Lukas Czerner <lczerner@xxxxxxxxxx> writes: >> No, no. Userland will know max-length from statvfs, right? So, let's >> assume it is 100 (->f_blocks) * 1024 (->f_bsize). > > You do not need to know the filesystem size to do the discard, it should > be adjusted within the kernel. Just specify ULLONG_MAX as a length. See > fstrim tool in util-linux-ng. > >> >> Now, userland know about max length, 102400, ok? Let's start to trim. >> >> Assume, userland want to trim whole. So, userland will specify like >> >> trim(0, 102400). >> >> What happen in kernel actually? >> >> Current implement doesn't map blocks. So, in the case of FAT, it adjusts >> from 0 to 2 * 1024. >> >> So, it trims between 2048 and 102400. The problem is here. FS layout is >> actually, 2048 and (102400 + 2048). I.e. actually userland has to do >> >> trim(2048, 102400 + 2048) >> >> to specify whole. How to know 2048? > > You do not need to know anything in userspace. If you want to trim the > whole filesystem you just do trim(0, ULLONG_MAX) - which is what fstrim > does when you do not specify range. And you just skip the filesystem > metadata obviously, regardless if they are at the beginning of the > filesystem or in the middle. Just do whatever you need to do within your > filesystem. > > What we do in ext4 is, that we convert length and start passed in struct > fstrim_range into filesystem block units and then get the last > allocation group and block offset within that group (we do the same for > the start block) and we try to discard free block ranges in from staring > block to the last block. > > It is really not a rocket science and since every filesystem is > different and has different internal data structures it is up to you how > to do this. And if you shift a block or two, it really does not matter > as much since user-land does not know about how the filesystem block are > laid out anyway, nor user land knows which are free and which are not. > > I agree that the interface is a little bit fuzzy, but that is mainly > because it is intended to be filesystem independent and we do have a lot > of various filesystems, so I wanted it to be as flexibile as it should, > hence the start, len in Bytes. > > Hope it helped. No. If you want to trim whole with some chunk like 1GB and periodically (IIRC in xfstest), what do? We have to trim until ULLONG_MAX for each 1GB? Thanks. -- OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx> -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
