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

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

fs-v1-metadata.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"bytes"
 	"encoding/json"
 	"path"
 	"sort"
@@ -57,16 +58,23 @@ func (m *fsMetaV1) AddObjectPart(partNumber int, partName string, partETag strin
 }
 
 // readFSMetadata - returns the object metadata `fs.json` content.
-func (fs fsObjects) readFSMetadata(bucket, object string) (fsMeta fsMetaV1, err error) {
-	var buffer []byte
-	buffer, err = readAll(fs.storage, bucket, path.Join(object, fsMetaJSONFile))
-	if err != nil {
+func readFSMetadata(disk StorageAPI, bucket, object string) (fsMeta fsMetaV1, err error) {
+	// 32KiB staging buffer for copying `fs.json`.
+	var buf = make([]byte, 32*1024)
+
+	// `fs.json` writer.
+	var buffer = new(bytes.Buffer)
+	if err = copyBuffer(buffer, disk, bucket, path.Join(object, fsMetaJSONFile), buf); err != nil {
 		return fsMetaV1{}, err
 	}
-	err = json.Unmarshal(buffer, &fsMeta)
-	if err != nil {
+
+	//  Decode `fs.json` into fsMeta structure.
+	d := json.NewDecoder(buffer)
+	if err = d.Decode(&fsMeta); err != nil {
 		return fsMetaV1{}, err
 	}
+
+	// Success.
 	return fsMeta, nil
 }