revert disabling FSync for non Mac (#3814)

weed/storage/backend/disk_file.go

@@ -4,6 +4,7 @@ import (
 	"github.com/seaweedfs/seaweedfs/weed/glog"
 	. "github.com/seaweedfs/seaweedfs/weed/storage/types"
 	"os"
+	"runtime"
 	"time"
 )
 
@@ -11,6 +12,8 @@ var (
 	_ BackendStorageFile = &DiskFile{}
 )
 
+const isMac = runtime.GOOS == "darwin"
+
 type DiskFile struct {
 	File         *os.File
 	fullFilePath string
@@ -81,6 +84,11 @@ func (df *DiskFile) Name() string {
 }
 
 func (df *DiskFile) Sync() error {
-	return nil
-	// return df.File.Sync()
+	if df.File == nil {
+		return os.ErrInvalid
+	}
+	if isMac {
+		return nil
+	}
+	return df.File.Sync()
 }