mount: improve read throughput with parallel chunk fetching (#7569)

* mount: improve read throughput with parallel chunk fetching

This addresses issue #7504 where a single weed mount FUSE instance
does not fully utilize node network bandwidth when reading large files.

Changes:
- Add -concurrentReaders mount option (default: 16) to control the
  maximum number of parallel chunk fetches during read operations
- Implement parallel section reading in ChunkGroup.ReadDataAt() using
  errgroup for better throughput when reading across multiple sections
- Enhance ReaderCache with MaybeCacheMany() to prefetch multiple chunks
  ahead in parallel during sequential reads (now prefetches 4 chunks)
- Increase ReaderCache limit dynamically based on concurrentReaders
  to support higher read parallelism

The bottleneck was that chunks were being read sequentially even when
they reside on different volume servers. By introducing parallel chunk
fetching, a single mount instance can now better saturate available
network bandwidth.

Fixes: #7504

* fmt

* Address review comments: make prefetch configurable, improve error handling

Changes:
1. Add DefaultPrefetchCount constant (4) to reader_at.go
2. Add GetPrefetchCount() method to ChunkGroup that derives prefetch count
   from concurrentReaders (1/4 ratio, min 1, max 8)
3. Pass prefetch count through NewChunkReaderAtFromClient
4. Fix error handling in readDataAtParallel to prioritize errgroup error
5. Update all callers to use DefaultPrefetchCount constant

For mount operations, prefetch scales with -concurrentReaders:
- concurrentReaders=16 (default) -> prefetch=4
- concurrentReaders=32 -> prefetch=8 (capped)
- concurrentReaders=4 -> prefetch=1

For non-mount paths (WebDAV, query engine, MQ), uses DefaultPrefetchCount.

* fmt

* Refactor: use variadic parameter instead of new function name

Use NewChunkGroup with optional concurrentReaders parameter instead of
creating a separate NewChunkGroupWithConcurrency function.

This maintains backward compatibility - existing callers without the
parameter get the default of 16 concurrent readers.

* Use explicit concurrentReaders parameter instead of variadic

* Refactor: use MaybeCache with count parameter instead of new MaybeCacheMany function

* Address nitpick review comments

- Add upper bound (128) on concurrentReaders to prevent excessive goroutine fan-out
- Cap readerCacheLimit at 256 accordingly
- Fix SetChunks: use Lock() instead of RLock() since we are writing to group.sections

weed/command/fuse_std.go

@@ -155,6 +155,13 @@ func runFuse(cmd *Command, args []string) bool {
 			} else {
 				panic(fmt.Errorf("concurrentWriters: %s", err))
 			}
+		case "concurrentReaders":
+			if parsed, err := strconv.ParseInt(parameter.value, 0, 32); err == nil {
+				intValue := int(parsed)
+				mountOptions.concurrentReaders = &intValue
+			} else {
+				panic(fmt.Errorf("concurrentReaders: %s", err))
+			}
 		case "cacheDir":
 			mountOptions.cacheDirForRead = &parameter.value
 		case "cacheCapacityMB":

weed/command/mount.go

@@ -17,6 +17,7 @@ type MountOptions struct {
 	ttlSec             *int
 	chunkSizeLimitMB   *int
 	concurrentWriters  *int
+	concurrentReaders  *int
 	cacheMetaTtlSec    *int
 	cacheDirForRead    *string
 	cacheDirForWrite   *string
@@ -65,6 +66,7 @@ func init() {
 	mountOptions.ttlSec = cmdMount.Flag.Int("ttl", 0, "file ttl in seconds")
 	mountOptions.chunkSizeLimitMB = cmdMount.Flag.Int("chunkSizeLimitMB", 2, "local write buffer size, also chunk large files")
 	mountOptions.concurrentWriters = cmdMount.Flag.Int("concurrentWriters", 32, "limit concurrent goroutine writers")
+	mountOptions.concurrentReaders = cmdMount.Flag.Int("concurrentReaders", 16, "limit concurrent chunk fetches for read operations")
 	mountOptions.cacheDirForRead = cmdMount.Flag.String("cacheDir", os.TempDir(), "local cache directory for file chunks and meta data")
 	mountOptions.cacheSizeMBForRead = cmdMount.Flag.Int64("cacheCapacityMB", 128, "file chunk read cache capacity in MB")
 	mountOptions.cacheDirForWrite = cmdMount.Flag.String("cacheDirWrite", "", "buffer writes mostly for large files")

