.EncodeToString(), err)
 	}
@@ -281,7 +299,7 @@ func (c *cache) flushObject(ctx context.Context, obj *object.Object, data []byte
 // Flush flushes all objects from the write-cache to the main storage.
 // Write-cache must be in readonly mode to ensure correctness of an operation and
 // to prevent interference with background flush workers.
-func (c *cache) Flush(ctx context.Context, ignoreErrors bool) error {
+func (c *Cache) Flush(ctx context.Context, ignoreErrors bool) error {
 	ctx, span := tracing.StartSpanFromContext(ctx, "writecache.Flush",
 		trace.WithAttributes(
 			attribute.Bool("ignore_errors", ignoreErrors),
@@ -294,21 +312,25 @@ func (c *cache) Flush(ctx context.Context, ignoreErrors bool) error {
 	return c.flush(ctx, ignoreErrors)
 }
 
-func (c *cache) flush(ctx context.Context, ignoreErrors bool) error {
+func (c *Cache) flush(ctx context.Context, ignoreErrors bool) error {
 	if err := c.flushFSTree(ctx, ignoreErrors); err != nil {
 		return err
 	}
 
-	return c.db.View(func(tx *bbolt.Tx) error {
+	var dbFunc func(func(*bbolt.Tx) error) error
+	if c.readOnly() {
+		dbFunc = c.db.View
+	} else {
+		dbFunc = c.db.Update
+	}
+
+	return dbFunc(func(tx *bbolt.Tx) error {
 		var addr oid.Address
 
 		b := tx.Bucket(defaultBucket)
 		cs := b.Cursor()
 		for k, data := cs.Seek(nil); k != nil; k, data = cs.Next() {
 			sa := string(k)
-			if _, ok := c.flushed.Peek(sa); ok {
-				continue
-			}
 
 			if err := addr.DecodeString(sa); err != nil {
 				c.reportFlushError("can't decode object address from the DB", sa, err)
@@ -327,10 +349,101 @@ func (c *cache) flush(ctx context.Context, ignoreErrors bool) error {
 				return err
 			}
 
-			if err := c.flushObject(ctx, &obj, data); err != nil {
+			err := c.flushObject(ctx, objWithData{obj: &obj, data: data})
+			if err != nil {
+				if ignoreErrors {
+					continue
+				}
+
 				return err
 			}
+
+			if c.readOnly() {
+				continue
+			}
+
+			removed, err := dropObject(tx, k)
+			if err != nil {
+				c.reportFlushError("can't drop an object from the DB", sa, err)
+				if ignoreErrors {
+					continue
+				}
+			}
+
+			storagelog.Write(c.log,
+				storagelog.AddressField(addr),
+				storagelog.StorageTypeField(wcStorageType),
+				storagelog.OpField("db DELETE"),
+			)
+			c.objCounters.decDB(removed)
 		}
 		return nil
 	})
 }
+
+func (c *Cache) dropSmallObject(ctx context.Context, addr oid.Address) error {
+	var removedBytes int
+	key := []byte(addr.EncodeToString())
+	var err error
+
+	err = c.db.Batch(func(tx *bbolt.Tx) error {
+		select {
+		case <-c.workersChan:
+			return nil
+		case <-ctx.Done():
+			return ctx.Err()
+		default:
+		}
+
+		removedBytes, err = dropObject(tx, key)
+
+		return err
+
+	})
+	if err != nil {
+		return err
+	}
+
+	storagelog.Write(c.log,
+		storagelog.AddressField(addr),
+		storagelog.StorageTypeField(wcStorageType),
+		storagelog.OpField("db DELETE"),
+	)
+
+	if removedBytes > 0 {
+		c.objCounters.decDB(removedBytes)
+	}
+
+	return nil
+}
+
+func dropObject(tx *bbolt.Tx, key []byte) (int, error) {
+	b := tx.Bucket(defaultBucket)
+
+	removedBytes := len(b.Get(key))
+	if removedBytes > 0 {
+		return removedBytes, b.Delete(key)
+	}
+
+	return 0, nil
+}
+
+func (c *Cache) dropBigObject(ctx context.Context, addr oid.Address, size int) error {
+	_, err := c.fsTree.Delete(ctx, common.DeletePrm{Address: addr})
+	if err != nil {
+		if errors.As(err, new(apistatus.ObjectNotFound)) {
+			return nil
+		}
+
+		return err
+	}
+
+	storagelog.Write(c.log,
+		storagelog.AddressField(addr),
+		storagelog.StorageTypeField(wcStorageType),
+		storagelog.OpField("fstree DELETE"),
+	)
+	c.objCounters.decFS(size)
+
+	return nil
+}

pkg/local_object_storage/writecache/flush_test.go

@@ -5,7 +5,6 @@ import (
 	"os"
 	"path/filepath"
 	"testing"
-	"time"
 
 	objectCore "git.frostfs.info/TrueCloudLab/frostfs-node/pkg/core/object"
 	"git.frostfs.info/TrueCloudLab/frostfs-node/pkg/local_object_storage/blobstor"
@@ -39,7 +38,7 @@ func TestFlush(t *testing.T) {
 		smallSize = 256
 	)
 
-	newCache := func(t *testing.T, opts ...Option) (Cache, *blobstor.BlobStor, *meta.DB) {
+	newCache := func(t *testing.T, opts ...Option) (*Cache, *blobstor.BlobStor, *meta.DB) {
 		dir := t.TempDir()
 		mb := meta.New(
 			meta.WithPath(filepath.Join(dir, "meta")),
@@ -76,7 +75,7 @@ func TestFlush(t *testing.T) {
 		return wc, bs, mb
 	}
 
-	putObjects := func(t *testing.T, c Cache) []objectPair {
+	putObjects := func(t *testing.T, c *Cache) []objectPair {
 		objects := make([]objectPair, objCount)
 		for i := range objects {
 			objects[i] = putObject(t, c, 1+(i%2)*smallSize)
@@ -106,12 +105,19 @@ func TestFlush(t *testing.T) {
 		wc, bs, mb := newCache(t)
 		objects := putObjects(t, wc)
 
