* Re: Question about xfs/440
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@ 2022-12-20 22:55 ` Darrick J. Wong
0 siblings, 0 replies; only message in thread
From: Darrick J. Wong @ 2022-12-20 22:55 UTC (permalink / raw)
To: yangx.jy@fujitsu.com; +Cc: xfs
On Fri, Dec 16, 2022 at 01:02:18PM +0000, yangx.jy@fujitsu.com 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
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