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

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Message-ID: <alpine.LFD.2.00.1206191413460.21961@dhcp-1-248.brq.redhat.com>
Date:	Tue, 19 Jun 2012 14:20:29 +0200 (CEST)
From:	Lukáš Czerner <lczerner@...hat.com>
To:	Spelic <spelic@...ftmail.org>
cc:	Lukáš Czerner <lczerner@...hat.com>,
	Mike Snitzer <snitzer@...hat.com>,
	Dave Chinner <david@...morbit.com>,
	device-mapper development <dm-devel@...hat.com>,
	linux-ext4@...r.kernel.org, xfs@....sgi.com
Subject: Re: Ext4 and xfs problems in dm-thin on allocation and discard

On Tue, 19 Jun 2012, Spelic wrote:

> Date: Tue, 19 Jun 2012 13:29:55 +0200
> From: Spelic <spelic@...ftmail.org>
> To: Lukáš Czerner <lczerner@...hat.com>
> Cc: Mike Snitzer <snitzer@...hat.com>, Dave Chinner <david@...morbit.com>,
>     Spelic <spelic@...ftmail.org>,
>     device-mapper development <dm-devel@...hat.com>,
>     linux-ext4@...r.kernel.org, xfs@....sgi.com
> Subject: Re: Ext4 and xfs problems in dm-thin on allocation and discard
> 
> 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)?
> 
> 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 ?

Yes, definitely this behaviour need to change in dm-thin. I do not
have a path, it was merely a proposal how thing could be done. Not
sure what Mike and rest of the dm folks think about this.

-Lukas

> 
> Thank you
> S.
>