Add async caching commit option in diskcache (#10742)

Add store and a forward option for a single part
uploads when an async mode is enabled with env
MINIO_CACHE_COMMIT=writeback 

It defaults to `writethrough` if unspecified.

cmd/config/cache/config.go

@@ -28,16 +28,17 @@ import (
 
 // Config represents cache config settings
 type Config struct {
-	Enabled       bool     `json:"-"`
+	Enabled         bool     `json:"-"`
-	Drives        []string `json:"drives"`
+	Drives          []string `json:"drives"`
-	Expiry        int      `json:"expiry"`
+	Expiry          int      `json:"expiry"`
-	MaxUse        int      `json:"maxuse"`
+	MaxUse          int      `json:"maxuse"`
-	Quota         int      `json:"quota"`
+	Quota           int      `json:"quota"`
-	Exclude       []string `json:"exclude"`
+	Exclude         []string `json:"exclude"`
-	After         int      `json:"after"`
+	After           int      `json:"after"`
-	WatermarkLow  int      `json:"watermark_low"`
+	WatermarkLow    int      `json:"watermark_low"`
-	WatermarkHigh int      `json:"watermark_high"`
+	WatermarkHigh   int      `json:"watermark_high"`
-	Range         bool     `json:"range"`
+	Range           bool     `json:"range"`
+	CommitWriteback bool     `json:"-"`
 }
 
 // UnmarshalJSON - implements JSON unmarshal interface for unmarshalling
@@ -152,3 +153,13 @@ func parseCacheExcludes(excludes string) ([]string, error) {
 
 	return excludesSlice, nil
 }
+
+func parseCacheCommitMode(commitStr string) (bool, error) {
+	switch strings.ToLower(commitStr) {
+	case "writeback":
+		return true, nil
+	case "writethrough":
+		return false, nil
+	}
+	return false, config.ErrInvalidCacheCommitValue(nil).Msg("cache commit value must be `writeback` or `writethrough`")
+}

cmd/config/cache/help.go

@@ -74,5 +74,11 @@ var (
 			Optional:    true,
 			Type:        "string",
 		},
+		config.HelpKV{
+			Key:         Commit,
+			Description: `set to control cache commit behavior, defaults to "writethrough"`,
+			Optional:    true,
+			Type:        "string",
+		},
 	}
 )

cmd/config/cache/lookup.go

@@ -35,6 +35,7 @@ const (
 	WatermarkLow  = "watermark_low"
 	WatermarkHigh = "watermark_high"
 	Range         = "range"
+	Commit        = "commit"
 
 	EnvCacheDrives        = "MINIO_CACHE_DRIVES"
 	EnvCacheExclude       = "MINIO_CACHE_EXCLUDE"
@@ -45,6 +46,7 @@ const (
 	EnvCacheWatermarkLow  = "MINIO_CACHE_WATERMARK_LOW"
 	EnvCacheWatermarkHigh = "MINIO_CACHE_WATERMARK_HIGH"
 	EnvCacheRange         = "MINIO_CACHE_RANGE"
+	EnvCacheCommit        = "MINIO_CACHE_COMMIT"
 
 	EnvCacheEncryptionMasterKey = "MINIO_CACHE_ENCRYPTION_MASTER_KEY"
 
@@ -53,6 +55,7 @@ const (
 	DefaultAfter         = "0"
 	DefaultWaterMarkLow  = "70"
 	DefaultWaterMarkHigh = "80"
+	DefaultCacheCommit   = "writethrough"
 )
 
 // DefaultKVS - default KV settings for caching.
@@ -90,6 +93,10 @@ var (
 			Key:   Range,
 			Value: config.EnableOn,
 		},
+		config.KV{
+			Key:   Commit,
+			Value: DefaultCacheCommit,
+		},
 	}
 )
 
