linux-kernel - Re: [RFC PATCH] block: xfs: dm thin: train XFS to give up on retrying IO if thinp is out of space

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Date:	Wed, 22 Jul 2015 10:09:23 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Mike Snitzer <snitzer@...hat.com>
Cc:	Eric Sandeen <sandeen@...hat.com>, axboe@...nel.dk, hch@....de,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	dm-devel@...hat.com, xfs@....sgi.com,
	Vivek Goyal <vgoyal@...hat.com>
Subject: Re: [RFC PATCH] block: xfs: dm thin: train XFS to give up on
 retrying IO if thinp is out of space

On Tue, Jul 21, 2015 at 01:47:53PM -0400, Mike Snitzer wrote:
> On Tue, Jul 21 2015 at 11:34am -0400, Eric Sandeen <sandeen@...hat.com> wrote:
> > On 7/20/15 5:36 PM, Dave Chinner wrote:
> > The issue we had discussed previously is that there is no agreement
> > across block devices about whether ENOSPC is a permanent or temporary
> > condition.  Asking the admin to tune  the fs to each block device's
> > behavior sucks, IMHO.
> 
> It does suck, but it beats the alternative of XFS continuing to do
> nothing about the problem.

Just a comment on that: doing nothing is better than doing the wrong
thing and being stuck with it forever. :)

> Disucssing more with Vivek, might be that XFS would be best served to
> model what dm-thinp has provided with its 'no_space_timeout'.  It
> defaults to queueing IO for 60 seconds, once the timeout expires the
> queued IOs getted errored.  If set to 0 dm-thinp will queue IO
> indefinitely.

Yes, that's exactly what I proposed in the thread I referenced in
my previous email, and what got stuck on the bikeshed wall because
of these concerns about knob twiddling:

http://oss.sgi.com/archives/xfs/2015-02/msg00346.html

| e.g. if we need configurable error handling, it needs to be
| configurable for different error types, and it needs to be
| configurable on a per-mount basis. And it needs to be configurable
| at runtime, not just at mount time. That kind of leads to using
| sysfs for this. e.g. for each error type we ned to handle different
| behaviour for:
| 
| $ cat /sys/fs/xfs/vda/meta_write_errors/enospc/type
| [transient] permanent
| $ cat /sys/fs/xfs/vda/meta_write_errors/enospc/perm_timeout_seconds
| 300
| $ cat
| /sys/fs/xfs/vda/meta_write_errors/enospc/perm_max_retry_attempts
| 50
| $ cat
| /sys/fs/xfs/vda/meta_write_errors/enospc/transient_fail_at_umount
| 1

I've rebased this patchset, and I'm cleaning it up now, so in a few
days I'll have something for review, likely for the 4.3 merge
window....

Cheers,

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