Re: [ASSERT failure] transaction reservations changes bad?

Dave Chinner
2013-03-27T02:03:38Z

On Tue, Mar 26, 2013 at 06:14:30PM +0800, Jeff Liu wrote:
> On 03/12/2013 08:05 PM, Dave Chinner wrote:
> > On Tue, Mar 12, 2013 at 07:56:35PM +0800, Jeff Liu wrote:
> >> More info, 3.7.0 is the oldest kernel on my environment, I ran into the
> >> same problem.
> >
> > Thanks for following up so quickly, Jeff. So the problem is that a
> > new test is tripping over a bug that has been around for a while,
> > not that it is a new regression.
> >
> > OK, so I'll expunge that from my testing for the moment as I don't
> > ahve time to dig in and find out what the cause is right now. If
> > anyone else wants to.... :)
>
> I did some further tests to nail down this issue, just posting the analysis result here,
> it might be of some use when we revising it again.
>
> The disk is formated with Dave's previous comments, i.e.
> mkfs.xfs -f -b size=512 -d agcount=16,su=256k,sw=12 -l su=256k,size=2560b /dev/xxx
>
> First of all, looks this bug stayed in hiding for years since I can reproduce it between upstream
> 3.0 to 3.9.0-rc3, the oldest kernel I have tried is 2.6.39 which has the same problem.

If you mount 2.6.39 with "-o nodelaylog", does the problem go away?

> IMHO, looks the major cause is related to the 'sunit' parameter,
> since it would affect the log space unit calculations by
> '2*log->l_mp->m_sb.sb_logsunit' at xlog_ticket_alloc(). However,
> we don't include this factor into consideration at mkfs or mount
> stage, should we take it into account?

That's what I suspected was the problem. i.e. that the log was too
small for the given configuration.

The question is this: how much space do we need to reserve. I'm
thinking a minimum of 4*lsu - 2*lsu for the existing CIL context, and
another 2*lsu for any queued ticket waiting for space to come
available.

I haven't thought a lot about it, though, and I have a little demon
sitting on my shoulder nagging me about specific thresholds whether
they need to play a part in this. e.g. no single transaction can be
larger than half the log; AIL push thresholds of 25% of log space;
background CIL commit threshold of 12.5% of the log...

So it's not immediately clear to me how much bigger the log needs to
be...

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com