It's free and open source!

It offers a free alternative to VMware. The license costs for VMware are quite large if you are running a big cluster. OpenStack on the other hand is free, open sourced solution. Of course there are some costs; hardware, electricity, administration, OS licence if you decide to run it on RHEL.

I was running CA ARCserve r16 on virtual Windows Server 2008 R2 machine. The environment it was running in had 3 physical VMware ESXi hosts and all of these were controlled by a single VMware vCenter server.