linux-kernel - Re: XFS mounted with 'discard' option

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Date:   Fri, 8 Sep 2017 09:34:02 +1000
From:   Dave Chinner <david@...morbit.com>
To:     "Lu, Qian" <luqia@...zon.com>
Cc:     "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: XFS mounted with 'discard' option - deleting fio test files slow


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On Thu, Sep 07, 2017 at 06:22:22PM +0000, Lu, Qian wrote:
> Recently we received a bug report in the XFS filesystem with
> 'discard' option. I have been able to reproduce this issue. I used
> XFS filesystem to format NVMe SSD and mounted with 'discard'
> option. When I tried to delete the test fio files, the session
> took long time. This issue is based on Linux 4.9 stable tree. I
> have also repeated this test with Linux 4.13, 4.12, and we are
> facing the same issue. Tests were repeated several times and it
> was consistent.

.....
> 
> # fio --name fio_test_file --direct=1 --rw=randwrite --bs=4k --size=5G --numjobs=8 --group_reporting
> --> Interrupt with Ctrl+C
> # time rm -rf fio_test_file.*
> real 2m2.242s
> user 0m0.000s
> sys 0m25.524s

4k random write with direct IO. 5GB file. Probably got a million 4k
extents in it. Which means XFS has sent a million tiny 4k discards
to the device. Run 'xfs_bmap -vvp fio_test_file.*' to confirm.

Don't use "-o discard" if you care about performance.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com