[linux-lvm] Why do lvcreate with clvmd insist on VG being available on all nodes?

[Jacek Konieczny]

On Thu, Nov 15, 2012 at 10:09:35AM +0100, Zdenek Kabelac wrote:
> > work properly, as I would expect (make the volume available/unavailable
> > on the node). But an attempt to create a new volume:
> >
> > lvcreate -n new_volume -L 1M shared_vg
> >
> > fails with:
> >
> > Error locking on node 1: Volume group for uuid not found: Hlk5NeaVF0qhDF20RBq61EZaIj5yyUJgGyMo5AQcLfZpJS0DZUcgj7QMd3QPWICL
> >
> 
> 
> Haven't really tried to understand what are you trying to achieve,
> but if you want to have node being activated only on one cluster node,
> you may easily use    lvcreate -aey  option.
> 
> If you are using default clustered operation - it's not surprising,
> the operation is refused if other nodes are not responding.

Hmmm didn't though about the initial activation. In fact, I don't need
that volume activated as soon as it is created. You are right, I should
try 'lvcreate -aey' or 'lvcreate -an'.

My stupid mistake, indeed.

'lvcreate -an -Z n' and 'lvcreate -aey' do work in such case.

Though, LVM have some problems with tracking the exclusive activations
later…

> > Indeed, the VG is not available at the standby node at that moment. But,
> > as it is not available there, I see no point in locking it there.
> 
> Well - you would need to write your own type of locking with support
> of 'standby'  currently clvmd doesn't work with such state (and it's not
> quite clear to me how it actually should even work).
> So far either node is in cluster or is fenced.

I will try to make this work somehow… I don't think a node in standby
(or when one of the VGs is not available there) should be quite
different that one fenced.

> > When clvmd is stopped on the inactive node and 'clvmd -S' has been run
> > on the active node, then both 'lvchange' and 'lvcreate' work as
> > expected, but that doesn't look like a graceful switch-over. And another
> > 'clvmd -S' stopped clvmd all together (this seems like a bug to me)
> >
> > And one more thing bothers me… my system would be very scalable to many
> > nodes, where only two share active storage (when using DRBD). But this
> > won't work if LVM would refuse some operations when any VG is not
> > available on all nodes.
> 
> Obviously using clustered VG in non-clustered environment isn't smart plan.
> What you could do - is to   disable clustering support on VG

Clusters do not have to be symmetrical. Cluster when different nodes
have a bit different set of resources available are still clusters.

I do need clustering locking – in case a volume group is available on
a few nodes it must not be possible to more than one node use any of the
logical volumes there.

> Note - you may always go around any locking problem with the above config 
> option - just do not report then problems with broken disk content and badly
> activated volumes on cluster nodes.

I am aware of the risks. That is why I do use clvmd. I don't quite
understand some of the clvmd behaviour and I wrote here to know if there
are some other risks taken into account by clvmd that I am not aware of.

It seem I still need some work and learning to make it work properly. 

Greets,
        Jacek