weed/command/mount_std.go

@@ -236,6 +236,7 @@ func RunMount(option *MountOptions, umask os.FileMode) bool {
 		DiskType:           types.ToDiskType(*option.diskType),
 		ChunkSizeLimit:     int64(chunkSizeLimitMB) * 1024 * 1024,
 		ConcurrentWriters:  *option.concurrentWriters,
+		ConcurrentReaders:  *option.concurrentReaders,
 		CacheDirForRead:    *option.cacheDirForRead,
 		CacheSizeMBForRead: *option.cacheSizeMBForRead,
 		CacheDirForWrite:   cacheDirForWrite,

weed/filer/filechunk_group.go

@@ -5,29 +5,64 @@ import (
 	"io"
 	"sync"
 
+	"golang.org/x/sync/errgroup"
+
 	"github.com/seaweedfs/seaweedfs/weed/pb/filer_pb"
 	"github.com/seaweedfs/seaweedfs/weed/util/chunk_cache"
 	"github.com/seaweedfs/seaweedfs/weed/wdclient"
 )
 
 type ChunkGroup struct {
-	lookupFn     wdclient.LookupFileIdFunctionType
-	sections     map[SectionIndex]*FileChunkSection
-	sectionsLock sync.RWMutex
-	readerCache  *ReaderCache
+	lookupFn          wdclient.LookupFileIdFunctionType
+	sections          map[SectionIndex]*FileChunkSection
+	sectionsLock      sync.RWMutex
+	readerCache       *ReaderCache
+	concurrentReaders int
 }
 
-func NewChunkGroup(lookupFn wdclient.LookupFileIdFunctionType, chunkCache chunk_cache.ChunkCache, chunks []*filer_pb.FileChunk) (*ChunkGroup, error) {
+// NewChunkGroup creates a ChunkGroup with configurable concurrency.
+// concurrentReaders controls:
+// - Maximum parallel chunk fetches during read operations
+// - Read-ahead prefetch parallelism
+// - Number of concurrent section reads for large files
+// If concurrentReaders <= 0, defaults to 16.
+func NewChunkGroup(lookupFn wdclient.LookupFileIdFunctionType, chunkCache chunk_cache.ChunkCache, chunks []*filer_pb.FileChunk, concurrentReaders int) (*ChunkGroup, error) {
+	if concurrentReaders <= 0 {
+		concurrentReaders = 16
+	}
+	if concurrentReaders > 128 {
+		concurrentReaders = 128 // Cap to prevent excessive goroutine fan-out
+	}
+	// ReaderCache limit should be at least concurrentReaders to allow parallel prefetching
+	readerCacheLimit := concurrentReaders * 2
+	if readerCacheLimit < 32 {
+		readerCacheLimit = 32
+	}
 	group := &ChunkGroup{
-		lookupFn:    lookupFn,
-		sections:    make(map[SectionIndex]*FileChunkSection),
-		readerCache: NewReaderCache(32, chunkCache, lookupFn),
+		lookupFn:          lookupFn,
+		sections:          make(map[SectionIndex]*FileChunkSection),
+		readerCache:       NewReaderCache(readerCacheLimit, chunkCache, lookupFn),
+		concurrentReaders: concurrentReaders,
 	}
 