+		for i := 0; i < 2; i++ {
+			var mPrm meta.GetPrm
+			mPrm.SetAddress(objects[i].addr)
+			_, err := mb.Get(context.Background(), mPrm)
+			require.Error(t, err)
+
+			_, err = bs.Get(context.Background(), common.GetPrm{Address: objects[i].addr})
+			require.Error(t, err)
+		}
+
 		require.NoError(t, bs.SetMode(mode.ReadWrite))
 		require.NoError(t, mb.SetMode(mode.ReadWrite))
 
-		wc.(*cache).flushed.Add(objects[0].addr.EncodeToString(), true)
-		wc.(*cache).flushed.Add(objects[1].addr.EncodeToString(), false)
-
 		require.NoError(t, wc.Flush(context.Background(), false))
 
 		for i := 0; i < 2; i++ {
@@ -121,7 +127,7 @@ func TestFlush(t *testing.T) {
 			require.Error(t, err)
 
 			_, err = bs.Get(context.Background(), common.GetPrm{Address: objects[i].addr})
-			require.Error(t, err)
+			require.NoError(t, err)
 		}
 
 		check(t, mb, bs, objects[2:])
@@ -131,19 +137,11 @@ func TestFlush(t *testing.T) {
 		wc, bs, mb := newCache(t)
 		objects := putObjects(t, wc)
 
-		// Blobstor is read-only, so we expect en error from `flush` here.
-		require.Error(t, wc.SetMode(mode.Degraded))
-
-		// First move to read-only mode to close background workers.
-		require.NoError(t, wc.SetMode(mode.ReadOnly))
-		require.NoError(t, bs.SetMode(mode.ReadWrite))
-		require.NoError(t, mb.SetMode(mode.ReadWrite))
-
-		wc.(*cache).flushed.Add(objects[0].addr.EncodeToString(), true)
-		wc.(*cache).flushed.Add(objects[1].addr.EncodeToString(), false)
-
+		// Moving to the degraded mode is called with `ignoreErrors` so
+		// we do not expect an error from `flush` here.
 		require.NoError(t, wc.SetMode(mode.Degraded))
 
+		// bs is read-only; so is can't get the objects
 		for i := 0; i < 2; i++ {
 			var mPrm meta.GetPrm
 			mPrm.SetAddress(objects[i].addr)
@@ -154,19 +152,29 @@ func TestFlush(t *testing.T) {
 			require.Error(t, err)
 		}
 
+		require.NoError(t, wc.SetMode(mode.ReadWrite))
+		require.NoError(t, bs.SetMode(mode.ReadWrite))
+		require.NoError(t, mb.SetMode(mode.ReadWrite))
+
+		require.NoError(t, wc.SetMode(mode.Degraded))
+
+		for i := 0; i < 2; i++ {
+			_, err := bs.Get(context.Background(), common.GetPrm{Address: objects[i].addr})
+			require.NoError(t, err)
+		}
+
 		check(t, mb, bs, objects[2:])
 	})
 
 	t.Run("ignore errors", func(t *testing.T) {
-		testIgnoreErrors := func(t *testing.T, f func(*cache)) {
+		testIgnoreErrors := func(t *testing.T, f func(*Cache)) {
 			var errCount atomic.Uint32
 			wc, bs, mb := newCache(t, WithReportErrorFunc(func(message string, err error) {
 				errCount.Inc()
 			}))
 			objects := putObjects(t, wc)
-			f(wc.(*cache))
+			f(wc)
 
-			require.NoError(t, wc.SetMode(mode.ReadOnly))
 			require.NoError(t, bs.SetMode(mode.ReadWrite))
 			require.NoError(t, mb.SetMode(mode.ReadWrite))
 
@@ -178,7 +186,7 @@ func TestFlush(t *testing.T) {
 			check(t, mb, bs, objects)
 		}
 		t.Run("db, invalid address", func(t *testing.T) {
-			testIgnoreErrors(t, func(c *cache) {
+			testIgnoreErrors(t, func(c *Cache) {
 				_, data := newObject(t, 1)
 				require.NoError(t, c.db.Batch(func(tx *bbolt.Tx) error {
 					b := tx.Bucket(defaultBucket)
@@ -187,7 +195,7 @@ func TestFlush(t *testing.T) {
 			})
 		})
 		t.Run("db, invalid object", func(t *testing.T) {
-			testIgnoreErrors(t, func(c *cache) {
+			testIgnoreErrors(t, func(c *Cache) {
 				require.NoError(t, c.db.Batch(func(tx *bbolt.Tx) error {
 					b := tx.Bucket(defaultBucket)
 					return b.Put([]byte(oidtest.Address().EncodeToString()), []byte{1, 2, 3})
@@ -195,7 +203,7 @@ func TestFlush(t *testing.T) {
 			})
 		})
 		t.Run("fs, read error", func(t *testing.T) {
-			testIgnoreErrors(t, func(c *cache) {
+			testIgnoreErrors(t, func(c *Cache) {
 				obj, data := newObject(t, 1)
 
 				var prm common.PutPrm
@@ -214,7 +222,7 @@ func TestFlush(t *testing.T) {
 			})
 		})
 		t.Run("fs, invalid object", func(t *testing.T) {
-			testIgnoreErrors(t, func(c *cache) {
+			testIgnoreErrors(t, func(c *Cache) {
 				var prm common.PutPrm
 				prm.Address = oidtest.Address()
 				prm.RawData = []byte{1, 2, 3}
@@ -224,7 +232,7 @@ func TestFlush(t *testing.T) {
 		})
 	})
 
-	t.Run("on init", func(t *testing.T) {
+	t.Run("flush", func(t *testing.T) {
 		wc, bs, mb := newCache(t)
 		objects := []objectPair{
 			// removed
@@ -260,9 +268,6 @@ func TestFlush(t *testing.T) {
 		_, err = mb.Delete(context.Background(), deletePrm)
 		require.NoError(t, err)
 
-		require.NoError(t, bs.SetMode(mode.ReadOnly))
-		require.NoError(t, mb.SetMode(mode.ReadOnly))
-
 		// Open in read-only: no error, nothing is removed.
 		require.NoError(t, wc.Open(true))
 		initWC(t, wc)
@@ -275,18 +280,22 @@ func TestFlush(t *testing.T) {
 		// Open in read-write: no error, something is removed.
 		require.NoError(t, wc.Open(false))
 		initWC(t, wc)
+
 		for i := range objects {
 			_, err := wc.Get(context.Background(), objects[i].addr)
-			if i < 2 {
-				require.ErrorAs(t, err, new(apistatus.ObjectNotFound), i)
-			} else {
-				require.NoError(t, err, i)
-			}
+			require.NoError(t, err, i)
+		}
+
+		require.NoError(t, wc.Flush(context.Background(), true))
+
+		for i := range objects {
+			_, err := wc.Get(context.Background(), objects[i].addr)
+			require.ErrorAs(t, err, new(apistatus.ObjectNotFound), i)
 		}
 	})
 }
 
