linux-ext4 - Re: Ext4 and xfs problems in dm-thin on allocation and discard

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Message-ID: <20120619133441.GA7001@redhat.com>
Date:	Tue, 19 Jun 2012 09:34:42 -0400
From:	Mike Snitzer <snitzer@...hat.com>
To:	Spelic <spelic@...ftmail.org>
Cc:	Lukáš Czerner <lczerner@...hat.com>,
	Dave Chinner <david@...morbit.com>, xfs@....sgi.com,
	device-mapper development <dm-devel@...hat.com>,
	linux-ext4@...r.kernel.org
Subject: Re: Ext4 and xfs problems in dm-thin on allocation and discard

On Tue, Jun 19 2012 at  7:29am -0400,
Spelic <spelic@...ftmail.org> wrote:

> On 06/19/12 08:32, Lukáš Czerner wrote:
> >
> >So do I understand correctly that even though the discard came
> >through and thinp took advantage of it it still returns EOPNOTSUPP ?
> >This seems rather suboptimal. IIRC there was a discussion to add an
> >option to enable/disable sending discard in thinp target down
> >to the device.
> 
> I'll ask this too...
> do I understand correctly that dm-thin returns EOPNOTSUPP to the
> filesystem layer even though it is using the discard to unmap
> blocks, and at that point XFS stops sending discards down there
> (while ext4 keeps sending them)?

Are you actually seeing that?  Or are you just seizing on Lukas'
misunderstanding?

> This looks like a bug of dm-thin to me. Discards are "supported" in
> such a scenario.
> 
> Do you have a patch for dm-thin so to prevent it sending EOPTNOTSUPP ?

thinp should _not_ be sending -EOPNOTSUPP unless 'ignore_discard' is
provided as a feature when loading thin-pool's DM  table.
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