@@ -210,6 +217,12 @@ func LookupConfig(kvs config.KVS) (Config, error) {
 		}
 		cfg.Range = rng
 	}
+	if commit := env.Get(EnvCacheCommit, kvs.Get(Commit)); commit != "" {
+		cfg.CommitWriteback, err = parseCacheCommitMode(commit)
+		if err != nil {
+			return cfg, err
+		}
+	}
 
 	return cfg, nil
 }

cmd/config/errors.go

@@ -102,6 +102,12 @@ var (
 		"MINIO_CACHE_RANGE: Valid expected value is `on` or `off`",
 	)
 
+	ErrInvalidCacheCommitValue = newErrFn(
+		"Invalid cache commit value",
+		"Please check the passed value",
+		"MINIO_CACHE_COMMIT: Valid expected value is `writeback` or `writethrough`",
+	)
+
 	ErrInvalidRotatingCredentialsBackendEncrypted = newErrFn(
 		"Invalid rotating credentials",
 		"Please set correct rotating credentials in the environment for decryption",

cmd/disk-cache-backend.go

@@ -19,7 +19,9 @@ package cmd
 import (
 	"bytes"
 	"context"
+	"crypto/md5"
 	"crypto/rand"
+	"encoding/base64"
 	"encoding/hex"
 	"errors"
 	"fmt"
@@ -70,7 +72,9 @@ type cacheMeta struct {
 	// Ranges maps cached range to associated filename.
 	Ranges map[string]string `json:"ranges,omitempty"`
 	// Hits is a counter on the number of times this object has been accessed so far.
-	Hits int `json:"hits,omitempty"`
+	Hits   int    `json:"hits,omitempty"`
+	Bucket string `json:"bucket,omitempty"`
+	Object string `json:"object,omitempty"`
 }
 
 // RangeInfo has the range, file and range length information for a cached range.
@@ -130,15 +134,17 @@ type diskCache struct {
 	online       uint32 // ref: https://golang.org/pkg/sync/atomic/#pkg-note-BUG
 	purgeRunning int32
 
-	triggerGC     chan struct{}
+	triggerGC        chan struct{}
-	dir           string         // caching directory
+	dir              string         // caching directory
-	stats         CacheDiskStats // disk cache stats for prometheus
+	stats            CacheDiskStats // disk cache stats for prometheus
-	quotaPct      int            // max usage in %
+	quotaPct         int            // max usage in %
-	pool          sync.Pool
+	pool             sync.Pool
-	after         int // minimum accesses before an object is cached.
+	after            int // minimum accesses before an object is cached.
-	lowWatermark  int
+	lowWatermark     int
-	highWatermark int
+	highWatermark    int
-	enableRange   bool
+	enableRange      bool
+	commitWriteback  bool
+	retryWritebackCh chan ObjectInfo
 	// nsMutex namespace lock
 	nsMutex *nsLockMap
 	// Object functions pointing to the corresponding functions of backend implementation.
@@ -156,15 +162,17 @@ func newDiskCache(ctx context.Context, dir string, config cache.Config) (*diskCa
 		return nil, fmt.Errorf("Unable to initialize '%s' dir, %w", dir, err)
 	}
 	cache := diskCache{
-		dir:           dir,
+		dir:              dir,
-		triggerGC:     make(chan struct{}, 1),
+		triggerGC:        make(chan struct{}, 1),
-		stats:         CacheDiskStats{Dir: dir},
+		stats:            CacheDiskStats{Dir: dir},
-		quotaPct:      quotaPct,
+		quotaPct:         quotaPct,
-		after:         config.After,
+		after:            config.After,
-		lowWatermark:  config.WatermarkLow,
+		lowWatermark:     config.WatermarkLow,
-		highWatermark: config.WatermarkHigh,
+		highWatermark:    config.WatermarkHigh,
-		enableRange:   config.Range,
+		enableRange:      config.Range,
-		online:        1,
+		commitWriteback:  config.CommitWriteback,
+		retryWritebackCh: make(chan ObjectInfo, 10000),
+		online:           1,
 		pool: sync.Pool{
 			New: func() interface{} {
 				b := disk.AlignedBlock(int(cacheBlkSize))
@@ -174,6 +182,9 @@ func newDiskCache(ctx context.Context, dir string, config cache.Config) (*diskCa
 		nsMutex: newNSLock(false),
 	}
 	go cache.purgeWait(ctx)
+	if cache.commitWriteback {
+		go cache.scanCacheWritebackFailures(ctx)
+	}
 	cache.diskSpaceAvailable(0) // update if cache usage is already high.
 	cache.NewNSLockFn = func(ctx context.Context, cachePath string) RWLocker {
 		return cache.nsMutex.NewNSLock(ctx, nil, cachePath, "")
@@ -323,6 +334,12 @@ func (c *diskCache) purge(ctx context.Context) {
 		// stat all cached file ranges and cacheDataFile.
 		cachedFiles := fiStatFn(meta.Ranges, cacheDataFile, pathJoin(c.dir, name))
 		objInfo := meta.ToObjectInfo("", "")
+		// prevent gc from clearing un-synced commits. This metadata is present when
+		// cache writeback commit setting is enabled.
+		status, ok := objInfo.UserDefined[writeBackStatusHeader]
+		if ok && status != CommitComplete.String() {
+			return nil
+		}
 		cc := cacheControlOpts(objInfo)
 		for fname, fi := range cachedFiles {
 			if cc != nil {
@@ -524,7 +541,11 @@ func (c *diskCache) saveMetadata(ctx context.Context, bucket, object string, met
 	}
 	defer f.Close()
 
