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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: Mike Snitzer <snitzer@xxxxxxxxxx>
Date: Tue, 19 Jun 2012 14:48:59 -0400
Cc: Lukáš Czerner <lczerner@xxxxxxxxxx>, device-mapper development <dm-devel@xxxxxxxxxx>, linux-ext4@xxxxxxxxxxxxxxx, Dave Chinner <david@xxxxxxxxxxxxx>, xfs@xxxxxxxxxxx
In-reply-to: <20120619144413.GA7225@xxxxxxxxxx>
References: <4FDF9EBE.2030809@xxxxxxxxxxxxx> <20120619015745.GJ25389@dastard> <20120619031241.GA3884@xxxxxxxxxx> <alpine.LFD.2.00.1206190816440.2241@xxxxxxxxxxxxxxxxxxxxxxxxx> <20120619131649.GA6811@xxxxxxxxxx> <alpine.LFD.2.00.1206191519320.21961@xxxxxxxxxxxxxxxxxxxxxxxxx> <20120619133041.GB6811@xxxxxxxxxx> <4FE0840F.2050704@xxxxxxxxxxxxx> <20120619144413.GA7225@xxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Tue, Jun 19 2012 at 10:44am -0400,
Mike Snitzer <snitzer@xxxxxxxxxx> wrote:

> On Tue, Jun 19 2012 at  9:52am -0400,
> Spelic <spelic@xxxxxxxxxxxxx> 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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