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

To: Spelic <spelic@xxxxxxxxxxxxx>
Subject: Re: Ext4 and xfs problems in dm-thin on allocation and discard
From: Lukáš Czerner <lczerner@xxxxxxxxxx>
Date: Tue, 19 Jun 2012 14:20:29 +0200 (CEST)
Cc: Lukáš Czerner <lczerner@xxxxxxxxxx>, Mike Snitzer <snitzer@xxxxxxxxxx>, Dave Chinner <david@xxxxxxxxxxxxx>, device-mapper development <dm-devel@xxxxxxxxxx>, linux-ext4@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx
In-reply-to: <4FE062B3.7080702@xxxxxxxxxxxxx>
References: <4FDF9EBE.2030809@xxxxxxxxxxxxx> <20120619015745.GJ25389@dastard> <20120619031241.GA3884@xxxxxxxxxx> <alpine.LFD.2.00.1206190816440.2241@xxxxxxxxxxxxxxxxxxxxxxxxx> <4FE062B3.7080702@xxxxxxxxxxxxx>
User-agent: Alpine 2.00 (LFD 1167 2008-08-23)
On Tue, 19 Jun 2012, Spelic wrote:

> Date: Tue, 19 Jun 2012 13:29:55 +0200
> From: Spelic <spelic@xxxxxxxxxxxxx>
> To: Lukáš Czerner <lczerner@xxxxxxxxxx>
> Cc: Mike Snitzer <snitzer@xxxxxxxxxx>, Dave Chinner <david@xxxxxxxxxxxxx>,
>     Spelic <spelic@xxxxxxxxxxxxx>,
>     device-mapper development <dm-devel@xxxxxxxxxx>,
>     linux-ext4@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx
> 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.
> 
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