[ClusterLabs] principal questions to a two-node cluster

[Lentes, Bernd]

Michael wrote:
> > >
> > > > Also the database and the vm's on the SAN ? Which fs would you
> > > > recommend for the SAN volumes ? OCFS2 ? Can I mount the same
> > >
> > > volume on
> > >
> > > > each node contemporarily ? Or do I have to use the ocfs2 as a
> > > > resource managed by pacemaker, so that the volume is only
> mounted
> > > > if it is
> > >
> > > necessary ?
> > >
> > > In your setup I'd avoid concurrent mounts of the columes on both
> > > servers. If you have concurrent mounts, you will have to use a
> > > cluster file system (OCFS2, GFS, ...). These file systems provide
> > > locking. But if pacemaker takes care, that the volumes are only
> > > mounted on one machine, you can go with a plain file system (ext4,
> > > efx).
> >
> > I thought ocfs2 would give me a further level of security. If,
> > somehow, although pacemaker takes care, two hosts try to mount
> > concurrently, with
> > ocfs2 nothing would happen. Right ? Is there any reason not to use
> ocfs2 ?
> > E.g. performance, stability ?
>
> Yes. Some people wear suspenders and a belt ;-)
>
> But seriously, OCFSs needs DLM. I'd try to avoid complex setups if
> possible.
> Make it as simple as possible. Complex setup tend to create the most
> weired errors.

OK. You propose a simple configuration. Seems to be a good idea. Is there a way to have snapshots ? OCFS2 offers snapshots for files. But it's a complex setup, as you write.
cLVM does not offer snapshots because of the cluster environment. Using a non-cluster fs would be simpler. But which one offers snapshots ? I know of btrfs taking snapshots,
but I don't like to use it on production systems. Ext4 snapshots have an "experimental" status. What to do ?

>
> > > if you need LVM, you anyway need LVM2 with Distributed Locking
> > > (DLM).
> >
> > Yes. I will not use LVM. But if I choose ocfs2, I also need DLM. Right ?
> > Or is there an advantage of choosing LVM ? Snapshots ? OCFS2 also
> > seems to be able to take snapshots.
>
> No LVM2 cannot take snapshots. If you use LVM in clusters you need
> LVM2 because it takes care for a consistent numbering of LVMs on all
> servers.
> otherwise You could run your LVMs only on the right or the left server.
> Not even a part here and a part there.

I read LVM2 offers snapshots, but not in a cluster environment.

> > > Please note that you need fencing in ANY case if you have shared
> > > storage.
> >
> > Yes. I have HP ProLiant servers with ILO cards, and also a
> > configureable (via LAN) power distributor from APC.
>
> APCs have an advantage over ILO. ILO will not answer if the power is
> gone. So the cluster will not continue. That missing feedback is much
> more unlikely from with an APC.
>
>
>
Bernd

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