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

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Date:	Tue, 19 Jun 2012 09:05:42 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Spelic <spelic@...ftmail.org>
CC:	Mike Snitzer <msnitzer@...hat.com>,
	Lukáš Czerner <lczerner@...hat.com>,
	Dave Chinner <david@...morbit.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 6/19/12 8:52 AM, Spelic wrote:
> On 06/19/12 15:30, Mike Snitzer wrote:
>> I don't recall Spelic saying anything about EOPNOTSUPP. So what has made you zero in on an -EOPNOTSUPP return (which should not be happening)? 
> 
> Exactly: I do not know if EOPNOTSUPP is being returned or not.
> 
> If this helps, I have configured dm-thin via lvm2
>   LVM version:     2.02.95(2) (2012-03-06)
>   Library version: 1.02.74 (2012-03-06)
>   Driver version:  4.22.0
> 
> from dmsetup table I only see one option : "skip_block_zeroing", if and only if I configure it with -Zn . I do not see anything regarding ignore_discard
> 
>      vg1-pooltry1-tpool: 0 20971520 thin-pool 252:1 252:2 2048 0 1 skip_block_zeroing
>      vg1-pooltry1_tdata: 0 20971520 linear 9:20 62922752
>      vg1-pooltry1_tmeta: 0 8192 linear 9:20 83894272
>      vg1-thinlv1: 0 31457280 thin 252:3 1
> 
> 
> and in dmesg:
> [   33.685200] device-mapper: thin: Discard unsupported by data device (dm-2): Disabling discard passdown.
> [   33.709586] device-mapper: thin: Discard unsupported by data device (dm-6): Disabling discard passdown.
> 
> 
> 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.

Might be worth seeing if xfs is ever getting to its discard code?  There is a tracepoint...

# mount -t debugfs none /sys/kernel/debug
# echo 1 > /sys/kernel/debug/tracing/tracing_enabled
# echo 1 > /sys/kernel/debug/tracing/events/xfs/xfs_discard_extent/enable

<run test>

# cat /sys/kernel/debug/tracing/trace

-Eric
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