-	m := &cacheMeta{Version: cacheMetaVersion}
+	m := &cacheMeta{
+		Version: cacheMetaVersion,
+		Bucket:  bucket,
+		Object:  object,
+	}
 	if err := jsonLoad(f, m); err != nil && err != io.EOF {
 		return err
 	}
@@ -561,7 +582,6 @@ func (c *diskCache) saveMetadata(ctx context.Context, bucket, object string, met
 			m.Meta["etag"] = etag
 		}
 	}
-
 	m.Hits++
 
 	m.Checksum = CacheChecksumInfoV1{Algorithm: HighwayHash256S.String(), Blocksize: cacheBlkSize}
@@ -573,22 +593,22 @@ func getCacheSHADir(dir, bucket, object string) string {
 }
 
 // Cache data to disk with bitrot checksum added for each block of 1MB
-func (c *diskCache) bitrotWriteToCache(cachePath, fileName string, reader io.Reader, size uint64) (int64, error) {
+func (c *diskCache) bitrotWriteToCache(cachePath, fileName string, reader io.Reader, size uint64) (int64, string, error) {
 	if err := os.MkdirAll(cachePath, 0777); err != nil {
-		return 0, err
+		return 0, "", err
 	}
 	filePath := pathJoin(cachePath, fileName)
 
 	if filePath == "" || reader == nil {
-		return 0, errInvalidArgument
+		return 0, "", errInvalidArgument
 	}
 
 	if err := checkPathLength(filePath); err != nil {
-		return 0, err
+		return 0, "", err
 	}
 	f, err := os.Create(filePath)
 	if err != nil {
-		return 0, osErrToFileErr(err)
+		return 0, "", osErrToFileErr(err)
 	}
 	defer f.Close()
 
@@ -598,12 +618,12 @@ func (c *diskCache) bitrotWriteToCache(cachePath, fileName string, reader io.Rea
 
