Re: [ASSERT failure] transaction reservations changes bad?

Jeff Liu
2013-03-28T15:16:48Z

On 03/27/2013 10:03 AM, Dave Chinner wrote:
> 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?
touch file is ok, but create directory still cause the assertion failure.
>
>> 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...
I still need some time to understand the space reservation strategy to
figure them out. :(

Thanks,
-Jeff