Re: High Availability options

On Thu, Jul 13, 2006 at 11:23:48AM -0500, Stephen Kirkpatrick wrote:
> I am seeking advice from those of you who have experience with high
> availability on RHEL3.  I have been researching this through Google
> and have observed several options, but don't know what would be most
> appropriate for our deployment.

I think you're simply out of luck with a Linux-based solution on your
current hardware.  All of the HA solutions essentially allow a stateless
application to fail over from node to node.  ssh requires an active
state and an active connection.  After all, the application has
permanent memory assigned.  Your hardware failure could be memory so you
have to realize that the only way you can survive this is if the memory
is redundant.

> 
> My main concern with providing HA for our environment is to not
> interrupt our SSH login sessions.  While there are other network
> services running, such as Apache, these services could tolerate a
> short interruption in the event of a failover.

To survive hardware failures for an ssh session, you'll need
fault-tolerant hardware like a Tandem system.  They're extremely pricey
but they do what they're designed to do - basically give you 100% uptime
for your front-end application so it can talk to back-end servers that
have failover functionality.

You might be cheaper off rewriting your application to eliminate the ssh
front-end, or else to allow that to fail and have a user sign back on to
another host in a cluster quickly.  The latter is what I do with my
VMScluster - the cluster has over 7 years of uptime, but if a node
crashes, the user loses his/her session and immediately signs back on
again and starts working again.  The application has to start up
quickly, and hopefully you've got some sort of journalling within your
app, but you don't have a lot of choices.

Your problem is not HA software - it's HA hardware.  No add-on software
that I'm aware of will maintain your ssh state while the system crashes
out from under it.

        .../Ed

-- 
Ed Wilts, RHCE
Mounds View, MN, USA
mailto:[email protected]
Member #1, Red Hat Community Ambassador Program

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