[Pacemaker] principal questions to a two-node cluster

Discussion:

Lentes, Bernd

2015-04-20 12:29:03 UTC

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 ? 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 ?

Thanks for any hint.

Bernd

--
Bernd Lentes

Systemadministration
Institut für Entwicklungsgenetik
Gebäude 35.34 - Raum 208
HelmholtzZentrum münchen
***@helmholtz-muenchen.de
phone: +49 89 3187 1241
fax: +49 89 3187 2294
http://www.helmholtz-muenchen.de/idg

Je suis Charlie

Helmholtz Zentrum München
Deutsches Forschungszentrum für Gesundheit und Umwelt (GmbH)
Ingolstädter Landstr. 1
85764 Neuherberg
www.helmholtz-muenchen.de
Aufsichtsratsvorsitzende: MinDir´in Bärbel Brumme-Bothe
Geschäftsführer: Prof. Dr. Günther Wess, Dr. Nikolaus Blum, Dr. Alfons Enhsen
Registergericht: Amtsgericht München HRB 6466
USt-IdNr: DE 129521671

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Digimer

2015-04-20 13:36:22 UTC

Permalink

You're trying to make your website HA, specifically?

Assuming so, you have two main options;

1. Application level HA
2. Server (VM) level HA

The benefit of #1 is that failover and recovery is usually faster, but
the downside is complexity. The benefits of #2 are that the HA is
obfuscated away from the application, migrating the service between
nodes is seamless/no interruption and the HA setup is portable to other
apps without modification. If you never plan to create another HA
anything, then part of the benefit of #2 goes away.

Personally, I am a big fan of keeping things as simple as possible. By
making the server HA, you need to change nothing about your application
stack. If the host fails, the cluster simply reboots the server on the
backup, done. Being a VM, reboot times are (in my experience, across
many OSes) 30~90 seconds to get back to the OS login screen (plus stack
startup, but that is negligible for most web server stacks).

Then you can create new servers on the same config (even OSes like MS
Windows) and they're magically HA as well, nothing more to do.

If you prefer #1 though, that is OK as well. The question then becomes
more about your particular needs, as the setup will be customized per
environment. Generally speaking, you want to avoid active/active if you
can avoid it. The reason being that clustered file systems, by the very
nature of their locking needing to be coordinated and clustered, comes
at a performance cost. People often think "well, I have two nodes, why
not double my performance?". It is sensible on the surface, until you
realize the complexity.

If you can get away with it, I'd use active/passive. In this case, only
one node will have your LUN at a time, hosting a traditional FS
ext4/xfs, and the data for your HA stuff on the LUN. The node that is
the current host would:

Connect the LUN -> mount the FS -> start the services -> take a
floating/virtual IP.

Migrate the service is:

Old Active; Take down the IP, stop the services -> dismount the FS ->
disconnect the LUN
New Active; Connect the LUN -> mount the FS -> start the services ->
take a floating/virtual IP.

(This is why having your service in an HA VM is better for migration; No
'stop' needed, live-migration causes no interruption).

Recovery from a crashed/failed active;

Fence the lost node -> Connect the LUN -> mount the FS -> start the
services -> take a floating/virtual IP.

To get into anything more specific, you will need to be more specific
about your priorities and the details of your setup.
--
Digimer
Papers and Projects: https://alteeve.ca/w/
What if the cure for cancer is trapped in the mind of a person without
access to education?

_______________________________________________
Pacemaker mailing list: ***@oss.clusterlabs.org
http://oss.clusterlabs.org/mailman/listinfo/pacemaker

Project Home: http://www.clusterlabs.org
Getting started: http://www.clusterlabs.org/doc/Cluster_from_Scratch.pdf
Bugs: http://bugs.clusterlabs.org

Digimer

2015-04-20 13:39:10 UTC

Permalink

Ignore this, I re-sent to ClusterLabs.

