Implement support for calculating disk usage per tenant (#5969)

Fixes #5961

pkg/disk/disk.go

@@ -28,4 +28,7 @@ type Info struct {
 	Files  uint64
 	Ffree  uint64
 	FSType string
+
+	// Usage is calculated per tenant.
+	Usage uint64
 }

pkg/madmin/examples/server-info.go

@@ -0,0 +1,44 @@
+// +build ignore
+
+/*
+ * 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 main
+
+import (
+	"log"
+
+	"github.com/minio/minio/pkg/madmin"
+)
+
+func main() {
+	// Note: YOUR-ACCESSKEYID, YOUR-SECRETACCESSKEY and my-bucketname are
+	// dummy values, please replace them with original values.
+
+	// API requests are secure (HTTPS) if secure=true and insecure (HTTPS) otherwise.
+	// New returns an Minio Admin client object.
+	madmClnt, err := madmin.New("your-minio.example.com:9000", "YOUR-ACCESSKEYID", "YOUR-SECRETACCESSKEY", true)
+	if err != nil {
+		log.Fatalln(err)
+	}
+
+	st, err := madmClnt.ServerInfo()
+	if err != nil {
+		log.Fatalln(err)
+	}
+	log.Println(st)
+}

pkg/madmin/info-commands.go

@@ -44,10 +44,10 @@ type DriveInfo HealDriveInfo
 
 // StorageInfo - represents total capacity of underlying storage.
 type StorageInfo struct {
-	// Total disk space.
-	Total int64
-	// Free available disk space.
-	Free int64
+	Total uint64 // Total disk space.
+	Free  uint64 // Free space available.
+	Used  uint64 // Total used spaced per tenant.
+
 	// Backend type.
 	Backend struct {
 		// Represents various backend types, currently on FS and Erasure.