refactor memory mapped file into backend storage

weed/storage/volume_create_linux.go

@@ -7,13 +7,14 @@ import (
 	"syscall"
 
 	"github.com/chrislusf/seaweedfs/weed/glog"
+	"github.com/chrislusf/seaweedfs/weed/storage/backend"
 )
 
-func createVolumeFile(fileName string, preallocate int64, memoryMapSizeMB uint32) (*os.File, error) {
+func createVolumeFile(fileName string, preallocate int64, memoryMapSizeMB uint32) (backend.DataStorageBackend, error) {
 	file, e := os.OpenFile(fileName, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0644)
 	if preallocate != 0 {
 		syscall.Fallocate(int(file.Fd()), 1, 0, preallocate)
 		glog.V(0).Infof("Preallocated %d bytes disk space for %s", preallocate, fileName)
 	}
-	return file, e
+	return backend.NewDiskFile(file), e
 }