diff --git a/go.mod b/go.mod
index fbb9764ee..21320ab0e 100644
--- a/go.mod
+++ b/go.mod
@@ -78,6 +78,7 @@ require (
 	gocloud.dev/pubsub/rabbitpubsub v0.16.0
 	golang.org/x/image v0.0.0-20200119044424-58c23975cae1 // indirect
 	golang.org/x/net v0.0.0-20200202094626-16171245cfb2
+	golang.org/x/sync v0.0.0-20200930132711-30421366ff76 // indirect
 	golang.org/x/sys v0.0.0-20200202164722-d101bd2416d5
 	golang.org/x/tools v0.0.0-20191029190741-b9c20aec41a5
 	google.golang.org/api v0.9.0
diff --git a/go.sum b/go.sum
index b9ceb80fc..3ac47ec22 100644
--- a/go.sum
+++ b/go.sum
@@ -615,6 +615,8 @@ golang.org/x/sync v0.0.0-20190227155943-e225da77a7e6/go.mod h1:RxMgew5VJxzue5/jJ
 golang.org/x/sync v0.0.0-20190412183630-56d357773e84/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
 golang.org/x/sync v0.0.0-20190423024810-112230192c58 h1:8gQV6CLnAEikrhgkHFbMAEhagSSnXWGV915qUMm9mrU=
 golang.org/x/sync v0.0.0-20190423024810-112230192c58/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
+golang.org/x/sync v0.0.0-20200930132711-30421366ff76 h1:JnxiSYT3Nm0BT2a8CyvYyM6cnrWpidecD1UuSYbhKm0=
+golang.org/x/sync v0.0.0-20200930132711-30421366ff76/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
 golang.org/x/sys v0.0.0-20180830151530-49385e6e1522/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
 golang.org/x/sys v0.0.0-20180905080454-ebe1bf3edb33/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
 golang.org/x/sys v0.0.0-20180909124046-d0be0721c37e/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
diff --git a/weed/filer/reader_at.go b/weed/filer/reader_at.go
index 5ffc3a024..1c11f718a 100644
--- a/weed/filer/reader_at.go
+++ b/weed/filer/reader_at.go
@@ -3,6 +3,7 @@ package filer
 import (
 	"context"
 	"fmt"
+	"github.com/golang/groupcache/singleflight"
 	"io"
 	"math/rand"
 	"sync"
@@ -18,12 +19,12 @@ type ChunkReadAt struct {
 	chunkViews   []*ChunkView
 	lookupFileId func(fileId string) (targetUrl string, err error)
 	readerLock   sync.Mutex
-	fetcherLock  sync.Mutex
 	fileSize     int64
 
-	lastChunkFileId string
-	lastChunkData   []byte
-	chunkCache      chunk_cache.ChunkCache
+	fetchGroup          singleflight.Group
+	lastChunkFileId     string
+	lastChunkData       []byte
+	chunkCache          chunk_cache.ChunkCache
 }
 
 // var _ = io.ReaderAt(&ChunkReadAt{})
@@ -88,10 +89,16 @@ func (c *ChunkReadAt) doReadAt(p []byte, offset int64) (n int, err error) {
 
 	var buffer []byte
 	startOffset, remaining := offset, int64(len(p))
+	var nextChunk *ChunkView
 	for i, chunk := range c.chunkViews {
 		if remaining <= 0 {
 			break
 		}
+		if i+1 < len(c.chunkViews) {
+			nextChunk = c.chunkViews[i+1]
+		} else {
+			nextChunk = nil
+		}
 		if startOffset < chunk.LogicOffset {
 			gap := int(chunk.LogicOffset - startOffset)
 			glog.V(4).Infof("zero [%d,%d)", startOffset, startOffset+int64(gap))
@@ -107,7 +114,7 @@ func (c *ChunkReadAt) doReadAt(p []byte, offset int64) (n int, err error) {
 			continue
 		}
 		glog.V(4).Infof("read [%d,%d), %d/%d chunk %s [%d,%d)", chunkStart, chunkStop, i, len(c.chunkViews), chunk.FileId, chunk.LogicOffset-chunk.Offset, chunk.LogicOffset-chunk.Offset+int64(chunk.Size))
-		buffer, err = c.readFromWholeChunkData(chunk)
+		buffer, err = c.readFromWholeChunkData(chunk, nextChunk)
 		if err != nil {
 			glog.Errorf("fetching chunk %+v: %v\n", chunk, err)
 			return
@@ -135,36 +142,63 @@ func (c *ChunkReadAt) doReadAt(p []byte, offset int64) (n int, err error) {
 
