On Thu, Sep 15, 2022 at 3:33 PM Arthur Outhenin-Chalandre
<arthur.outhenin-chalan...@cern.ch> wrote:
>
> Hi Ronny,
>
> > On 15/09/2022 14:32 ronny.lippold <c...@spark5.de> wrote:
> > hi arthur, some time went ...
> >
> > i would like to know, if there are some news of your setup.
> > do you have replication active running?
>
> No, there was no change at CERN. I am switching jobs as well actually so I
> won't have much news for you on CERN infra in the future. I know other people
> from the Ceph team at CERN watch this ml so you might hear from them as well
> I guess.
>
> > we are using actually snapshot based and had last time a move of both
> > clusters.
> > after that, we had some damaged filesystems ind the kvm vms.
> > did you ever had such a problems in your tests.
> >
> > i think, there are not so many people, how are using ceph replication.
> > for me its hard to find the right way.
> > can a snapshot based ceph replication be crash consisten? i think no.
>
> I never noticed it myself, but yes it's written on the docs actually
> https://docs.ceph.com/en/quincy/rbd/rbd-snapshot/ (but on the mirroring docs
> this is not actually explained). I never tested that super carefully though
> and thought this was more a rare occurence than anything else.
>
> I heard a while back (maybe a year-ish ago) that there was some long term
> plan to automatically trigger an fsfreeze for librbd/qemu on a snapshot which
> would probably solve your issue (and also allow application level consistency
> via fsfreeze custom hooks). But this was apparently a tricky feature to add.
> I cc'ed Illya maybe he would know more about that or if something else could
> have caused your issue.
The underlying support for snapshot quiescing is there in librbd, but
the only thing that can currently take advantage of it is rbd-nbd. The
default hook is shipped in /usr/libexec/rbd-nbd/rbd-nbd_quiesce and it
just calls "fsfreeze -f" on the filesystem mounted on the device before
the snapshot is taken and follows up with "fsfreeze -u" after. This
provides filesystem-level consistency. Application-level consistency
can be achieved with a custom hook.
Unfortunately, neither QEMU nor the kernel client have that integrated
at present.
Thanks,
Ilya
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