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

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Message-ID: <20120619200631.GL25389@dastard>
Date:	Wed, 20 Jun 2012 06:06:31 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Mike Snitzer <snitzer@...hat.com>
Cc:	Spelic <spelic@...ftmail.org>,
	Lukáš Czerner <lczerner@...hat.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, Jun 19, 2012 at 02:48:59PM -0400, Mike Snitzer wrote:
> 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.:

THat's normal when you use -o discard - XFS sends extremely
fine-grained discards as the have to be issued during the checkpoint
commit that frees the extent. Hence they can't be aggregated like is
done in ext4.

As it is, no-one really should be using -o discard - it is extremely
inefficient compared to a background fstrim run given that discards
are unqueued, blocking IOs. It's just a bad idea until the lower
layers get fixed to allow asynchronous, vectored discards and SATA
supports queued discards...

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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