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

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Message-ID: <20170908071630.GB9869@infradead.org>
Date:   Fri, 8 Sep 2017 00:16:30 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Dave Chinner <david@...morbit.com>
Cc:     "Lu, Qian" <luqia@...zon.com>,
        "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

On Fri, Sep 08, 2017 at 09:34:02AM +1000, Dave Chinner wrote:
> 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.

And your device sucks :)

Some numbers here:

https://events.linuxfoundation.org/sites/events/files/slides/discard_0.pdf

Discard will cause extra work, and for worst case sceanarios lots of
it.  But some device handle that extra load very well.