[ClusterLabs] principal questions to a two-node cluster

Lentes, Bernd bernd.lentes at helmholtz-muenchen.de
Mon Apr 20 13:12:01 EDT 2015

Michael wrote:
> Am Montag, 20. April 2015, 15:23:28 schrieb Lentes, Bernd:
> > Hi,
> >
> > we'd like to create a two-node cluster for our services (web,
> > database, virtual machines). We will have two servers and a shared
> fiberchannel SAN.
> > What would you do e.g. with the content of the webpages we offer ?
> Put
> > them on the SAN so we don't need to synchronize them between the
> two nodes ?
> Yes. That seems to be a good idea.
> > 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 ?

> 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.

> Please also consider NFSv4 if your SAN box offers it. NFS has file locking
> included.

The SAN does not offer NFS.

> Please do not hesitate to mail to me or to the list, if there are any other
> problems.
> For the databases, you also could consider using a Master/Slave setup.
> So the data replication does happen on application level and no shared
> filesystems are needed. pacemaker handles the state (Master / Slave) of
> the database application. Otherwise the database would need share
> storage.
> 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.

> Greetings,

Bist Du der Autor von "Clusterbau: Hochverfügbarkeit mit Linux" ? Tolles Buch.


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