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[cinder] Ceph, active-active and no coordination


On 17/11, Giulio Fidente wrote:
> I am leaving some comments inline and adding on CC some cinder folks who
> know better
>
> On 11/17/20 9:27 PM, RadosÅ?aw Piliszek wrote:
> > Dear Cinder Masters,
> >
> > I have a question for you. (or two, or several; well, actually the
> > whole Kolla team has :-) )
> >
> > The background is that Kolla has been happily deploying multinode
> > cinder-volume with Ceph RBD backend, with no coordination configured,
> > cluster parameter unset, host properly set per host and backend_host
> > normalised (as well as any other relevant config) between the
> > cinder-volume hosts.
> >
> > The first question is: do we correctly understand that this was an
> > active-active deployment? Or really something else?

Hi,

That is an Active-Active deployment with an Active-Passive
configuration, so it's a PROBLEM waiting to happen.

Besides races that could happen in the code because there is no
coordinator configured (this is less of an issue for the RBD driver than
for other drivers, but it's still an issue), there's also a problem
whenever a cinder-volume service starts.

Any time a cinder-volume service starts it will mess many of the
resources that are being worked on (those with status ending in 'ing',
such as 'creating') because it thinks those are resources that it left
in that state and need to be cleaned.

My recommendation is to do this right: configure the cluster option,
remove the backend_host, and configure the coordinator.  Upgrading a
deployment from that configuration to clustered is relatively easy, we
just need to leave one of the cinder-volume services with the
backend_host as it was before; that way when it starts it will
automatically migrate all the resources from being non-clustered to
being clustered (an alternative would be to add this command to
cinder-manage, because I don't think the current "cinder-manage cluster
rename" will work).

If deploying the coordinator is an issue, we should at least do the
other 2 steps.  That way we'll get rid of the cleanup issue even if we
still have the race conditions.

Cheers,
Gorka.

>
> this configuration is similar to that deployed by tripleo, except
> tripleo would use pacemaker to have always a single cinder-volume running
>
> the reason being that, as far as I understand, without a coordinator the
> first cinder-volume within a given 'backend_host' group to consume the
> message from the amqp queue will start executing the task ... so if
> another task is queued (or is in progress), for the same volume, there
> is risk of data corruption
>
> > Now, there have been no reports that it misbehaved for anyone. It
> > certainly has not for any Kolla core. The fact is it was brought to
> > our attention because due to the drop of Kolla-deployed Ceph, the
> > recommendation to set backend_host was not present and users tripped
> > over non-uniform backend_host. And this is expected, of course.
> >
> > The second and final question is, building up on the first one, were
> > we doing it wrong all the time?
> > (plus extras: Why did it work? Were there any quirks? What should we do?)
> I think the correct setup for active/active should be
>
> - do not use same host or backend_host
> - do set cluster to same value across cluster members
> - use a coordinator
>
> > PS: Please let me know if this thought process is actually
> > Ceph-independent as well.
> I don't think it's Ceph dependent, my understanding is that
> active/active is only possible with some drivers because not every
> driver is safe to use in active/active configuration; some can, for
> example, have issues handling the database
>
> Ceph is just one of those drivers which behaves correctly in
> active/active configuration
> --
> Giulio Fidente
> GPG KEY: 08D733BA
>