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

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Message-ID: <20120619184858.GA8841@redhat.com>
Date:	Tue, 19 Jun 2012 14:48:59 -0400
From:	Mike Snitzer <snitzer@...hat.com>
To:	Spelic <spelic@...ftmail.org>
Cc:	Lukáš Czerner <lczerner@...hat.com>,
	device-mapper development <dm-devel@...hat.com>,
	linux-ext4@...r.kernel.org, Dave Chinner <david@...morbit.com>,
	xfs@....sgi.com
Subject: Re: Ext4 and xfs problems in dm-thin on allocation and discard

On Tue, Jun 19 2012 at 10:44am -0400,
Mike Snitzer <snitzer@...hat.com> wrote:

> On Tue, Jun 19 2012 at  9:52am -0400,
> Spelic <spelic@...ftmail.org> wrote:
>
> > I do not know what is the mechanism for which xfs cannot unmap
> > blocks from dm-thin, but it really can't.
> > If anyone has dm-thin installed he can try. This is 100%
> > reproducible for me.
> 
> I was initially surprised by this considering the thinp-test-suite does
> test a compilebench workload against xfs and ext4 using online discard
> (-o discard).
> 
> But I just modified that test to use a thin-pool with 'ignore_discard'
> and the test still passed on both ext4 and xfs.
> 
> So there is more work needed in the thinp-test-suite to use blktrace
> hooks to verify that discards are occuring when the compilebench
> generated files are removed.
> 
> I'll work through that and report back.

blktrace shows discards for both xfs and ext4.

But in general xfs is issuing discards with much smaller extents than
ext4 does, e.g.:

to the thin device:
+ 128 vs + 32

to the thin-pool's data device:
+ 120 vs + 16
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