 	bufp := c.pool.Get().(*[]byte)
 	defer c.pool.Put(bufp)
-
+	md5Hash := md5.New()
 	var n, n2 int
 	for {
 		n, err = io.ReadFull(reader, *bufp)
 		if err != nil && err != io.EOF && err != io.ErrUnexpectedEOF {
-			return 0, err
+			return 0, "", err
 		}
 		eof := err == io.EOF || err == io.ErrUnexpectedEOF
 		if n == 0 && size != 0 {
@@ -612,21 +632,28 @@ func (c *diskCache) bitrotWriteToCache(cachePath, fileName string, reader io.Rea
 		}
 		h.Reset()
 		if _, err = h.Write((*bufp)[:n]); err != nil {
-			return 0, err
+			return 0, "", err
 		}
 		hashBytes := h.Sum(nil)
+		// compute md5Hash of original data stream if writeback commit to cache
+		if c.commitWriteback {
+			if _, err = md5Hash.Write((*bufp)[:n]); err != nil {
+				return 0, "", err
+			}
+		}
 		if _, err = f.Write(hashBytes); err != nil {
-			return 0, err
+			return 0, "", err
 		}
 		if n2, err = f.Write((*bufp)[:n]); err != nil {
-			return 0, err
+			return 0, "", err
 		}
 		bytesWritten += int64(n2)
 		if eof {
 			break
 		}
 	}
-	return bytesWritten, nil
+
+	return bytesWritten, base64.StdEncoding.EncodeToString(md5Hash.Sum(nil)), nil
 }
 
 func newCacheEncryptReader(content io.Reader, bucket, object string, metadata map[string]string) (r io.Reader, err error) {
@@ -663,41 +690,41 @@ func newCacheEncryptMetadata(bucket, object string, metadata map[string]string)
 }
 
 // Caches the object to disk
-func (c *diskCache) Put(ctx context.Context, bucket, object string, data io.Reader, size int64, rs *HTTPRangeSpec, opts ObjectOptions, incHitsOnly bool) error {
+func (c *diskCache) Put(ctx context.Context, bucket, object string, data io.Reader, size int64, rs *HTTPRangeSpec, opts ObjectOptions, incHitsOnly bool) (oi ObjectInfo, err error) {
 	if !c.diskSpaceAvailable(size) {
 		io.Copy(ioutil.Discard, data)
-		return errDiskFull
+		return oi, errDiskFull
 	}
 	cachePath := getCacheSHADir(c.dir, bucket, object)
 	cLock := c.NewNSLockFn(ctx, cachePath)
 	if err := cLock.GetLock(globalOperationTimeout); err != nil {
-		return err
+		return oi, err
 	}
 	defer cLock.Unlock()
 
 	meta, _, numHits, err := c.statCache(ctx, cachePath)
 	// Case where object not yet cached
 	if os.IsNotExist(err) && c.after >= 1 {
-		return c.saveMetadata(ctx, bucket, object, opts.UserDefined, size, nil, "", false)
+		return oi, c.saveMetadata(ctx, bucket, object, opts.UserDefined, size, nil, "", false)
 	}
 	// Case where object already has a cache metadata entry but not yet cached
 	if err == nil && numHits < c.after {
 		cETag := extractETag(meta.Meta)
 		bETag := extractETag(opts.UserDefined)
 		if cETag == bETag {
-			return c.saveMetadata(ctx, bucket, object, opts.UserDefined, size, nil, "", false)
+			return oi, c.saveMetadata(ctx, bucket, object, opts.UserDefined, size, nil, "", false)
 		}
 		incHitsOnly = true
 	}
 
 	if rs != nil {
-		return c.putRange(ctx, bucket, object, data, size, rs, opts)
+		return oi, c.putRange(ctx, bucket, object, data, size, rs, opts)
 	}
 	if !c.diskSpaceAvailable(size) {
-		return errDiskFull
+		return oi, errDiskFull
 	}
 	if err := os.MkdirAll(cachePath, 0777); err != nil {
-		return err
+		return oi, err
 	}
 	var metadata = cloneMSS(opts.UserDefined)
 	var reader = data
@@ -705,25 +732,39 @@ func (c *diskCache) Put(ctx context.Context, bucket, object string, data io.Read
 	if globalCacheKMS != nil {
 		reader, err = newCacheEncryptReader(data, bucket, object, metadata)
 		if err != nil {
-			return err
+			return oi, err
 		}
 		actualSize, _ = sio.EncryptedSize(uint64(size))
 	}
-	n, err := c.bitrotWriteToCache(cachePath, cacheDataFile, reader, actualSize)
+	n, md5sum, err := c.bitrotWriteToCache(cachePath, cacheDataFile, reader, actualSize)
 	if IsErr(err, baseErrs...) {
 		// take the cache drive offline
 		c.setOffline()
 	}
 	if err != nil {
 		removeAll(cachePath)
-		return err
+		return oi, err
 	}
 
