Add ServerDrivesPerfInfo() admin API (#6969)

This is part of implementation for mc admin health command. The
ServerDrivesPerfInfo() admin API returns read and write speed
information for all the drives (local and remote) in a given Minio
server deployment.

Part of minio/mc#2606

pkg/disk/helpers.go

@@ -0,0 +1,52 @@
+/*
+ * Minio Cloud Storage, (C) 2018 Minio, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package disk
+
+import (
+	"os"
+)
+
+func readFile(path string, buf []byte) (int, error) {
+	f, err := os.Open(path)
+	if err != nil {
+		return 0, err
+	}
+	defer f.Close()
+
+	n, err := f.Read(buf)
+	if err != nil {
+		return 0, err
+	}
+
+	return n, nil
+}
+
+func writeFile(path string, data []byte) (int, error) {
+	f, err := os.Create(path)
+	if err != nil {
+		return 0, err
+	}
+	defer f.Close()
+
+	n, err := f.Write(data)
+	if err != nil {
+		return 0, err
+	}
+
+	f.Sync()
+	return n, nil
+}