DSXchange

If we start with a basic installation topology where the Services, Repository and Engine tiers are all on the same server, how easy is it to separate the Engine tier from the other two at a later date?

They would all be on a single VM to start with and if there are performance or capacity issues (e.g. users experiencing slow response when the engine is busy), then separating the tiers could help resolve this without requiring additional licences.

I'd like some idea of how big a task that would be and if anyone has had experience of doing this?

Well, it's a reinstall if you do that.

IMHO, keep Websphere side by side with your Engine. I have seen components fail because of assumptions IBM made. The IRULES stuff for instance. Failed when it didn't have some of the WAS jar files in the path. So I would stick with a 3 tier install instead of 4.

Splitting it up later is not trivial and is time consuming. You did mention they are VM's. Virtual Machines are usually very cheap, and as you say, there's no licensing issue for splitting out the Services and Repository tiers.

I'd recommend doing it up front. You can always start out with smaller VM's and add CPU / memory later if it turns out you need more horsepower. That's trivial and can be easily adjusted.

Note: Be aware of IBM's licensing requirements for VM's. There are rules you must follow or you might be charged for every CPU in the entire Virtual Rack!