 	if actualSize != uint64(n) {
 		removeAll(cachePath)
-		return IncompleteBody{Bucket: bucket, Object: object}
+		return oi, IncompleteBody{Bucket: bucket, Object: object}
 	}
-	return c.saveMetadata(ctx, bucket, object, metadata, n, nil, "", incHitsOnly)
+	if c.commitWriteback {
+		metadata["content-md5"] = md5sum
+		if md5bytes, err := base64.StdEncoding.DecodeString(md5sum); err == nil {
+			metadata["etag"] = hex.EncodeToString(md5bytes)
+		}
+		metadata[writeBackStatusHeader] = CommitPending.String()
+	}
+	return ObjectInfo{
+			Bucket:      bucket,
+			Name:        object,
+			ETag:        metadata["etag"],
+			Size:        n,
+			UserDefined: metadata,
+		},
+		c.saveMetadata(ctx, bucket, object, metadata, n, nil, "", incHitsOnly)
 }
 
 // Caches the range to disk
@@ -754,7 +795,7 @@ func (c *diskCache) putRange(ctx context.Context, bucket, object string, data io
 
 	}
 	cacheFile := MustGetUUID()
-	n, err := c.bitrotWriteToCache(cachePath, cacheFile, reader, actualSize)
+	n, _, err := c.bitrotWriteToCache(cachePath, cacheFile, reader, actualSize)
 	if IsErr(err, baseErrs...) {
 		// take the cache drive offline
 		c.setOffline()
@@ -954,3 +995,36 @@ func (c *diskCache) Exists(ctx context.Context, bucket, object string) bool {
 	}
 	return true
 }
+
+// queues writeback upload failures on server startup
+func (c *diskCache) scanCacheWritebackFailures(ctx context.Context) {
+	defer close(c.retryWritebackCh)
+	filterFn := func(name string, typ os.FileMode) error {
+		if name == minioMetaBucket {
+			// Proceed to next file.
+			return nil
+		}
+		cacheDir := pathJoin(c.dir, name)
+		meta, _, _, err := c.statCachedMeta(ctx, cacheDir)
+		if err != nil {
+			return nil
+		}
+
+		objInfo := meta.ToObjectInfo("", "")
+		status, ok := objInfo.UserDefined[writeBackStatusHeader]
+		if !ok || status == CommitComplete.String() {
+			return nil
+		}
+		select {
+		case c.retryWritebackCh <- objInfo:
+		default:
+		}
+
+		return nil
+	}
+
+	if err := readDirFilterFn(c.dir, filterFn); err != nil {
+		logger.LogIf(ctx, err)
+		return
+	}
+}

cmd/disk-cache.go

@@ -22,24 +22,47 @@ import (
 	"fmt"
 	"io"
 	"net/http"
+	"strconv"
 	"strings"
 	"sync"
 	"sync/atomic"
 	"time"
 
 	"github.com/minio/minio/cmd/config/cache"
+	xhttp "github.com/minio/minio/cmd/http"
 	"github.com/minio/minio/cmd/logger"
 	objectlock "github.com/minio/minio/pkg/bucket/object/lock"
 	"github.com/minio/minio/pkg/color"
+	"github.com/minio/minio/pkg/hash"
 	"github.com/minio/minio/pkg/sync/errgroup"
 	"github.com/minio/minio/pkg/wildcard"
 )
 
 const (
-	cacheBlkSize    = int64(1 * 1024 * 1024)
+	cacheBlkSize          = 1 << 20
-	cacheGCInterval = time.Minute * 30
+	cacheGCInterval       = time.Minute * 30
+	writeBackStatusHeader = ReservedMetadataPrefixLower + "write-back-status"
+	writeBackRetryHeader  = ReservedMetadataPrefixLower + "write-back-retry"
 )
 
