@yangdongsheng I use it because I try some bytes in guest with fstrim, and I create tmp file in guest with 5M, it is larger than it and the result is that the disk map will no change. For really part fstrim( like the fstrim result is > ori and < full) I can not find the suitable size value for it. I refer to the help command of fstrim with --minimum option. I also don't know more about it.