On Fri, Dec 16, 2022 at 01:02:18PM +0000, yangx.jy@xxxxxxxxxxx wrote: > Hi Darrick, > > As you know, the following steps comes from xfs/440. I have two > questions about these steps. > The first one: > Why repquota -u command shows "128" rather than "64" for fsgqa user > after cp --reflink command is completed? Quota resource usage are charged for each file, even for shared blocks. There's no easy way to amortize the cost of a shared block among its owners unless you can walk backwards from a piece of storage back to its owners, which is too much work to make it worth the effort. Hence reflinking a 64k file to another file creates 128k worth of usage. > The second one: > Why repquota -u command shows "1152" for fsgqa user after CoW(Copy on > Write) is completed? The space extents used to stage COW operations are separate space allocations, which means they're charged to the quota and hang around until writeback happens. In xfs/440, we set a cow extent size hint of 1mb, so we've actually staged 1mb of cow so that appending writes to /b will end up in contiguous space. That's why usage goes from 128 to 1152. --D > > How does the kernel process reflink and CoW? > How does the kernel calculate the number(e.g. 128，1152)? > > # mount -o usrquota,grpquota /dev/pmem1 /mnt/scratch/ > # touch /mnt/scratch/a /mnt/scratch/force_fsgqa > # chown fsgqa /mnt/scratch/a /mnt/scratch/force_fsgqa > # xfs_io -c "pwrite -S 0x58 0 64k" /mnt/scratch/a > # repquota -u /mnt/scratch/ > ... > Block limits File limits > User used soft hard grace used soft hard grace > ---------------------------------------------------------------------- > root -- 0 0 0 3 0 0 > fsgqa -- 64 0 0 2 0 0 > > # cp --reflink=always -p /mnt/scratch/a /mnt/scratch/b > # repquota -u /mnt/scratch/ > ... > Block limits File limits > User used soft hard grace used soft hard grace > ---------------------------------------------------------------------- > root -- 0 0 0 3 0 0 > fsgqa -- 128 0 0 3 0 0 > > # xfs_io -c "pwrite -S 0x59 0 64k" /mnt/scratch/a > # repquota -u /mnt/scratch/ > ... > Block limits File limits > User used soft hard grace used soft hard grace > ---------------------------------------------------------------------- > root -- 0 0 0 3 0 0 > fsgqa -- 1152 0 0 3 0 0 > > Best Regards, > Xiao Yang