+type cacheCommitStatus string
+
+const (
+	// CommitPending - cache writeback with backend is pending.
+	CommitPending cacheCommitStatus = "pending"
+
+	// CommitComplete - cache writeback completed ok.
+	CommitComplete cacheCommitStatus = "complete"
+
+	// CommitFailed - cache writeback needs a retry.
+	CommitFailed cacheCommitStatus = "failed"
+)
+
+// String returns string representation of status
+func (s cacheCommitStatus) String() string {
+	return string(s)
+}
+
 // CacheStorageInfo - represents total, free capacity of
 // underlying cache storage.
 type CacheStorageInfo struct {
@@ -69,11 +92,14 @@ type cacheObjects struct {
 	exclude []string
 	// number of accesses after which to cache an object
 	after int
+	// commit objects in async manner
+	commitWriteback bool
 	// if true migration is in progress from v1 to v2
 	migrating bool
 	// mutex to protect migration bool
 	migMutex sync.Mutex
-
+	// retry queue for writeback cache mode to reattempt upload to backend
+	wbRetryCh chan ObjectInfo
 	// Cache stats
 	cacheStats *CacheStats
 
@@ -333,7 +359,7 @@ func (c *cacheObjects) GetObjectNInfo(ctx context.Context, bucket, object string
 	teeReader := io.TeeReader(bkReader, pipeWriter)
 	userDefined := getMetadata(bkReader.ObjInfo)
 	go func() {
-		putErr := dcache.Put(ctx, bucket, object,
+		_, putErr := dcache.Put(ctx, bucket, object,
 			io.LimitReader(pipeReader, bkReader.ObjInfo.Size),
 			bkReader.ObjInfo.Size, rs, ObjectOptions{
 				UserDefined: userDefined,
@@ -629,7 +655,14 @@ func (c *cacheObjects) PutObject(ctx context.Context, bucket, object string, r *
 		dcache.Delete(ctx, bucket, object)
 		return putObjectFn(ctx, bucket, object, r, opts)
 	}
-
+	if c.commitWriteback {
+		oi, err := dcache.Put(ctx, bucket, object, r, r.Size(), nil, opts, false)
+		if err != nil {
+			return ObjectInfo{}, err
+		}
+		go c.uploadObject(GlobalContext, oi)
+		return oi, nil
+	}
 	objInfo, err = putObjectFn(ctx, bucket, object, r, opts)
 
 	if err == nil {
@@ -647,9 +680,68 @@ func (c *cacheObjects) PutObject(ctx context.Context, bucket, object string, r *
 			}
 		}()
 	}
-
 	return objInfo, err
+}
 
