 | | | High Availability options | High Availability options 2006-07-13 - By Stephen Kirkpatrick
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Ed Wilts wrote:
> 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
>
>
Thanks for the reply. We are using proprietary third party
development tools, so we are stuck with using SSH for the
login session (or rewriting nearly 20 years worth of development
in another language). While no interruption of the sessions would
be desired, a brief interupption would be tolerable. I will
then plan on using the scheme you mentioned where the users log
back into the system after a node goes down.
With that being said, any recommendations for HA software?
Thanks,
Stephen Kirkpatrick
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