-func putObject(t *testing.T, c Cache, size int) objectPair {
+func putObject(t *testing.T, c *Cache, size int) objectPair {
 	obj, data := newObject(t, size)
 
 	var prm common.PutPrm
@@ -319,13 +328,8 @@ func newObject(t *testing.T, size int) (*object.Object, []byte) {
 	return obj, data
 }
 
-func initWC(t *testing.T, wc Cache) {
+func initWC(t *testing.T, wc *Cache) {
 	require.NoError(t, wc.Init())
-
-	require.Eventually(t, func() bool {
-		rawWc := wc.(*cache)
-		return rawWc.initialized.Load()
-	}, 100*time.Second, 1*time.Millisecond)
 }
 
 type dummyEpoch struct{}

pkg/local_object_storage/writecache/get.go

@@ -18,7 +18,7 @@ import (
 // Get returns object from write-cache.
 //
 // Returns an error of type apistatus.ObjectNotFound if the requested object is missing in write-cache.
-func (c *cache) Get(ctx context.Context, addr oid.Address) (*objectSDK.Object, error) {
+func (c *Cache) Get(ctx context.Context, addr oid.Address) (*objectSDK.Object, error) {
 	saddr := addr.EncodeToString()
 
 	ctx, span := tracing.StartSpanFromContext(ctx, "writecache.Get",
@@ -30,7 +30,6 @@ func (c *cache) Get(ctx context.Context, addr oid.Address) (*objectSDK.Object, e
 	value, err := Get(c.db, []byte(saddr))
 	if err == nil {
 		obj := objectSDK.New()
-		c.flushed.Get(saddr)
 		return obj, obj.Unmarshal(value)
 	}
 
@@ -39,14 +38,13 @@ func (c *cache) Get(ctx context.Context, addr oid.Address) (*objectSDK.Object, e
 		return nil, logicerr.Wrap(apistatus.ObjectNotFound{})
 	}
 
-	c.flushed.Get(saddr)
 	return res.Object, nil
 }
 
 // Head returns object header from write-cache.
 //
 // Returns an error of type apistatus.ObjectNotFound if the requested object is missing in write-cache.
-func (c *cache) Head(ctx context.Context, addr oid.Address) (*objectSDK.Object, error) {
+func (c *Cache) Head(ctx context.Context, addr oid.Address) (*objectSDK.Object, error) {
 	ctx, span := tracing.StartSpanFromContext(ctx, "writecache.Head",
 		trace.WithAttributes(
 			attribute.String("address", addr.EncodeToString()),

pkg/local_object_storage/writecache/init.go

@@ -2,191 +2,33 @@ package writecache
 
 import (
 	"context"
-	"errors"
-	"sync"
+	"fmt"
 
-	"git.frostfs.info/TrueCloudLab/frostfs-node/internal/logs"
-	"git.frostfs.info/TrueCloudLab/frostfs-node/pkg/local_object_storage/blobstor/common"
-	storagelog "git.frostfs.info/TrueCloudLab/frostfs-node/pkg/local_object_storage/internal/log"
-	meta "git.frostfs.info/TrueCloudLab/frostfs-node/pkg/local_object_storage/metabase"
-	apistatus "git.frostfs.info/TrueCloudLab/frostfs-sdk-go/client/status"
-	oid "git.frostfs.info/TrueCloudLab/frostfs-sdk-go/object/id"
-	"go.etcd.io/bbolt"
-	"go.uber.org/zap"
+	"git.frostfs.info/TrueCloudLab/frostfs-api-go/v2/pkg/tracing"
+	"git.frostfs.info/TrueCloudLab/frostfs-node/pkg/local_object_storage/shard/mode"
 )
 
-func (c *cache) initFlushMarks(ctx context.Context) {
-	var localWG sync.WaitGroup
+// Init runs necessary services.
+func (c *Cache) Init() error {
+	ctx, span := tracing.StartSpanFromContext(context.TODO(), "writecache.Init")
+	defer span.End()
 
-	localWG.Add(1)
-	go func() {
-		defer localWG.Done()
+	c.modeMtx.Lock()
+	defer c.modeMtx.Unlock()
 
-		c.fsTreeFlushMarkUpdate(ctx)
-	}()
-
-	localWG.Add(1)
-	go func() {
-		defer localWG.Done()
-
-		c.dbFlushMarkUpdate(ctx)
-	}()
-
-	c.initWG.Add(1)
-	c.wg.Add(1)
-	go func() {
-		defer c.wg.Done()
-		defer c.initWG.Done()
-
-		localWG.Wait()
-
-		select {
-		case <-c.stopInitCh:
-			return
-		case <-c.closeCh:
-			return
-		default:
-		}
-
-		c.initialized.Store(true)
-	}()
-}
-
-var errStopIter = errors.New("stop iteration")
-
-func (c *cache) fsTreeFlushMarkUpdate(ctx context.Context) {
-	c.log.Info(logs.WritecacheFillingFlushMarksForObjectsInFSTree)
-
-	var prm common.IteratePrm
-	prm.LazyHandler = func(addr oid.Address, _ func() ([]byte, error)) error {
-		select {
-		case <-c.closeCh:
-			return errStopIter
-		case <-c.stopInitCh:
-			return errStopIter
-		default:
-		}
-
-		flushed, needRemove := c.flushStatus(ctx, addr)
-		if flushed {
-			c.store.flushed.Add(addr.EncodeToString(), true)
-			if needRemove {
-				var prm common.DeletePrm
-				prm.Address = addr
-
-				_, err := c.fsTree.Delete(ctx, prm)
-				if err == nil {
-					storagelog.Write(c.log,
-						storagelog.AddressField(addr),
-						storagelog.StorageTypeField(wcStorageType),
-						storagelog.OpField("fstree DELETE"),
-					)
-				}
-			}
-		}
+	if c.mode.NoMetabase() {
 		return nil
 	}
 