+// upload cached object to backend in async commit mode.
+func (c *cacheObjects) uploadObject(ctx context.Context, oi ObjectInfo) {
+	dcache, err := c.getCacheToLoc(ctx, oi.Bucket, oi.Name)
+	if err != nil {
+		// disk cache could not be located.
+		logger.LogIf(ctx, fmt.Errorf("Could not upload %s/%s to backend: %w", oi.Bucket, oi.Name, err))
+		return
+	}
+	cReader, _, bErr := dcache.Get(ctx, oi.Bucket, oi.Name, nil, http.Header{}, ObjectOptions{})
+	if bErr != nil {
+		return
+	}
+	defer cReader.Close()
+
+	if cReader.ObjInfo.ETag != oi.ETag {
+		return
+	}
+	st := cacheCommitStatus(oi.UserDefined[writeBackStatusHeader])
+	if st == CommitComplete || st.String() == "" {
+		return
+	}
+	hashReader, err := hash.NewReader(cReader, oi.Size, "", "", oi.Size, globalCLIContext.StrictS3Compat)
+	if err != nil {
+		return
+	}
+	var opts ObjectOptions
+	opts.UserDefined = make(map[string]string)
+	opts.UserDefined[xhttp.ContentMD5] = oi.UserDefined["content-md5"]
+	objInfo, err := c.InnerPutObjectFn(ctx, oi.Bucket, oi.Name, NewPutObjReader(hashReader, nil, nil), opts)
+	wbCommitStatus := CommitComplete
+	if err != nil {
+		wbCommitStatus = CommitFailed
+	}
+
+	meta := cloneMSS(cReader.ObjInfo.UserDefined)
+	retryCnt := 0
+	if wbCommitStatus == CommitFailed {
+		retryCnt, _ = strconv.Atoi(meta[writeBackRetryHeader])
+		retryCnt++
+		meta[writeBackRetryHeader] = strconv.Itoa(retryCnt)
+	} else {
+		delete(meta, writeBackRetryHeader)
+	}
+	meta[writeBackStatusHeader] = wbCommitStatus.String()
+	meta["etag"] = oi.ETag
+	dcache.SaveMetadata(ctx, oi.Bucket, oi.Name, meta, objInfo.Size, nil, "", false)
+	if retryCnt > 0 {
+		// slow down retries
+		time.Sleep(time.Second * time.Duration(retryCnt%10+1))
+		c.queueWritebackRetry(oi)
+	}
+}
+
+func (c *cacheObjects) queueWritebackRetry(oi ObjectInfo) {
+	select {
+	case c.wbRetryCh <- oi:
+		c.uploadObject(GlobalContext, oi)
+	default:
+	}
 }
 
 // Returns cacheObjects for use by Server.
@@ -660,12 +752,13 @@ func newServerCacheObjects(ctx context.Context, config cache.Config) (CacheObjec
 		return nil, err
 	}
 	c := &cacheObjects{
-		cache:      cache,
+		cache:           cache,
-		exclude:    config.Exclude,
+		exclude:         config.Exclude,
-		after:      config.After,
+		after:           config.After,
-		migrating:  migrateSw,
+		migrating:       migrateSw,
-		migMutex:   sync.Mutex{},
+		migMutex:        sync.Mutex{},
-		cacheStats: newCacheStats(),
+		commitWriteback: config.CommitWriteback,
+		cacheStats:      newCacheStats(),
 		InnerGetObjectInfoFn: func(ctx context.Context, bucket, object string, opts ObjectOptions) (ObjectInfo, error) {
 			return newObjectLayerFn().GetObjectInfo(ctx, bucket, object, opts)
 		},
@@ -699,6 +792,15 @@ func newServerCacheObjects(ctx context.Context, config cache.Config) (CacheObjec
 		go c.migrateCacheFromV1toV2(ctx)
 	}
 	go c.gc(ctx)
+	if c.commitWriteback {
+		c.wbRetryCh = make(chan ObjectInfo, 10000)
+		go func() {
+			<-GlobalContext.Done()
+			close(c.wbRetryCh)
+		}()
+		go c.queuePendingWriteback(ctx)
+	}
+
 	return c, nil
 }
 
@@ -724,3 +826,25 @@ func (c *cacheObjects) gc(ctx context.Context) {
 		}
 	}
 }
+
+// queues any pending or failed async commits when server restarts
+func (c *cacheObjects) queuePendingWriteback(ctx context.Context) {
+	for _, dcache := range c.cache {
+		if dcache != nil {
+			for {
+				select {
+				case <-ctx.Done():
+					return
+				case oi, ok := <-dcache.retryWritebackCh:
+					if !ok {
+						goto next
+					}
+					c.queueWritebackRetry(oi)
+				default:
+					time.Sleep(time.Second * 1)
+				}
+			}
+		next:
+		}
+	}
+}

cmd/format-disk-cache.go

@@ -365,7 +365,7 @@ func migrateCacheData(ctx context.Context, c *diskCache, bucket, object, oldfile
 		}
 		actualSize, _ = sio.EncryptedSize(uint64(st.Size()))
 	}
-	_, err = c.bitrotWriteToCache(destDir, cacheDataFile, reader, actualSize)
+	_, _, err = c.bitrotWriteToCache(destDir, cacheDataFile, reader, actualSize)
 	return err
 }