@LAH3385 might be good to start a thread and try to determine what your needs are before going down the path of technology. By the time you were asking this question, you were already in pretty deep assuming certain products, product categories and platform HA. We should start with a business needs analysis, use that to set goals and then use the goals to select technology approaches.

Exactly - two 6 TB HDD's may be a lot less expensive, but the performance compared to SSD would be insanely different.

How many HDDs would you need to reach your IOPS requirements?

I've never monitored them.

The work is being done by Dom0, not the VM.

And with the resources being statically assigned I can't imagine that there is much of a hit to the performance of the VM's them self.

Quite a while means several months, not years.

http://www.xenserver.org/