 	err := group.SetChunks(chunks)
 	return group, err
 }
 
+// GetPrefetchCount returns the number of chunks to prefetch ahead during sequential reads.
+// This is derived from concurrentReaders to keep the network pipeline full.
+func (group *ChunkGroup) GetPrefetchCount() int {
+	// Prefetch at least 1, and scale with concurrency (roughly 1/4 of concurrent readers)
+	prefetch := group.concurrentReaders / 4
+	if prefetch < 1 {
+		prefetch = 1
+	}
+	if prefetch > 8 {
+		prefetch = 8 // Cap at 8 to avoid excessive memory usage
+	}
+	return prefetch
+}
+
 func (group *ChunkGroup) AddChunk(chunk *filer_pb.FileChunk) error {
 
 	group.sectionsLock.Lock()
@@ -54,6 +89,19 @@ func (group *ChunkGroup) ReadDataAt(ctx context.Context, fileSize int64, buff []
 	defer group.sectionsLock.RUnlock()
 
 	sectionIndexStart, sectionIndexStop := SectionIndex(offset/SectionSize), SectionIndex((offset+int64(len(buff)))/SectionSize)
+	numSections := int(sectionIndexStop - sectionIndexStart + 1)
+
+	// For single section or when concurrency is disabled, use sequential reading
+	if numSections <= 1 || group.concurrentReaders <= 1 {
+		return group.readDataAtSequential(ctx, fileSize, buff, offset, sectionIndexStart, sectionIndexStop)
+	}
+
+	// For multiple sections, use parallel reading
+	return group.readDataAtParallel(ctx, fileSize, buff, offset, sectionIndexStart, sectionIndexStop)
+}
+
+// readDataAtSequential reads sections sequentially (original behavior)
+func (group *ChunkGroup) readDataAtSequential(ctx context.Context, fileSize int64, buff []byte, offset int64, sectionIndexStart, sectionIndexStop SectionIndex) (n int, tsNs int64, err error) {
 	for si := sectionIndexStart; si < sectionIndexStop+1; si++ {
 		section, found := group.sections[si]
 		rangeStart, rangeStop := max(offset, int64(si*SectionSize)), min(offset+int64(len(buff)), int64((si+1)*SectionSize))
@@ -78,9 +126,98 @@ func (group *ChunkGroup) ReadDataAt(ctx context.Context, fileSize int64, buff []
 	return
 }
 
+// sectionReadResult holds the result of a section read operation
+type sectionReadResult struct {
+	sectionIndex SectionIndex
+	n            int
+	tsNs         int64
+	err          error
+}
+
+// readDataAtParallel reads multiple sections in parallel for better throughput
+func (group *ChunkGroup) readDataAtParallel(ctx context.Context, fileSize int64, buff []byte, offset int64, sectionIndexStart, sectionIndexStop SectionIndex) (n int, tsNs int64, err error) {
+	numSections := int(sectionIndexStop - sectionIndexStart + 1)
+
+	// Limit concurrency to the smaller of concurrentReaders and numSections
+	maxConcurrent := group.concurrentReaders
+	if numSections < maxConcurrent {
+		maxConcurrent = numSections
+	}
+
+	g, gCtx := errgroup.WithContext(ctx)
+	g.SetLimit(maxConcurrent)
+
+	results := make([]sectionReadResult, numSections)
+
+	for i := 0; i < numSections; i++ {
+		si := sectionIndexStart + SectionIndex(i)
+		idx := i
+
+		section, found := group.sections[si]
+		rangeStart, rangeStop := max(offset, int64(si*SectionSize)), min(offset+int64(len(buff)), int64((si+1)*SectionSize))
+		if rangeStart >= rangeStop {
+			continue
+		}
+
+		if !found {
+			// Zero-fill missing sections synchronously
+			rangeStop = min(rangeStop, fileSize)
+			for j := rangeStart; j < rangeStop; j++ {
+				buff[j-offset] = 0
+			}
+			results[idx] = sectionReadResult{
+				sectionIndex: si,
+				n:            int(rangeStop - rangeStart),
+				tsNs:         0,
+				err:          nil,
+			}
+			continue
+		}
+
+		// Capture variables for closure
+		sectionCopy := section
+		buffSlice := buff[rangeStart-offset : rangeStop-offset]
+		rangeStartCopy := rangeStart
+
+		g.Go(func() error {
+			xn, xTsNs, xErr := sectionCopy.readDataAt(gCtx, group, fileSize, buffSlice, rangeStartCopy)
+			results[idx] = sectionReadResult{
+				sectionIndex: si,
+				n:            xn,
+				tsNs:         xTsNs,
+				err:          xErr,
+			}
+			if xErr != nil && xErr != io.EOF {
+				return xErr
+			}
+			return nil
+		})
+	}
+
+	// Wait for all goroutines to complete
+	groupErr := g.Wait()
+
+	// Aggregate results
+	for _, result := range results {
+		n += result.n
+		tsNs = max(tsNs, result.tsNs)
+		// Collect first non-EOF error from results as fallback
+		if result.err != nil && result.err != io.EOF && err == nil {
+			err = result.err
+		}
+	}
+
+	// Prioritize errgroup error (first error that cancelled context)
+	if groupErr != nil {
+		err = groupErr
+	}
+
+	return
+}
+
 func (group *ChunkGroup) SetChunks(chunks []*filer_pb.FileChunk) error {
-	group.sectionsLock.RLock()
-	defer group.sectionsLock.RUnlock()
+	group.sectionsLock.Lock()
+	defer group.sectionsLock.Unlock()
 
