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

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Date:	Tue, 19 Jun 2012 08:32:50 +0200 (CEST)
From:	Lukáš Czerner <lczerner@...hat.com>
To:	Mike Snitzer <snitzer@...hat.com>
cc:	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 Mon, 18 Jun 2012, Mike Snitzer wrote:

> Date: Mon, 18 Jun 2012 23:12:42 -0400
> From: Mike Snitzer <snitzer@...hat.com>
> To: Dave Chinner <david@...morbit.com>
> Cc: 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 Mon, Jun 18 2012 at  9:57pm -0400,
> Dave Chinner <david@...morbit.com> 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.

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.

So maybe it might be a bit smarter than that and actually
enable/disable discard pass through depending on the underlying
support, so we do not blindly send discard down to the device even
though it does not support it.

So we'll have three options: 

pass through - always send discard down
backstop - never send discard down to the device
auto - send discard only if the underlying device supports it

What do you think ?

-Lukas

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