use json format to get rbd image size

pkg/volume/rbd/rbd_test.go

@@ -682,3 +682,28 @@ func TestRequiresRemount(t *testing.T) {
 		t.Errorf("Exepcted RequiresRemount to be false, got %t", has)
 	}
 }
+
+func TestGetRbdImageSize(t *testing.T) {
+	for i, c := range []struct {
+		Output     string
+		TargetSize int
+	}{
+		{
+			Output:     `{"name":"kubernetes-dynamic-pvc-18e7a4d9-050d-11e9-b905-548998f3478f","size":10737418240,"objects":2560,"order":22,"object_size":4194304,"block_name_prefix":"rbd_data.9f4ff7238e1f29","format":2}`,
+			TargetSize: 10240,
+		},
+		{
+			Output:     `{"name":"kubernetes-dynamic-pvc-070635bf-e33f-11e8-aab7-548998f3478f","size":1073741824,"objects":256,"order":22,"object_size":4194304,"block_name_prefix":"rbd_data.670ac4238e1f29","format":2}`,
+			TargetSize: 1024,
+		},
+	} {
+		size, err := getRbdImageSize([]byte(c.Output))
+		if err != nil {
+			t.Errorf("Case %d: getRbdImageSize failed: %v", i, err)
+			continue
+		}
+		if size != c.TargetSize {
+			t.Errorf("Case %d: unexpected size, wanted %d, got %d", i, c.TargetSize, size)
+		}
+	}
+}