wal: support fadatasync on linux

pkg/fileutil/sync.go

@@ -0,0 +1,26 @@
+// Copyright 2016 CoreOS, 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.
+
+// +build !linux
+
+package fileutil
+
+import "os"
+
+// Fdatasync is similar to fsync(), but does not flush modified metadata
+// unless that metadata is needed in order to allow a subsequent data retrieval
+// to be correctly handled.
+func Fdatasync(f *os.File) error {
+	return f.Sync()
+}

pkg/fileutil/sync_linux.go

@@ -0,0 +1,29 @@
+// Copyright 2016 CoreOS, 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.
+
+// +build linux
+
+package fileutil
+
+import (
+	"os"
+	"syscall"
+)
+
+// Fdatasync is similar to fsync(), but does not flush modified metadata
+// unless that metadata is needed in order to allow a subsequent data retrieval
+// to be correctly handled.
+func Fdatasync(f *os.File) error {
+	return syscall.Fdatasync(int(f.Fd()))
+}

wal/wal.go

@@ -399,7 +399,7 @@ func (w *WAL) sync() error {
 		}
 	}
 	start := time.Now()
-	err := w.f.Sync()
+	err := fileutil.Fdatasync(w.f)
 	syncDurations.Observe(float64(time.Since(start)) / float64(time.Second))
 	return err
 }