Tips and tricks for Redhat Cluster

Redhat Cluster is a nice HA product. I have been implementing it for a while now, lecturing about it, and yes – I like it. But like any other software product, it has few flaws and issues which you should take under consideration – especially when you create custom “agents” – plugins to control (start/stop/status) your 3rd party application.

I want to list several tips and good practices which will help you create your own agent or custom script, and will help you sleep better at night.

Nighty Night: Sleeping is easier when your cluster is quiet. It usually means that  you don’t want the cluster to suddenly failover during night time, or – for that matter, during any hour, unexpectedly.
Below are some tips to help you sleep better, or to perform an easier postmortem of any cluster failure.

Chop the Logs: Since RedHat Cluster logging might be hidden and filled with lots of irrelevant information, you want your agents to be nice about it. Let them log out somewhere the result of running “status” or “stop” or even “start”. Of course – either recycle the output logs, or rotate them away. You could use

exec &>/tmp/my_script_name.out

much like HACMP does (or at least – behaves as if it does). You can also use specific logging facility for different subsystems of the cluster (cman, rg, qdiskd)

Mind the Gap: Don’t trust unknown scripts or applications’ return codes. Your cluster will fail miserably if a script or a file you expect to run will not be there. Do not automatically assume that the vmware script, for example, will return normal values. Check the return codes and decide how to respond accordingly.

Speak the Language: A service in RedHat Cluster is a combination of one or more resources. This can be somewhat confusing as we tend to refer to resources as a (system) service. Use the correct lingo.  I will try to do just that in this document, so heed the difference between the terms “service” and “system service”, which can be a cluster resource.

Divide and Conquer: Split your services to the minimal set of resources possible. If your service consists of hundreds of resources failure to one of them could cause the entire service to restart, taking down all other working resources. If you keep it to the minimum, you actually protect yourself.

Trust No One: To stress out the “Mind the Gap” point above – don’t trust 3rd party scripts or applications to return a correct error code. Don’t trust their configuration files, and don’t trust the users to “do it right”. They will not. Try to create your own services as fault-protected as possible. Don’t crash because some stupid user (or a stupid administrator – for a cluster implementer both are the same, right?) used incorrect input parameters, or because he has kept  an important configuration file in a different name than was required.

I have some special things I want to do with regard to RedHat Cluster Suite. Stay tuned 🙂

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