[ClusterLabs] principal questions to a two-node cluster

[Andrei Borzenkov]

В Mon, 20 Apr 2015 09:37:24 -0400
Digimer <lists at alteeve.ca> пишет:

> On 20/04/15 09:23 AM, Lentes, Bernd wrote:
> > 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
> 
> 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.

TBH this is oversimplification. There are often hidden dependencies on
host name scattered around applications and nobody knows they are there
until someone tries to move installation to another host. In the past
sometimes it was necessary to actually change host name on failover.

Things did become better over time, so for common cases that are
implemented often it is probably no more an issue.