Re: rbd map command hangs for 15 minutes during system start up

[Alex Elder <elder@inktank.com>]

On 12/18/2012 12:05 PM, Nick Bartos wrote:
> I've added the output of "ps -ef" in addition to triggering a trace
> when a hang is detected.  Not much is generally running at that point,
> but you can have a look:
> 
> https://gist.github.com/raw/4330223/2f131ee312ee43cb3d8c307a9bf2f454a7edfe57/rbd-hang-1355851498.txt

This helped a lot.  I updated the bug with a little more info.

    http://tracker.newdream.net/issues/3519

I also think I have now found something that could explain what you
are seeing, and am developing a fix.  I'll provide you an update
as soon as I have tested what I come up with, almost certainly
this afternoon.

					-Alex

> Is it possible that there is some sort of deadlock going on?  We are
> doing the rbd maps (and subsequent filesystem mounts) on the same
> systems which are running the ceph-osd and ceph-mon processes.  To get
> around the 'sync' deadlock problem, we are using a patch from Sage
> which ignores system wide sync's on filesystems mounted with the
> 'mand' option (and we mount the underlying osd filesystems with
> 'mand').  However I am wondering if there is potential for other types
> of deadlocks in this environment.
> 
> Also, we recently saw an rbd hang in a much older version, running
> kernel 3.5.3 with only the sync hack patch, along side ceph 0.48.1.
> It's possible that this issue was around for some time, just the
> recent patches made it happen more often (and thus more reproducible)
> for us.
> 
> 
> On Tue, Dec 18, 2012 at 8:09 AM, Alex Elder <elder@inktank.com> wrote:
>> On 12/17/2012 11:12 AM, Nick Bartos wrote:
>>> Here's a log with the rbd debugging enabled:
>>>
>>> https://gist.github.com/raw/4319962/d9690fd92c169198efc5eecabf275ef1808929d2/rbd-hang-test-1355763470.log
>>>
>>> On Fri, Dec 14, 2012 at 10:03 AM, Alex Elder <elder@inktank.com> wrote:
>>>> On 12/14/2012 10:53 AM, Nick Bartos wrote:
>>>>> Yes I was only enabling debugging for libceph.  I'm adding debugging
>>>>> for rbd as well.  I'll do a repro later today when a test cluster
>>>>> opens up.
>>>>
>>>> Excellent, thank you.   -Alex
>>
>> I looked through these debugging messages.  Looking only at the
>> rbd debugging, what I see seems to indicate that rbd is idle at
>> the point the "hang" seems to start.  This suggests that the hang
>> is not due to rbd itself, but rather whatever it is that might
>> be responsible for using the rbd image once it has been mapped.
>>
>> Is that possible?  I don't know what process you have that is
>> mapping the rbd image, and what is supposed to be the next thing
>> it does.  (I realize this may not make a lot of sense, given
>> a patch in rdb seems to have caused the hang to begin occurring.)
>>
>> Also note that the debugging information available (i.e., the
>> lines in the code that can output debugging information) may
>> well be incomplete.  So if you don't find anything it may be
>> necessary to provide you with another update which might include
>> more debugging.
>>
>> Anyway, could you provide a little more context about what
>> is going on sort of *around* rbd when activity seems to stop?
>>
>> Thanks a lot.
>>
>>                                         -Alex