-	c.modeMtx.RLock()
-	defer c.modeMtx.RUnlock()
-
-	_, _ = c.fsTree.Iterate(prm)
-
-	c.log.Info(logs.WritecacheFinishedUpdatingFSTreeFlushMarks)
-}
-
-func (c *cache) dbFlushMarkUpdate(ctx context.Context) {
-	c.log.Info(logs.WritecacheFillingFlushMarksForObjectsInDatabase)
-
-	c.modeMtx.RLock()
-	defer c.modeMtx.RUnlock()
-
-	var m []string
-	var indices []int
-	var lastKey []byte
-	var batchSize = flushBatchSize
-	for {
-		select {
-		case <-c.closeCh:
-			return
-		case <-c.stopInitCh:
-			return
-		default:
-		}
-
-		m = m[:0]
-		indices = indices[:0]
-
-		// We put objects in batches of fixed size to not interfere with main put cycle a lot.
-		_ = c.db.View(func(tx *bbolt.Tx) error {
-			b := tx.Bucket(defaultBucket)
-			cs := b.Cursor()
-			for k, _ := cs.Seek(lastKey); k != nil && len(m) < batchSize; k, _ = cs.Next() {
-				m = append(m, string(k))
-			}
-			return nil
-		})
-
-		var addr oid.Address
-		for i := range m {
-			if err := addr.DecodeString(m[i]); err != nil {
-				continue
-			}
-
-			flushed, needRemove := c.flushStatus(ctx, addr)
-			if flushed {
-				c.store.flushed.Add(addr.EncodeToString(), true)
-				if needRemove {
-					indices = append(indices, i)
-				}
-			}
-		}
-
-		if len(m) == 0 {
-			break
-		}
-
-		err := c.db.Batch(func(tx *bbolt.Tx) error {
-			b := tx.Bucket(defaultBucket)
-			for _, j := range indices {
-				if err := b.Delete([]byte(m[j])); err != nil {
-					return err
-				}
-			}
-			return nil
-		})
-		if err == nil {
-			for _, j := range indices {
-				storagelog.Write(c.log,
-					zap.String("address", m[j]),
-					storagelog.StorageTypeField(wcStorageType),
-					storagelog.OpField("db DELETE"),
-				)
-			}
-		}
-		lastKey = append([]byte(m[len(m)-1]), 0)
-	}
-
-	c.log.Info(logs.WritecacheFinishedUpdatingFlushMarks)
-}
-
-// flushStatus returns info about the object state in the main storage.
-// First return value is true iff object exists.
-// Second return value is true iff object can be safely removed.
-func (c *cache) flushStatus(ctx context.Context, addr oid.Address) (bool, bool) {
-	var existsPrm meta.ExistsPrm
-	existsPrm.SetAddress(addr)
-
-	_, err := c.metabase.Exists(ctx, existsPrm)
+	err := c.initCounters(ctx)
 	if err != nil {
-		needRemove := errors.Is(err, meta.ErrObjectIsExpired) || errors.As(err, new(apistatus.ObjectAlreadyRemoved))
-		return needRemove, needRemove
+		return fmt.Errorf("initializing write-cache size: %w", err)
 	}
 
-	var prm meta.StorageIDPrm
-	prm.SetAddress(addr)
+	if c.mode == mode.ReadWrite {
+		c.workersChan = make(chan struct{})
+		c.runFlushLoop()
+	}
 
-	mRes, _ := c.metabase.StorageID(ctx, prm)
-	res, err := c.blobstor.Exists(ctx, common.ExistsPrm{Address: addr, StorageID: mRes.StorageID()})
-	return err == nil && res.Exists, false
+	return nil
 }

pkg/local_object_storage/writecache/iterate.go

@@ -31,7 +31,7 @@ func (p *IterationPrm) WithIgnoreErrors(ignore bool) {
 // Iterate iterates over all objects present in write cache.
 // This is very difficult to do correctly unless write-cache is put in read-only mode.
 // Thus we silently fail if shard is not in read-only mode to avoid reporting misleading results.
-func (c *cache) Iterate(prm IterationPrm) error {
+func (c *Cache) Iterate(prm IterationPrm) error {
 	c.modeMtx.RLock()
 	defer c.modeMtx.RUnlock()
 	if !c.readOnly() {
@@ -41,9 +41,6 @@ func (c *cache) Iterate(prm IterationPrm) error {
 	err := c.db.View(func(tx *bbolt.Tx) error {
 		b := tx.Bucket(defaultBucket)
 		return b.ForEach(func(k, data []byte) error {
-			if _, ok := c.flushed.Peek(string(k)); ok {
-				return nil
-			}
 			return prm.handler(data)
 		})
 	})
@@ -54,9 +51,6 @@ func (c *cache) Iterate(prm IterationPrm) error {
 	var fsPrm common.IteratePrm
 	fsPrm.IgnoreErrors = prm.ignoreErrors
 	fsPrm.LazyHandler = func(addr oid.Address, f func() ([]byte, error)) error {
-		if _, ok := c.flushed.Peek(addr.EncodeToString()); ok {
-			return nil
-		}
 		data, err := f()
 		if err != nil {
 			if prm.ignoreErrors {

pkg/local_object_storage/writecache/mode.go

@@ -16,13 +16,10 @@ import (
 // ErrReadOnly is returned when Put/Write is performed in a read-only mode.
 var ErrReadOnly = logicerr.New("write-cache is in read-only mode")
 
-// ErrNotInitialized is returned when write-cache is initializing.
-var ErrNotInitialized = logicerr.New("write-cache is not initialized yet")
-
 // SetMode sets write-cache mode of operation.
 // When shard is put in read-only mode all objects in memory are flushed to disk
 // and all background jobs are suspended.
-func (c *cache) SetMode(m mode.Mode) error {
+func (c *Cache) SetMode(m mode.Mode) error {
 	ctx, span := tracing.StartSpanFromContext(context.TODO(), "writecache.SetMode",
 		trace.WithAttributes(
 			attribute.String("mode", m.String()),
@@ -33,31 +30,30 @@ func (c *cache) SetMode(m mode.Mode) error {
 }
 
