diff --git a/pkg/blobserver/diskpacked/diskpacked.go b/pkg/blobserver/diskpacked/diskpacked.go
index d6d204914..01ea73feb 100644
--- a/pkg/blobserver/diskpacked/diskpacked.go
+++ b/pkg/blobserver/diskpacked/diskpacked.go
@@ -324,6 +324,19 @@ func (s *storage) Close() error {
 }
 
 func (s *storage) Fetch(br blob.Ref) (io.ReadCloser, uint32, error) {
+	return s.fetch(br, 0, -1)
+}
+
+func (s *storage) SubFetch(br blob.Ref, offset, length int64) (io.ReadCloser, error) {
+	if offset < 0 || length < 0 {
+		return nil, errors.New("invalid offset or length")
+	}
+	rc, _, err := s.fetch(br, offset, length)
+	return rc, err
+}
+
+// length of -1 means all
+func (s *storage) fetch(br blob.Ref, offset, length int64) (rc io.ReadCloser, size uint32, err error) {
 	meta, err := s.meta(br)
 	if err != nil {
 		return nil, 0, err
@@ -333,7 +346,19 @@ func (s *storage) Fetch(br blob.Ref) (io.ReadCloser, uint32, error) {
 		return nil, 0, fmt.Errorf("diskpacked: attempt to fetch blob from out of range pack file %d > %d", meta.file, len(s.fds))
 	}
 	rac := s.fds[meta.file]
-	var rs io.ReadSeeker = io.NewSectionReader(rac, meta.offset, int64(meta.size))
+	var rs io.ReadSeeker
+	if length == -1 {
+		// normal Fetch mode
+		rs = io.NewSectionReader(rac, meta.offset, int64(meta.size))
+	} else {
+		if offset > int64(meta.size) {
+			offset = int64(meta.size)
+			length = 0
+		} else if offset+length > int64(meta.size) {
+			length = int64(meta.size) - offset
+		}
+		rs = io.NewSectionReader(rac, meta.offset+offset, length)
+	}
 	fn := rac.Name()
 	// Ensure entry is in map.
 	readVar.Add(fn, 0)
diff --git a/pkg/blobserver/localdisk/localdisk.go b/pkg/blobserver/localdisk/localdisk.go
index c68172a3f..d11b469f1 100644
--- a/pkg/blobserver/localdisk/localdisk.go
+++ b/pkg/blobserver/localdisk/localdisk.go
@@ -31,6 +31,7 @@ Example low-level config:
 package localdisk
 
 import (
+	"errors"
 	"fmt"
 	"io"
 	"os"
@@ -116,13 +117,26 @@ func (ds *DiskStorage) tryRemoveDir(dir string) {
 	os.Remove(dir) // ignore error
 }
 
-func (ds *DiskStorage) Fetch(blob blob.Ref) (io.ReadCloser, uint32, error) {
-	fileName := ds.blobPath(blob)
+func (ds *DiskStorage) Fetch(br blob.Ref) (io.ReadCloser, uint32, error) {
+	return ds.fetch(br, 0, -1)
+}
+
+func (ds *DiskStorage) SubFetch(br blob.Ref, offset, length int64) (io.ReadCloser, error) {
+	if offset < 0 || length < 0 {
+		return nil, errors.New("invalid offset or length")
+	}
+	rc, _, err := ds.fetch(br, offset, length)
+	return rc, err
+}
+
+// length -1 means entire file
+func (ds *DiskStorage) fetch(br blob.Ref, offset, length int64) (rc io.ReadCloser, size uint32, err error) {
+	fileName := ds.blobPath(br)
 	stat, err := os.Stat(fileName)
 	if os.IsNotExist(err) {
 		return nil, 0, os.ErrNotExist
 	}
-	size := types.U32(stat.Size())
+	size = types.U32(stat.Size())
 	file, err := os.Open(fileName)
 	if err != nil {
 		if os.IsNotExist(err) {
@@ -130,7 +144,18 @@ func (ds *DiskStorage) Fetch(blob blob.Ref) (io.ReadCloser, uint32, error) {
 		}
 		return nil, 0, err
 	}
-	return file, size, nil
+	// normal Fetch:
+	if length < 0 {
+		return file, size, nil
+	}
+	// SubFetch:
+	return struct {
+		io.Reader
+		io.Closer
+	}{
+		io.NewSectionReader(file, offset, length),
+		file,
+	}, 0 /* unused */, err
 }
 
 func (ds *DiskStorage) RemoveBlobs(blobs []blob.Ref) error {
diff --git a/pkg/test/fetcher.go b/pkg/test/fetcher.go
index d49fed1de..97a4c9ec4 100644
--- a/pkg/test/fetcher.go
+++ b/pkg/test/fetcher.go
@@ -98,6 +98,30 @@ func (tf *Fetcher) Fetch(ref blob.Ref) (file io.ReadCloser, size uint32, err err
 	}, size, nil
 }
 
+func (tf *Fetcher) SubFetch(ref blob.Ref, offset, length int64) (io.ReadCloser, error) {
+	if tf.FetchErr != nil {
+		if err := tf.FetchErr(); err != nil {
+			return nil, err
+		}
+	}
+	tf.mu.RLock()
+	defer tf.mu.RUnlock()
+	tb, ok := tf.m[ref.String()]
+	if !ok {
+		return nil, os.ErrNotExist
+	}
+	atomic.AddInt64(&tf.blobsFetched, 1)
+	atomic.AddInt64(&tf.bytesFetched, length)
+
+	return struct {
+		*io.SectionReader
+		io.Closer
+	}{
+		io.NewSectionReader(strings.NewReader(tb.Contents), offset, int64(length)),
+		types.NopCloser,
+	}, nil
+}
+
 func (tf *Fetcher) BlobContents(br blob.Ref) (contents string, ok bool) {
 	tf.mu.RLock()
 	defer tf.mu.RUnlock()