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

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

format-config-v1.go

@@ -17,6 +17,7 @@
 package main
 
 import (
+	"bytes"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -346,9 +347,10 @@ func reorderDisks(bootstrapDisks []StorageAPI, formatConfigs []*formatConfigV1)
 
 // loadFormat - loads format.json from disk.
 func loadFormat(disk StorageAPI) (format *formatConfigV1, err error) {
-	var buffer []byte
-	buffer, err = readAll(disk, minioMetaBucket, formatConfigFile)
-	if err != nil {
+	// Allocate staging buffer of 32KiB for copyBuffer.
+	buf := make([]byte, 32*1024)
+	var buffer = new(bytes.Buffer)
+	if err = copyBuffer(buffer, disk, minioMetaBucket, formatConfigFile, buf); err != nil {
 		// 'file not found' and 'volume not found' as
 		// same. 'volume not found' usually means its a fresh disk.
 		if err == errFileNotFound || err == errVolumeNotFound {
@@ -366,11 +368,15 @@ func loadFormat(disk StorageAPI) (format *formatConfigV1, err error) {
 		}
 		return nil, err
 	}
+
+	// Try to decode format json into formatConfigV1 struct.
 	format = &formatConfigV1{}
-	err = json.Unmarshal(buffer, format)
-	if err != nil {
+	d := json.NewDecoder(buffer)
+	if err = d.Decode(format); err != nil {
 		return nil, err
 	}
+
+	// Success.
 	return format, nil
 }