 // setMode applies new mode. Must be called with cache.modeMtx lock taken.
-func (c *cache) setMode(ctx context.Context, m mode.Mode) error {
+func (c *Cache) setMode(ctx context.Context, m mode.Mode) error {
 	var err error
-	turnOffMeta := m.NoMetabase()
-
-	if !c.initialized.Load() {
-		close(c.stopInitCh)
-
-		c.initWG.Wait()
-		c.stopInitCh = make(chan struct{})
-
-		defer func() {
-			if err == nil && !turnOffMeta {
-				c.initFlushMarks(ctx)
-			}
-		}()
-	}
 
 	c.modeMtx.Lock()
 	defer c.modeMtx.Unlock()
 
-	if turnOffMeta && !c.mode.NoMetabase() {
+	var workersActive bool
+	select {
+	case <-c.workersChan:
+	default:
+		workersActive = true
+	}
+
+	stopWorkers := m.NoMetabase() && !c.mode.NoMetabase() || c.mode.ReadWrite() && !m.ReadWrite()
+	if stopWorkers {
 		err = c.flush(ctx, true)
 		if err != nil {
 			return err
 		}
+
+		if workersActive {
+			close(c.workersChan)
+			c.wg.Wait()
+		}
 	}
 
 	if c.db != nil {
@@ -67,14 +63,14 @@ func (c *cache) setMode(ctx context.Context, m mode.Mode) error {
 	}
 
 	// Suspend producers to ensure there are channel send operations in fly.
-	// flushCh is populated by `flush` with `modeMtx` taken, thus waiting until it is empty
+	// smallFlushCh is populated by `flush` with `modeMtx` taken, thus waiting until it is empty
 	// guarantees that there are no in-fly operations.
-	for len(c.flushCh) != 0 {
+	for len(c.smallFlushCh) != 0 {
 		c.log.Info(logs.WritecacheWaitingForChannelsToFlush)
 		time.Sleep(time.Second)
 	}
 
-	if turnOffMeta {
+	if m.NoMetabase() {
 		c.mode = m
 		return nil
 	}
@@ -84,11 +80,21 @@ func (c *cache) setMode(ctx context.Context, m mode.Mode) error {
 	}
 
 	c.mode = m
+
+	if m == mode.ReadWrite {
+		select {
+		case <-c.workersChan:
+			c.workersChan = make(chan struct{})
+			c.runFlushLoop()
+		default:
+		}
+	}
+
 	return nil
 }
 
 // readOnly returns true if current mode is read-only.
 // `c.modeMtx` must be taken.
-func (c *cache) readOnly() bool {
+func (c *Cache) readOnly() bool {
 	return c.mode.ReadOnly()
 }

pkg/local_object_storage/writecache/options.go

@@ -48,8 +48,6 @@ type options struct {
 	// maxCacheSize is the maximum total size of all objects saved in cache (DB + FS).
 	// 1 GiB by default.
 	maxCacheSize uint64
-	// objCounters contains atomic counters for the number of objects stored in cache.
-	objCounters counters
 	// maxBatchSize is the maximum batch size for the small object database.
 	maxBatchSize int
 	// maxBatchDelay is the maximum batch wait time for the small object database.

pkg/local_object_storage/writecache/put.go

@@ -3,6 +3,7 @@ package writecache
 import (
 	"context"
 	"errors"
+	"fmt"
 
 	"git.frostfs.info/TrueCloudLab/frostfs-api-go/v2/pkg/tracing"
 	"git.frostfs.info/TrueCloudLab/frostfs-node/pkg/local_object_storage/blobstor/common"
@@ -25,7 +26,7 @@ var (
 // Returns ErrNotInitialized if write-cache has not been initialized yet.
 // Returns ErrOutOfSpace if saving an object leads to WC's size overflow.
 // Returns ErrBigObject if an objects exceeds maximum object size.
-func (c *cache) Put(ctx context.Context, prm common.PutPrm) (common.PutRes, error) {
+func (c *Cache) Put(ctx context.Context, prm common.PutPrm) (common.PutRes, error) {
 	ctx, span := tracing.StartSpanFromContext(ctx, "writecache.Put",
 		trace.WithAttributes(
 			attribute.String("address", prm.Address.EncodeToString()),
@@ -37,8 +38,6 @@ func (c *cache) Put(ctx context.Context, prm common.PutPrm) (common.PutRes, erro
 	defer c.modeMtx.RUnlock()
 	if c.readOnly() {
 		return common.PutRes{}, ErrReadOnly
-	} else if !c.initialized.Load() {
-		return common.PutRes{}, ErrNotInitialized
 	}
 
 	sz := uint64(len(prm.RawData))
@@ -52,23 +51,42 @@ func (c *cache) Put(ctx context.Context, prm common.PutPrm) (common.PutRes, erro
 		data: prm.RawData,
 	}
 
-	if sz <= c.smallObjectSize {
-		return common.PutRes{}, c.putSmall(oi)
+	if c.maxCacheSize < c.sizeIfAdd(sz) {
+		return common.PutRes{}, ErrOutOfSpace
 	}
-	return common.PutRes{}, c.putBig(ctx, oi.addr, prm)
+
+	if sz <= c.smallObjectSize {
+		err := c.putSmall(oi)
+		if err != nil {
+			err = fmt.Errorf("could not put small object to DB: %w", err)
+		}
+
+		return common.PutRes{}, err
+	}
+
+	err := c.putBig(ctx, oi.addr, prm)
+	if err != nil {
+		err = fmt.Errorf("could not put big object to FSTree: %w", err)
+	}
+
+	return common.PutRes{}, err
 }
 
 // putSmall persists small objects to the write-cache database and
 // pushes the to the flush workers queue.
-func (c *cache) putSmall(obj objectInfo) error {
-	cacheSize := c.estimateCacheSize()
-	if c.maxCacheSize < c.incSizeDB(cacheSize) {
-		return ErrOutOfSpace
-	}
+func (c *Cache) putSmall(obj objectInfo) error {
+	var alreadyExists bool
 
 	err := c.db.Batch(func(tx *bbolt.Tx) error {
 		b := tx.Bucket(defaultBucket)
-		return b.Put([]byte(obj.addr), obj.data)
+		addr := []byte(obj.addr)
+
+		alreadyExists = len(b.Get(addr)) != 0
+		if alreadyExists {
+			return nil
+		}
+
+		return b.Put(addr, obj.data)
 	})
 	if err == nil {
 		storagelog.Write(c.log,
@@ -76,29 +94,22 @@ func (c *cache) putSmall(obj objectInfo) error {
 			storagelog.StorageTypeField(wcStorageType),
 			storagelog.OpField("db PUT"),
 		)
-		c.objCounters.IncDB()
+
+		if !alreadyExists {
+			c.objCounters.incDB(len(obj.data))
+		}
 	}
-	return nil
+	return err
 }
 
