Re: Still seeing hangs in xlog_grant_log_space

[Dave Chinner <david@fromorbit.com>]

On Mon, Apr 23, 2012 at 05:33:40PM +0200, Juerg Haefliger wrote:
> Hi Dave,
> 
> 
> On Mon, Apr 23, 2012 at 4:38 PM, Dave Chinner <david@fromorbit.com> wrote:
> > On Mon, Apr 23, 2012 at 02:09:53PM +0200, Juerg Haefliger wrote:
> >> Hi,
> >>
> >> I have a test system that I'm using to try to force an XFS filesystem
> >> hang since we're encountering that problem sporadically in production
> >> running a 2.6.38-8 Natty kernel. The original idea was to use this
> >> system to find the patches that fix the issue but I've tried a whole
> >> bunch of kernels and they all hang eventually (anywhere from 5 to 45
> >> mins) with the stack trace shown below.
> >
> > If you kill the workload, does the file system recover normally?
> 
> The workload can't be killed.

OK.

> >> Only an emergency flush will
> >> bring the filesystem back. I tried kernels 3.0.29, 3.1.10, 3.2.15,
> >> 3.3.2. From reading through the mail archives, I get the impression
> >> that this should be fixed in 3.1.
> >
> > What you see is not necessarily a hang. It may just be that you've
> > caused your IO subsystem to have so much IO queued up it's completely
> > overwhelmed. How much RAM do you have in the machine?
> 
> When it hangs, there are zero IOs going to the disk. The machine has
> 100GB of RAM.

Can you get an event trace across the period where the hang occurs?

....

> >> I can't seem to hit the problem without the above modifications.
> >
> > How on earth did you come up with this configuration?
> 
> Just plain ol' luck. I was looking for a configuration that would
> allow me to reproduce the hangs and I accidentally picked a machine
> with a faulty controller battery which disabled the cache.

Wonderful.

> >> For the IO workload I pre-create 8000 files with random content and
> >> sizes between 1k and 128k on the test partition. Then I run a tool
> >> that spawns a bunch of threads which just copy these files to a
> >> different directory on the same partition.
> >
> > So, your workload also has a significant amount parallelism and
> > concurrency on a filesytsem with only 4 AGs?
> 
> Yes. Excuse my ignorance but what are AGs?

Allocation groups.

> >> At the same time there are
> >> other threads that rename, remove and overwrite random files in the
> >> destination directory keeping the file count at around 500.
> >
> > And you've added as much concurrent metadata modification as
> > possible, too, which makes me wonder.....
> >
> >> Let me know what other information I can provide to pin this down.
> >
> > .... exactly what are you trying to acheive with this test?  From my
> > point of view, you're doing something completely and utterly insane.
> > You filesystem config and workload is so far outside normal
> > configurations and workloads that I'm not surprised you're seeing
> > some kind of problem.....
> 
> No objection from my side. It's a silly configuration but it's the
> only one I've found that lets me reproduce a hang at will.

Ok, that's fair enough - it's handy to tell us that up front,
though.  ;)

Alright, then I need all the usual information. I suspect an event
trace is the only way I'm going to see what is happening. I just
updated the FAQ entry, so all the necessary info for gathering a
trace should be there now.

http://xfs.org/index.php/XFS_FAQ#Q:_What_information_should_I_include_when_reporting_a_problem.3F

-- 
Dave Chinner
david@fromorbit.com

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