[ceph-users] How to debug problem in MDS ?

Yan, Zheng ukernel at gmail.com
Wed Oct 17 18:40:57 PDT 2018


On Thu, Oct 18, 2018 at 3:59 AM Florent B <florent at coppint.com> wrote:
>
> Hi,
> I'm not running multiple active MDS (1 active & 7 standby).
> I know about debug_mds 20, is it the only log you need to see bugs ?
>
> On 16/10/2018 18:32, Sergey Malinin wrote:
> > Are you running multiple active MDS daemons?
> > On MDS host issue "ceph-daemon mds.X config set debug_mds 20" for maximum logging verbosity.
> >
> >> On 16.10.2018, at 19:23, Florent B <florent at coppint.com> wrote:
> >>
> >> Hi,
> >>
> >> A few months ago I sent a message to that list about a problem with a
> >> Ceph + Dovecot setup.
> >>
> >> Bug disappeared and I didn't answer to the thread.
> >>
> >> Now the bug has come again (Luminous up-to-date cluster + Dovecot
> >> up-to-date + Debian Stretch up-to-date).
> >>
> >> I know how to reproduce it, but it seems very related to my user's
> >> Dovecot data (few GB) and is related to file locking system (bug occurs
> >> when I set locking method to "fcntl" or "flock" in Dovecot, but not with
> >> "dotlock".
> >>
> >> It ends to a unresponsive MDS (100% CPU hang, switching to another MDS
> >> but always staying at 100% CPU usage). I can't even use the admin socket
> >> when MDS is hanged.
> >>

For issue like this, gdb is the most convenient way to debug. After
finding where the buggy code is, set debug_mds=20 and restart mds,
check the log to find how the bug was triggered.

Regards
Yan, Zheng


> >> I would like to know *exactly* which information do you need to
> >> investigate that bug ? (which commands, when, how to report large log
> >> files...)
> >>
> >> Thank you.
> >>
> >> Florent
> >>
> >>
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>
>
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