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

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Message-id: <4FE0F132.2080207@shiftmail.org>
Date:	Tue, 19 Jun 2012 23:37:54 +0200
From:	Spelic <spelic@...ftmail.org>
To:	Dave Chinner <david@...morbit.com>
Cc:	Mike Snitzer <snitzer@...hat.com>, 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 06/19/12 22:06, Dave Chinner wrote:
> 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...
>

Could it be that the thin blocksize is larger than the discard 
granularity by xfs so nothing ever gets unmapped?
I have tried thin pools with the default blocksize (64k afair with lvm2) 
and 1MB.
HOWEVER I also have tried fstrim on xfs, and that is also not capable to 
unmap things from the dm-thin.
What is the granularity with fstrim in xfs?
Sorry I can't access the machine right now; maybe tomorrow, or in the 
weekend.

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