 	var dataChunks []*filer_pb.FileChunk
 	for _, chunk := range chunks {

weed/filer/filechunk_section.go

@@ -85,7 +85,7 @@ func (section *FileChunkSection) setupForRead(ctx context.Context, group *ChunkG
 	}
 
 	if section.reader == nil {
-		section.reader = NewChunkReaderAtFromClient(ctx, group.readerCache, section.chunkViews, min(int64(section.sectionIndex+1)*SectionSize, fileSize))
+		section.reader = NewChunkReaderAtFromClient(ctx, group.readerCache, section.chunkViews, min(int64(section.sectionIndex+1)*SectionSize, fileSize), group.GetPrefetchCount())
 	}
 
 	section.isPrepared = true

weed/filer/reader_at.go

@@ -13,6 +13,12 @@ import (
 	"github.com/seaweedfs/seaweedfs/weed/wdclient"
 )
 
+// DefaultPrefetchCount is the default number of chunks to prefetch ahead during
+// sequential reads. This value is used when prefetch count is not explicitly
+// configured (e.g., WebDAV, query engine, message queue). For mount operations,
+// the prefetch count is derived from the -concurrentReaders option.
+const DefaultPrefetchCount = 4
+
 type ChunkReadAt struct {
 	masterClient  *wdclient.MasterClient
 	chunkViews    *IntervalList[*ChunkView]
@@ -20,6 +26,7 @@ type ChunkReadAt struct {
 	readerCache   *ReaderCache
 	readerPattern *ReaderPattern
 	lastChunkFid  string
+	prefetchCount int             // Number of chunks to prefetch ahead during sequential reads
 	ctx           context.Context // Context used for cancellation during chunk read operations
 }
 
@@ -35,8 +42,9 @@ var _ = io.Closer(&ChunkReadAt{})
 //   - No high availability (single filer address, no automatic failover)
 //
 // For NEW code, especially mount operations, use wdclient.FilerClient instead:
-//   filerClient := wdclient.NewFilerClient(filerAddresses, grpcDialOption, dataCenter, opts)
-//   lookupFn := filerClient.GetLookupFileIdFunction()
+//
+//	filerClient := wdclient.NewFilerClient(filerAddresses, grpcDialOption, dataCenter, opts)
+//	lookupFn := filerClient.GetLookupFileIdFunction()
 //
 // This provides:
 //   - Bounded cache with configurable size
@@ -56,7 +64,7 @@ func LookupFn(filerClient filer_pb.FilerClient) wdclient.LookupFileIdFunctionTyp
 	var vidCacheLock sync.RWMutex
 	cacheSize := 0
 	const maxCacheSize = 10000 // Simple bound to prevent unbounded growth
-	
+
 	return func(ctx context.Context, fileId string) (targetUrls []string, err error) {
 		vid := VolumeId(fileId)
 		vidCacheLock.RLock()
@@ -123,13 +131,14 @@ func LookupFn(filerClient filer_pb.FilerClient) wdclient.LookupFileIdFunctionTyp
 	}
 }
 