 // putBig writes object to FSTree and pushes it to the flush workers queue.
-func (c *cache) putBig(ctx context.Context, addr string, prm common.PutPrm) error {
-	cacheSz := c.estimateCacheSize()
-	if c.maxCacheSize < c.incSizeFS(cacheSz) {
-		return ErrOutOfSpace
-	}
-
+func (c *Cache) putBig(ctx context.Context, addr string, prm common.PutPrm) error {
 	_, err := c.fsTree.Put(ctx, prm)
 	if err != nil {
 		return err
 	}
 
-	if c.blobstor.NeedsCompression(prm.Object) {
-		c.mtx.Lock()
-		c.compressFlags[addr] = struct{}{}
-		c.mtx.Unlock()
-	}
-	c.objCounters.IncFS()
+	c.objCounters.incFS(len(prm.RawData))
 	storagelog.Write(c.log,
 		storagelog.AddressField(addr),
 		storagelog.StorageTypeField(wcStorageType),

pkg/local_object_storage/writecache/state.go

@@ -1,72 +1,134 @@
 package writecache
 
 import (
+	"context"
+	"errors"
 	"fmt"
+	"sync"
 
+	"git.frostfs.info/TrueCloudLab/frostfs-node/pkg/local_object_storage/blobstor/common"
+	oid "git.frostfs.info/TrueCloudLab/frostfs-sdk-go/object/id"
 	"go.etcd.io/bbolt"
 	"go.uber.org/atomic"
 )
 
-func (c *cache) estimateCacheSize() uint64 {
-	return c.objCounters.DB()*c.smallObjectSize + c.objCounters.FS()*c.maxObjectSize
-}
-
-func (c *cache) incSizeDB(sz uint64) uint64 {
-	return sz + c.smallObjectSize
-}
-
-func (c *cache) incSizeFS(sz uint64) uint64 {
-	return sz + c.maxObjectSize
+func (c *Cache) sizeIfAdd(delta uint64) uint64 {
+	return delta + c.objCounters.fstreeSize.Load() + c.objCounters.dbSize.Load()
 }
 
 type counters struct {
-	cDB, cFS atomic.Uint64
+	dbSize, fstreeSize atomic.Uint64
 }
 
-func (x *counters) IncDB() {
-	x.cDB.Inc()
+func (x *counters) incDB(delta int) {
+	x.dbSize.Add(uint64(delta))
 }
 
-func (x *counters) DecDB() {
-	x.cDB.Dec()
+func (x *counters) decDB(delta int) {
+	x.dbSize.Sub(uint64(delta))
 }
 
-func (x *counters) DB() uint64 {
-	return x.cDB.Load()
+func (x *counters) incFS(delta int) {
+	x.fstreeSize.Add(uint64(delta))
 }
 
-func (x *counters) IncFS() {
-	x.cFS.Inc()
+func (x *counters) decFS(delta int) {
+	x.fstreeSize.Sub(uint64(delta))
 }
 
-func (x *counters) DecFS() {
-	x.cFS.Dec()
+func (c *Cache) initCounters(ctx context.Context) error {
+	var wg sync.WaitGroup
+	var dbErr error
+	var fsErr error
+
+	wg.Add(1)
+	go func() {
+		dbErr = c.initDBSizeCounter(ctx)
+		wg.Done()
+	}()
+
+	wg.Add(1)
+	go func() {
+		fsErr = c.initFSSizeCounter(ctx)
+		wg.Done()
+	}()
+
+	wg.Wait()
+
+	switch {
+	case dbErr != nil:
+		return fmt.Errorf("database counter initialization: %w", dbErr)
+	case fsErr != nil:
+		return fmt.Errorf("FSTree counter initialization: %w", fsErr)
+	default:
+		return nil
+	}
 }
 
-func (x *counters) FS() uint64 {
-	return x.cFS.Load()
-}
+var stopIter = errors.New("stop")
 
-func (c *cache) initCounters() error {
-	var inDB uint64
+func (c *Cache) initDBSizeCounter(ctx context.Context) error {
+	var inDB int
 	err := c.db.View(func(tx *bbolt.Tx) error {
 		b := tx.Bucket(defaultBucket)
-		if b != nil {
-			inDB = uint64(b.Stats().KeyN)
+		if b == nil {
+			return nil
 		}
-		return nil
+
+		return b.ForEach(func(_, v []byte) error {
+			select {
+			case <-ctx.Done():
+				return ctx.Err()
+			case <-c.workersChan:
+				return stopIter
+			default:
+			}
+
+			inDB += len(v)
+			return nil
+		})
 	})
-	if err != nil {
+	if err != nil && !errors.Is(err, stopIter) {
 		return fmt.Errorf("could not read write-cache DB counter: %w", err)
 	}
 
-	inFS, err := c.fsTree.NumberOfObjects()
-	if err != nil {
-		return fmt.Errorf("could not read write-cache FS counter: %w", err)
-	}
-
-	c.objCounters.cDB.Store(inDB)
-	c.objCounters.cFS.Store(inFS)
+	c.objCounters.dbSize.Store(uint64(inDB))
+
+	return nil
+}
+
+func (c *Cache) initFSSizeCounter(ctx context.Context) error {
+	var inFSTree int
+
+	var prm common.IteratePrm
+	prm.LazyHandler = func(address oid.Address, f func() ([]byte, error)) error {
+		select {
+		case <-ctx.Done():
+			return ctx.Err()
+		case <-c.workersChan:
+			return stopIter
+		default:
+		}
+
+		data, err := f()
+		if err != nil {
+			return err
+		}
+
+		// write-cache is a temporary storage on a fast disk,
+		// so it is not expected to be configured with any
+		// compressor ever
+		inFSTree += len(data)
+
+		return nil
+	}
+
+	_, err := c.fsTree.Iterate(prm)
+	if err != nil && !errors.Is(err, stopIter) {
+		return fmt.Errorf("could not read write-cache FSTree counter: %w", err)
+	}
+
+	c.objCounters.fstreeSize.Store(uint64(inFSTree))
 
 	return nil
 }

pkg/local_object_storage/writecache/storage.go

@@ -1,45 +1,23 @@
 package writecache
 
 import (
-	"context"
-	"errors"
 	"fmt"
 	"os"
 
