func (fb *FileBackend) TrimAt(ctx context.Context, length int, offset int64) (int, error) {
	return length, nil
}
TrimAt implements Backend.TrimAt
package nbd
import (
	"golang.org/x/net/context"
	"os"
)
type FileBackend struct {
	file *os.File
	size uint64
}
func (fb *FileBackend) WriteAt(ctx context.Context, b []byte, offset int64, fua bool) (int, error) {
	n, err := fb.file.WriteAt(b, offset)
	if err != nil || !fua {
		return n, err
	}
	err = fb.file.Sync()
	if err != nil {
		return 0, err
	}
	return n, err
}
func (fb *FileBackend) ReadAt(ctx context.Context, b []byte, offset int64) (int, error) {
	return fb.file.ReadAt(b, offset)
}