Post by Digimer

You're trying to make your website HA, specifically?
Assuming so, you have two main options;
1. Application level HA
2. Server (VM) level HA
The benefit of #1 is that failover and recovery is usually faster, but
the downside is complexity. The benefits of #2 are that the HA is
obfuscated away from the application, migrating the service between
nodes is seamless/no interruption and the HA setup is portable to other
apps without modification. If you never plan to create another HA
anything, then part of the benefit of #2 goes away.
Personally, I am a big fan of keeping things as simple as possible. By
making the server HA, you need to change nothing about your application
stack. If the host fails, the cluster simply reboots the server on the
backup, done. Being a VM, reboot times are (in my experience, across
many OSes) 30~90 seconds to get back to the OS login screen (plus stack
startup, but that is negligible for most web server stacks).
Then you can create new servers on the same config (even OSes like MS
Windows) and they're magically HA as well, nothing more to do.
If you prefer #1 though, that is OK as well. The question then becomes
more about your particular needs, as the setup will be customized per
environment. Generally speaking, you want to avoid active/active if you
can avoid it. The reason being that clustered file systems, by the very
nature of their locking needing to be coordinated and clustered, comes
at a performance cost. People often think "well, I have two nodes, why
not double my performance?". It is sensible on the surface, until you
realize the complexity.
If you can get away with it, I'd use active/passive. In this case, only
one node will have your LUN at a time, hosting a traditional FS
ext4/xfs, and the data for your HA stuff on the LUN. The node that is
Connect the LUN -> mount the FS -> start the services -> take a
floating/virtual IP.
Old Active; Take down the IP, stop the services -> dismount the FS ->
disconnect the LUN
New Active; Connect the LUN -> mount the FS -> start the services ->
take a floating/virtual IP.
(This is why having your service in an HA VM is better for migration; No
'stop' needed, live-migration causes no interruption).
Recovery from a crashed/failed active;
Fence the lost node -> Connect the LUN -> mount the FS -> start the
services -> take a floating/virtual IP.
To get into anything more specific, you will need to be more specific
about your priorities and the details of your setup.

--
Digimer
Papers and Projects: https://alteeve.ca/w/
What if the cure for cancer is trapped in the mind of a person without
access to education?

_______________________________________________
Pacemaker mailing list: ***@oss.clusterlabs.org
http://oss.clusterlabs.org/mailman/listinfo/pacemaker

Project Home: http://www.clusterlabs.org
Getting started: http://www.clusterlabs.org/doc/Cluster_from_Scratch.pdf
Bugs: http://bugs.clusterlabs.org

A.Rubio

2015-04-20 13:37:21 UTC

Permalink

if resource is only mounted in a node, you can use ext4, xfs or any
filesystem.
if it is mounted in more nodes simultaneously, you should use ocfs2,
gfs2 or other cluster filesystem.

Post by 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 ? 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 ?
Thanks for any hint.
Bernd
--
Bernd Lentes
Systemadministration
Institut für Entwicklungsgenetik
Gebäude 35.34 - Raum 208
HelmholtzZentrum münchen
phone: +49 89 3187 1241
fax: +49 89 3187 2294
http://www.helmholtz-muenchen.de/idg
Je suis Charlie
Helmholtz Zentrum München
Deutsches Forschungszentrum für Gesundheit und Umwelt (GmbH)
Ingolstädter Landstr. 1
85764 Neuherberg
www.helmholtz-muenchen.de
Aufsichtsratsvorsitzende: MinDir´in Bärbel Brumme-Bothe
Geschäftsführer: Prof. Dr. Günther Wess, Dr. Nikolaus Blum, Dr. Alfons Enhsen
Registergericht: Amtsgericht München HRB 6466
USt-IdNr: DE 129521671
_______________________________________________
http://oss.clusterlabs.org/mailman/listinfo/pacemaker
Project Home: http://www.clusterlabs.org
Getting started: http://www.clusterlabs.org/doc/Cluster_from_Scratch.pdf
Bugs: http://bugs.clusterlabs.org

_______________________________________________
Pacemaker mailing list: ***@oss.clusterlabs.org
http://oss.clusterlabs.org/mailman/listinfo/pacemaker

Project Home: http://www.clusterlabs.org
Getting started: http://www.clusterlabs.org/doc/Cluster_from_Scratch.pdf
Bugs: http://bugs.clusterlabs.org