filer: stream read from volume server, reduce memory usage

weed/filer/filechunk_manifest.go

@@ -132,6 +132,41 @@ func retriedFetchChunkData(urlStrings []string, cipherKey []byte, isGzipped bool
 
 }
 
+func retriedStreamFetchChunkData(writer io.Writer, urlStrings []string, cipherKey []byte, isGzipped bool, isFullChunk bool, offset int64, size int) (err error) {
+
+	var shouldRetry bool
+	var written int
+
+	for waitTime := time.Second; waitTime < util.RetryWaitTime; waitTime += waitTime / 2 {
+		for _, urlString := range urlStrings {
+			shouldRetry, err = util.ReadUrlAsStream(urlString+"?readDeleted=true", cipherKey, isGzipped, isFullChunk, offset, size, func(data []byte) {
+				writer.Write(data)
+				written += len(data)
+			})
+			if !shouldRetry {
+				break
+			}
+			if err != nil {
+				glog.V(0).Infof("read %s failed, err: %v", urlString, err)
+				if written > 0 {
+					break
+				}
+			} else {
+				break
+			}
+		}
+		if err != nil && shouldRetry && written > 0 {
+			glog.V(0).Infof("retry reading in %v", waitTime)
+			time.Sleep(waitTime)
+		} else {
+			break
+		}
+	}
+
+	return err
+
+}
+
 func MaybeManifestize(saveFunc SaveDataAsChunkFunctionType, inputChunks []*filer_pb.FileChunk) (chunks []*filer_pb.FileChunk, err error) {
 	return doMaybeManifestize(saveFunc, inputChunks, ManifestBatch, mergeIntoManifest)
 }