-func NewChunkReaderAtFromClient(ctx context.Context, readerCache *ReaderCache, chunkViews *IntervalList[*ChunkView], fileSize int64) *ChunkReadAt {
+func NewChunkReaderAtFromClient(ctx context.Context, readerCache *ReaderCache, chunkViews *IntervalList[*ChunkView], fileSize int64, prefetchCount int) *ChunkReadAt {
 
 	return &ChunkReadAt{
 		chunkViews:    chunkViews,
 		fileSize:      fileSize,
 		readerCache:   readerCache,
 		readerPattern: NewReaderPattern(),
+		prefetchCount: prefetchCount,
 		ctx:           ctx,
 	}
 }
@@ -246,8 +255,10 @@ func (c *ChunkReadAt) readChunkSliceAt(ctx context.Context, buffer []byte, chunk
 			if c.lastChunkFid != "" {
 				c.readerCache.UnCache(c.lastChunkFid)
 			}
-			if nextChunkViews != nil {
-				c.readerCache.MaybeCache(nextChunkViews) // just read the next chunk if at the very beginning
+			if nextChunkViews != nil && c.prefetchCount > 0 {
+				// Prefetch multiple chunks ahead for better sequential read throughput
+				// This keeps the network pipeline full with parallel chunk fetches
+				c.readerCache.MaybeCache(nextChunkViews, c.prefetchCount)
 			}
 		}
 	}

weed/filer/reader_cache.go

@@ -46,10 +46,16 @@ func NewReaderCache(limit int, chunkCache chunk_cache.ChunkCache, lookupFileIdFn
 	}
 }
 
-func (rc *ReaderCache) MaybeCache(chunkViews *Interval[*ChunkView]) {
+// MaybeCache prefetches up to 'count' chunks ahead in parallel.
+// This improves read throughput for sequential reads by keeping the
+// network pipeline full with parallel chunk fetches.
+func (rc *ReaderCache) MaybeCache(chunkViews *Interval[*ChunkView], count int) {
 	if rc.lookupFileIdFn == nil {
 		return
 	}
+	if count <= 0 {
+		count = 1
+	}
 
 	rc.Lock()
 	defer rc.Unlock()
@@ -58,7 +64,8 @@ func (rc *ReaderCache) MaybeCache(chunkViews *Interval[*ChunkView]) {
 		return
 	}
 
-	for x := chunkViews; x != nil; x = x.Next {
+	cached := 0
+	for x := chunkViews; x != nil && cached < count; x = x.Next {
 		chunkView := x.Value
 		if _, found := rc.downloaders[chunkView.FileId]; found {
 			continue
@@ -80,7 +87,7 @@ func (rc *ReaderCache) MaybeCache(chunkViews *Interval[*ChunkView]) {
 		go cacher.startCaching()
 		<-cacher.cacheStartedCh
 		rc.downloaders[chunkView.FileId] = cacher
-
+		cached++
 	}
 
 	return

weed/mount/filehandle.go

@@ -81,7 +81,7 @@ func (fh *FileHandle) SetEntry(entry *filer_pb.Entry) {
 		fileSize := filer.FileSize(entry)
 		entry.Attributes.FileSize = fileSize
 		var resolveManifestErr error
-		fh.entryChunkGroup, resolveManifestErr = filer.NewChunkGroup(fh.wfs.LookupFn(), fh.wfs.chunkCache, entry.Chunks)
+		fh.entryChunkGroup, resolveManifestErr = filer.NewChunkGroup(fh.wfs.LookupFn(), fh.wfs.chunkCache, entry.Chunks, fh.wfs.option.ConcurrentReaders)
 		if resolveManifestErr != nil {
 			glog.Warningf("failed to resolve manifest chunks in %+v", entry)
 		}

