Add support of fallocate for FS and XL backends (#3032)

cmd/fallocate_linux.go

@@ -0,0 +1,31 @@
+// +build linux
+
+/*
+ * Minio Cloud Storage, (C) 2016 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 cmd
+
+import "syscall"
+
+// Fallocate uses the linux Fallocate syscall, which helps us to be
+// sure that subsequent writes on a file just created will not fail,
+// in addition, file allocation will be contigous on the disk
+func Fallocate(fd int, offset int64, len int64) error {
+	return syscall.Fallocate(fd,
+		1, // FALLOC_FL_KEEP_SIZE
+		offset,
+		len)
+}