[Linux-cluster] When to use more than one cluster?

[isplist at logicore.net]

Currently, all nodes, no matter what they are doing are all part of one single 
cluster. So, even though they aren't sharing services or storage, they are all 
part of the same cluster.

I could have one cluster for web servers and their storage, one for mail 
servers, one cluster for sql servers, etc. 

Am I complicating things for myself by having one single cluster. Which would 
be best in my case, multiple clusters or one single cluster?

I do see this as an easier method by which to grow in the future and right 
now, things get complex when it comes to turning certain servers on and off, 
changing the count of servers thereby sometimes causing the cluster not to 
boot until I change the config file. 

Any thoughts or input would be very appreciated and I'm sure be of help to 
some others as well.

Thanks.

Mike