-	"git.frostfs.info/TrueCloudLab/frostfs-node/internal/logs"
-	"git.frostfs.info/TrueCloudLab/frostfs-node/pkg/local_object_storage/blobstor/common"
 	"git.frostfs.info/TrueCloudLab/frostfs-node/pkg/local_object_storage/blobstor/fstree"
-	storagelog "git.frostfs.info/TrueCloudLab/frostfs-node/pkg/local_object_storage/internal/log"
 	"git.frostfs.info/TrueCloudLab/frostfs-node/pkg/util"
-	apistatus "git.frostfs.info/TrueCloudLab/frostfs-sdk-go/client/status"
-	oid "git.frostfs.info/TrueCloudLab/frostfs-sdk-go/object/id"
-	lru "github.com/hashicorp/golang-lru/v2"
-	"github.com/hashicorp/golang-lru/v2/simplelru"
 	"go.etcd.io/bbolt"
-	"go.uber.org/zap"
 )
 
-// store represents persistent storage with in-memory LRU cache
+// smallStore represents persistent storage with in-memory LRU cache
 // for flushed items on top of it.
-type store struct {
-	maxFlushedMarksCount int
-	maxRemoveBatchSize   int
-
-	// flushed contains addresses of objects that were already flushed to the main storage.
-	// We use LRU cache instead of map here to facilitate removing of unused object in favour of
-	// frequently read ones.
-	// MUST NOT be used inside bolt db transaction because it's eviction handler
-	// removes untracked items from the database.
-	flushed simplelru.LRUCache[string, bool]
-	db      *bbolt.DB
-
-	dbKeysToRemove []string
-	fsKeysToRemove []string
+type smallStore struct {
+	db *bbolt.DB
 }
 
 const dbName = "small.bolt"
 
-func (c *cache) openStore(readOnly bool) error {
+func (c *Cache) openStore(readOnly bool) error {
 	err := util.MkdirAllX(c.path, os.ModePerm)
 	if err != nil {
 		return err
@@ -69,101 +47,9 @@ func (c *cache) openStore(readOnly bool) error {
 		fstree.WithDepth(1),
 		fstree.WithDirNameLen(1),
 		fstree.WithNoSync(c.noSync))
-	if err := c.fsTree.Open(readOnly); err != nil {
+	if err = c.fsTree.Open(readOnly); err != nil {
 		return fmt.Errorf("could not open FSTree: %w", err)
 	}
 
-	// Write-cache can be opened multiple times during `SetMode`.
-	// flushed map must not be re-created in this case.
-	if c.flushed == nil {
-		c.flushed, _ = lru.NewWithEvict[string, bool](c.maxFlushedMarksCount, c.removeFlushed)
-	}
-
-	c.initialized.Store(false)
-
 	return nil
 }
-
-// removeFlushed removes an object from the writecache.
-// To minimize interference with the client operations, the actual removal
-// is done in batches.
-// It is not thread-safe and is used only as an evict callback to LRU cache.
-func (c *cache) removeFlushed(key string, value bool) {
-	fromDatabase := value
-	if fromDatabase {
-		c.dbKeysToRemove = append(c.dbKeysToRemove, key)
-	} else {
-		c.fsKeysToRemove = append(c.fsKeysToRemove, key)
-	}
-
-	if len(c.dbKeysToRemove)+len(c.fsKeysToRemove) >= c.maxRemoveBatchSize {
-		c.dbKeysToRemove = c.deleteFromDB(c.dbKeysToRemove)
-		c.fsKeysToRemove = c.deleteFromDisk(c.fsKeysToRemove)
-	}
-}
-
-func (c *cache) deleteFromDB(keys []string) []string {
-	if len(keys) == 0 {
-		return keys
-	}
-
-	var errorIndex int
-	err := c.db.Batch(func(tx *bbolt.Tx) error {
-		b := tx.Bucket(defaultBucket)
-		for errorIndex = range keys {
-			if err := b.Delete([]byte(keys[errorIndex])); err != nil {
-				return err
-			}
-		}
-		return nil
-	})
-	for i := 0; i < errorIndex; i++ {
-		c.objCounters.DecDB()
-		storagelog.Write(c.log,
-			storagelog.AddressField(keys[i]),
-			storagelog.StorageTypeField(wcStorageType),
-			storagelog.OpField("db DELETE"),
-		)
-	}
-	if err != nil {
-		c.log.Error(logs.WritecacheCantRemoveObjectsFromTheDatabase, zap.Error(err))
-	}
-
-	copy(keys, keys[errorIndex:])
-	return keys[:len(keys)-errorIndex]
-}
-
-func (c *cache) deleteFromDisk(keys []string) []string {
-	if len(keys) == 0 {
-		return keys
-	}
-
-	var copyIndex int
-	var addr oid.Address
-
-	for i := range keys {
-		if err := addr.DecodeString(keys[i]); err != nil {
-			c.log.Error(logs.WritecacheCantParseAddress, zap.String("address", keys[i]))
-			continue
-		}
-
-		_, err := c.fsTree.Delete(context.TODO(), common.DeletePrm{Address: addr})
-		if err != nil && !errors.As(err, new(apistatus.ObjectNotFound)) {
-			c.log.Error(logs.WritecacheCantRemoveObjectFromWritecache, zap.Error(err))
-
-			// Save the key for the next iteration.
-			keys[copyIndex] = keys[i]
-			copyIndex++
-			continue
-		} else if err == nil {
-			storagelog.Write(c.log,
-				storagelog.AddressField(keys[i]),
-				storagelog.StorageTypeField(wcStorageType),
-				storagelog.OpField("fstree DELETE"),
-			)
-			c.objCounters.DecFS()
-		}
-	}
-
-	return keys[:copyIndex]
-}

pkg/local_object_storage/writecache/writecache.go

@@ -5,15 +5,11 @@ import (
 	"os"
 	"sync"
 