 }
 
-func (c *ChunkReadAt) readFromWholeChunkData(chunkView *ChunkView) (chunkData []byte, err error) {
-
-	c.fetcherLock.Lock()
-	defer c.fetcherLock.Unlock()
+func (c *ChunkReadAt) readFromWholeChunkData(chunkView, nextChunkView *ChunkView) (chunkData []byte, err error) {
 
 	if c.lastChunkFileId == chunkView.FileId {
 		return c.lastChunkData, nil
 	}
 
-	glog.V(4).Infof("readFromWholeChunkData %s offset %d [%d,%d) size at least %d", chunkView.FileId, chunkView.Offset, chunkView.LogicOffset, chunkView.LogicOffset+int64(chunkView.Size), chunkView.ChunkSize)
+	v, doErr := c.readOneWholeChunk(chunkView)
 
-	chunkData = c.chunkCache.GetChunk(chunkView.FileId, chunkView.ChunkSize)
-	if chunkData != nil {
-		glog.V(4).Infof("cache hit %s [%d,%d)", chunkView.FileId, chunkView.LogicOffset-chunkView.Offset, chunkView.LogicOffset-chunkView.Offset+int64(len(chunkData)))
-	} else {
-		chunkData, err = c.doFetchFullChunkData(chunkView)
-		if err != nil {
-			return
-		}
-		c.chunkCache.SetChunk(chunkView.FileId, chunkData)
-		c.lastChunkData = chunkData
-		c.lastChunkFileId = chunkView.FileId
+	if doErr != nil {
+		return
 	}
 
+	chunkData = v.([]byte)
+
+	c.lastChunkData = chunkData
+	c.lastChunkFileId = chunkView.FileId
+
+	go func() {
+		if c.chunkCache != nil && nextChunkView != nil {
+			c.readOneWholeChunk(nextChunkView)
+		}
+	}()
+
 	return
 }
 
+func (c *ChunkReadAt) readOneWholeChunk(chunkView *ChunkView) (interface{}, error) {
+
+	var err error
+
+	return c.fetchGroup.Do(chunkView.FileId, func() (interface{}, error) {
+
+		glog.V(4).Infof("readFromWholeChunkData %s offset %d [%d,%d) size at least %d", chunkView.FileId, chunkView.Offset, chunkView.LogicOffset, chunkView.LogicOffset+int64(chunkView.Size), chunkView.ChunkSize)
+
+		data := c.chunkCache.GetChunk(chunkView.FileId, chunkView.ChunkSize)
+		if data != nil {
+			glog.V(4).Infof("cache hit %s [%d,%d)", chunkView.FileId, chunkView.LogicOffset-chunkView.Offset, chunkView.LogicOffset-chunkView.Offset+int64(len(data)))
+		} else {
+			var err error
+			data, err = c.doFetchFullChunkData(chunkView)
+			if err != nil {
+				return data, err
+			}
+			c.chunkCache.SetChunk(chunkView.FileId, data)
+		}
+		return data, err
+	})
+}
+
 func (c *ChunkReadAt) doFetchFullChunkData(chunkView *ChunkView) ([]byte, error) {
+
+	glog.V(2).Infof("+ doFetchFullChunkData %s", chunkView.FileId)
+
 	data, err := fetchChunk(c.lookupFileId, chunkView.FileId, chunkView.CipherKey, chunkView.IsGzipped)
 
+	glog.V(2).Infof("- doFetchFullChunkData %s", chunkView.FileId)
+
 	return data, err
 
 }