weed/mount/weedfs.go

@@ -42,6 +42,7 @@ type Option struct {
 	DiskType           types.DiskType
 	ChunkSizeLimit     int64
 	ConcurrentWriters  int
+	ConcurrentReaders  int
 	CacheDirForRead    string
 	CacheSizeMBForRead int64
 	CacheDirForWrite   string

weed/mq/logstore/read_parquet_to_log.go

@@ -101,7 +101,7 @@ func GenParquetReadFunc(filerClient filer_pb.FilerClient, t topic.Topic, p topic
 		visibleIntervals, _ := filer.NonOverlappingVisibleIntervals(context.Background(), lookupFileIdFn, entry.Chunks, 0, int64(fileSize))
 		chunkViews := filer.ViewFromVisibleIntervals(visibleIntervals, 0, int64(fileSize))
 		readerCache := filer.NewReaderCache(32, chunkCache, lookupFileIdFn)
-		readerAt := filer.NewChunkReaderAtFromClient(context.Background(), readerCache, chunkViews, int64(fileSize))
+		readerAt := filer.NewChunkReaderAtFromClient(context.Background(), readerCache, chunkViews, int64(fileSize), filer.DefaultPrefetchCount)
 
 		// create parquet reader
 		parquetReader := parquet.NewReader(readerAt)

weed/query/engine/hybrid_message_scanner.go

@@ -1286,7 +1286,7 @@ func (h *HybridMessageScanner) extractParquetFileStats(entry *filer_pb.Entry, lo
 	visibleIntervals, _ := filer.NonOverlappingVisibleIntervals(context.Background(), lookupFileIdFn, entry.Chunks, 0, int64(fileSize))
 	chunkViews := filer.ViewFromVisibleIntervals(visibleIntervals, 0, int64(fileSize))
 	readerCache := filer.NewReaderCache(32, chunkCache, lookupFileIdFn)
-	readerAt := filer.NewChunkReaderAtFromClient(context.Background(), readerCache, chunkViews, int64(fileSize))
+	readerAt := filer.NewChunkReaderAtFromClient(context.Background(), readerCache, chunkViews, int64(fileSize), filer.DefaultPrefetchCount)
 
 	// Create parquet reader - this only reads metadata, not data
 	parquetReader := parquet.NewReader(readerAt)

weed/query/engine/parquet_scanner.go

@@ -182,7 +182,7 @@ func (ps *ParquetScanner) scanParquetFile(ctx context.Context, entry *filer_pb.E
 	visibleIntervals, _ := filer.NonOverlappingVisibleIntervals(ctx, lookupFileIdFn, entry.Chunks, 0, int64(fileSize))
 	chunkViews := filer.ViewFromVisibleIntervals(visibleIntervals, 0, int64(fileSize))
 	readerCache := filer.NewReaderCache(32, ps.chunkCache, lookupFileIdFn)
-	readerAt := filer.NewChunkReaderAtFromClient(ctx, readerCache, chunkViews, int64(fileSize))
+	readerAt := filer.NewChunkReaderAtFromClient(ctx, readerCache, chunkViews, int64(fileSize), filer.DefaultPrefetchCount)
 
 	// Create Parquet reader
 	parquetReader := parquet.NewReader(readerAt)

weed/server/webdav_server.go

@@ -566,7 +566,7 @@ func (f *WebDavFile) Read(p []byte) (readSize int, err error) {
 	}
 	if f.reader == nil {
 		chunkViews := filer.ViewFromVisibleIntervals(f.visibleIntervals, 0, fileSize)
-		f.reader = filer.NewChunkReaderAtFromClient(f.ctx, f.fs.readerCache, chunkViews, fileSize)
+		f.reader = filer.NewChunkReaderAtFromClient(f.ctx, f.fs.readerCache, chunkViews, fileSize, filer.DefaultPrefetchCount)
 	}
 
 	readSize, err = f.reader.ReadAt(p, f.off)