-	"git.frostfs.info/TrueCloudLab/frostfs-api-go/v2/pkg/tracing"
-	"git.frostfs.info/TrueCloudLab/frostfs-node/pkg/local_object_storage/blobstor/common"
 	"git.frostfs.info/TrueCloudLab/frostfs-node/pkg/local_object_storage/blobstor/fstree"
 	"git.frostfs.info/TrueCloudLab/frostfs-node/pkg/local_object_storage/shard/mode"
 	"git.frostfs.info/TrueCloudLab/frostfs-node/pkg/util/logger"
 	"git.frostfs.info/TrueCloudLab/frostfs-sdk-go/object"
-	oid "git.frostfs.info/TrueCloudLab/frostfs-sdk-go/object/id"
 	"go.etcd.io/bbolt"
-	"go.uber.org/atomic"
 	"go.uber.org/zap"
 )
 
@@ -24,52 +20,24 @@ type Info struct {
 }
 
 // Cache represents write-cache for objects.
-type Cache interface {
-	Get(ctx context.Context, address oid.Address) (*object.Object, error)
-	Head(context.Context, oid.Address) (*object.Object, error)
-	// Delete removes object referenced by the given oid.Address from the
-	// Cache. Returns any error encountered that prevented the object to be
-	// removed.
-	//
-	// Returns apistatus.ObjectNotFound if object is missing in the Cache.
-	// Returns ErrReadOnly if the Cache is currently in the read-only mode.
-	Delete(context.Context, oid.Address) error
-	Iterate(IterationPrm) error
-	Put(context.Context, common.PutPrm) (common.PutRes, error)
-	SetMode(mode.Mode) error
-	SetLogger(*logger.Logger)
-	DumpInfo() Info
-	Flush(context.Context, bool) error
-
-	Init() error
-	Open(readOnly bool) error
-	Close() error
-}
-
-type cache struct {
+type Cache struct {
 	options
 
-	// mtx protects statistics, counters and compressFlags.
-	mtx sync.RWMutex
+	// objCounters contains atomic counters for the number of objects stored in cache.
+	objCounters counters
 
-	mode        mode.Mode
-	initialized atomic.Bool
-	stopInitCh  chan struct{}  // used to sync initWG initialisation routines and _only_ them
-	initWG      sync.WaitGroup // for initialisation routines only
-	modeMtx     sync.RWMutex
-
-	// compressFlags maps address of a big object to boolean value indicating
-	// whether object should be compressed.
-	compressFlags map[string]struct{}
+	modeMtx sync.RWMutex
+	mode    mode.Mode
 
 	// flushCh is a channel with objects to flush.
-	flushCh chan *object.Object
-	// closeCh is close channel, protected by modeMtx.
-	closeCh chan struct{}
+	smallFlushCh chan objWithData
+	// workersChan is close channel, protected by modeMtx.
+	// It indicates status of the background workers.
+	workersChan chan struct{}
 	// wg is a wait group for flush workers.
 	wg sync.WaitGroup
 	// store contains underlying database.
-	store
+	smallStore
 	// fsTree contains big files stored directly on file-system.
 	fsTree *fstree.FSTree
 }
@@ -94,13 +62,16 @@ var (
 )
 
 // New creates new writecache instance.
-func New(opts ...Option) Cache {
-	c := &cache{
-		flushCh:    make(chan *object.Object),
-		mode:       mode.ReadWrite,
-		stopInitCh: make(chan struct{}),
+// The value must not be copied after creation.
+func New(opts ...Option) *Cache {
+	closeCh := make(chan struct{})
+	close(closeCh)
+
+	c := &Cache{
+		smallFlushCh: make(chan objWithData),
+		mode:         mode.ReadWrite,
+		workersChan:  closeCh,
 
-		compressFlags: make(map[string]struct{}),
 		options: options{
 			log:             &logger.Logger{Logger: zap.NewNop()},
 			maxObjectSize:   defaultMaxObjectSize,
@@ -117,67 +88,52 @@ func New(opts ...Option) Cache {
 		opts[i](&c.options)
 	}
 
-	// Make the LRU cache contain which take approximately 3/4 of the maximum space.
-	// Assume small and big objects are stored in 50-50 proportion.
-	c.maxFlushedMarksCount = int(c.maxCacheSize/c.maxObjectSize+c.maxCacheSize/c.smallObjectSize) / 2 * 3 / 4
-	// Trigger the removal when the cache is 7/8 full, so that new items can still arrive.
-	c.maxRemoveBatchSize = c.maxFlushedMarksCount / 8
-
 	return c
 }
 
 // SetLogger sets logger. It is used after the shard ID was generated to use it in logs.
-func (c *cache) SetLogger(l *logger.Logger) {
+func (c *Cache) SetLogger(l *logger.Logger) {
 	c.log = l
 }
 
-func (c *cache) DumpInfo() Info {
+func (c *Cache) DumpInfo() Info {
 	return Info{
 		Path: c.path,
 	}
 }
 
 // Open opens and initializes database. Reads object counters from the ObjectCounters instance.
-func (c *cache) Open(readOnly bool) error {
+func (c *Cache) Open(readOnly bool) error {
 	err := c.openStore(readOnly)
 	if err != nil {
 		return err
 	}
 
-	// Opening after Close is done during maintenance mode,
-	// thus we need to create a channel here.
-	c.closeCh = make(chan struct{})
+	c.modeMtx.Lock()
+	defer c.modeMtx.Unlock()
 
-	return c.initCounters()
-}
+	if readOnly {
+		c.mode = mode.ReadOnly
+	} else {
+		c.mode = mode.ReadWrite
+	}
 
-// Init runs necessary services.
-func (c *cache) Init() error {
-	ctx, span := tracing.StartSpanFromContext(context.TODO(), "writecache.Init")
-	defer span.End()
-
-	c.initFlushMarks(ctx)
-	c.runFlushLoop()
 	return nil
 }
 
 // Close closes db connection and stops services. Executes ObjectCounters.FlushAndClose op.
-func (c *cache) Close() error {
-	// Finish all in-progress operations.
-	if err := c.setMode(context.TODO(), mode.ReadOnly); err != nil {
-		return err
+func (c *Cache) Close() error {
+	// Finish all in-progress operations if they are
+	// in progress.
+	select {
+	case <-c.workersChan:
+	default:
+		err := c.setMode(context.TODO(), mode.ReadOnly)
+		if err != nil {
+			return err
+		}
 	}
 
-	if c.closeCh != nil {
-		close(c.closeCh)
-	}
-	c.wg.Wait()
-	if c.closeCh != nil {
-		c.closeCh = nil
-	}
-
-	c.initialized.Store(false)
-
 	var err error
 	if c.db != nil {
 		err = c.db.Close()