[CentOS] Clustering solutions - mail, www, storage.

Hi all.

I am currently working for a hosting provider in a 100+ linux hosts'
environment. We have www, mail HA solutions, as storage we mainly use
NFS at the moment. We are also using DRBD, Heartbeat, Corosync.

I am now gathering info to make a cluster with:
- two virtualization nodes (active master and passive slave);
- two storage nodes (for vm files) used by mentioned virtualization
nodes (also active/passive).

For virtualization I am thinking to use OpenVZ or KVM. For storage NFS
or iSCSI. Could you please share your experiences with these
technologies? Which one would you use and why? Are there any good
alternatives in CentOS?

Thanks for the info,
Rafal.