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

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: Ext4 and xfs problems in dm-thin on allocation and discard
From: Mike Snitzer <snitzer@xxxxxxxxxx>
Date: Mon, 18 Jun 2012 23:12:42 -0400
Cc: Spelic <spelic@xxxxxxxxxxxxx>, device-mapper development <dm-devel@xxxxxxxxxx>, linux-ext4@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx
In-reply-to: <20120619015745.GJ25389@dastard>
References: <4FDF9EBE.2030809@xxxxxxxxxxxxx> <20120619015745.GJ25389@dastard>
User-agent: Mutt/1.5.21 (2010-09-15)
On Mon, Jun 18 2012 at  9:57pm -0400,
Dave Chinner <david@xxxxxxxxxxxxx> wrote:

> On Mon, Jun 18, 2012 at 11:33:50PM +0200, Spelic wrote:
>
> > Please note that since I am above MD raid5 (I believe this is the
> > reason), the passdown of discards does not work, as my dmesg says:
> > [160508.497879] device-mapper: thin: Discard unsupported by data
> > device (dm-1): Disabling discard passdown.
> > but AFAIU, unless there is a thinp bug, this should not affect the
> > unmapping of thin blocks by fstrimming xfs... and in fact ext4 is
> > able to do that.
> 
> Does ext4 report that same error?

That message says the underlying device doesn't support discards
(because it is an MD device).  But the thinp device still has discards
enabled -- it just won't pass the discards down to the underlying data
device.

So yes, it'll happen with ext4 -- it is generated when the thin-pool
device is loaded (which happens independent of the filesystem that is
layered ontop).

The discards still inform the thin-pool that the corresponding extents
are no longer allocated.

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