XL: Make allocations simpler avoid redundant allocs. (#1961)

- Reduce 10MiB buffers for loopy calls to use 128KiB.
- start using 128KiB buffer where needed.

xl-v1-multipart-common.go

@@ -17,6 +17,7 @@
 package main
 
 import (
+	"bytes"
 	"encoding/json"
 	"path"
 	"sort"
@@ -69,16 +70,25 @@ func (u uploadsV1) Index(uploadID string) int {
 
 // readUploadsJSON - get all the saved uploads JSON.
 func readUploadsJSON(bucket, object string, disk StorageAPI) (uploadIDs uploadsV1, err error) {
+	// Staging buffer of 128KiB kept for reading `uploads.json`.
+	var buf = make([]byte, 128*1024)
+
+	// Writer holding `uploads.json` content.
+	var buffer = new(bytes.Buffer)
+
 	uploadJSONPath := path.Join(mpartMetaPrefix, bucket, object, uploadsJSONFile)
-	// Read all of 'uploads.json'
-	buffer, rErr := readAll(disk, minioMetaBucket, uploadJSONPath)
-	if rErr != nil {
-		return uploadsV1{}, rErr
+	// Reads entire `uploads.json`.
+	if err = copyBuffer(buffer, disk, minioMetaBucket, uploadJSONPath, buf); err != nil {
+		return uploadsV1{}, err
 	}
-	rErr = json.Unmarshal(buffer, &uploadIDs)
-	if rErr != nil {
-		return uploadsV1{}, rErr
+
+	// Decode `uploads.json`.
+	d := json.NewDecoder(buffer)
+	if err = d.Decode(&uploadIDs); err != nil {
+		return uploadsV1{}, err
 	}
+
+	// Success.
 	return uploadIDs, nil
 }