Hi Mouhamad,
Very broadly stated, it is recommended that hosts in a cluster have the same configuration. Now, there are several things to think about here, though. First and foremost, you want hosts in the same cluster that have processors that come from the same vendor class, i.e. Intel or AMD, for vMotioning (and thus DRS) purposes, and the same processor family. Slightly different processors may be vMotion compatible if Enhanced vMotion Compatibility (EVC) is enabled at the cluster level. See page 55 in the vSphere 5 Resource Management guide here:
http://pubs.vmware.com/vsphere-50/topic/com.vmware.ICbase/PDF/vsphere-esxi-vcenter-server-50-resource-management-guide.pdf
You also want to have the same about of physical RAM installed in each server in a cluster. This will allow you to better plan for HA events and get the most out of your hardware. We're talking about slot sizes here (specifically the RAM portion of slots) and how many resources are reserved (based on slot sizes) on other hosts in the cluster. Say, for instance, you configure an HA cluster to tolerate the failure of one host. You have three hosts with unbalanced amounts of RAM, say 16 GB, 16 GB, and 32 GB. If you're using close the entire 32 GB of RAM on the one host, then HA will need to reserve enough resources on the other two hosts to start up all the VMs running on the larger host, which amounts to nearly all the RAM in the remaining two hosts - leaving no room for VMs to run on the other two hosts. So your'e wasting two entire hosts' worth of resources with unbalanced RAM across hosts in a cluster. Duncan Epping and James Bowling talk about these topics here:
http://www.yellow-bricks.com/2009/08/12/ha-and-slot-sizes/
http://vsential.com/2011/03/should-servers-in-a-cluster-have-the-same-everything/
Page 31 of the vSphere 5 Availability Guide recommends as a best practice setting the Percentage of Cluster Resources Reserved as the admission control policy to 1/N, where N is the number of hosts in the cluster. http://pubs.vmware.com/vsphere-50/topic/com.vmware.ICbase/PDF/vsphere-esxi-vcenter-server-50-availability-guide.pdf
It's okay to mix server vendors as long as the processors are similar enough to allow vMotion between the hosts. As with the above paragraphs, the closer matched each host is in a cluster, the better.
And finally, the best place to look for hardware compatibility is VMware's own HCL. Many big name vendors will already have their hardware certified and you can probably push the burden on them to prove their hardware is on the HCL. For everyone else, there's the VMware HCL here
http://www.vmware.com/resources/compatibility/search.php?rct=j&q=&esrc=s&source=web&cd=1&ved=0CCgQFjAA&url=http://www.vmware.com/go/hcl&ei=iAhKT9SxLoLStAahzJ2UBQ&usg=AFQjCNEXuH3H1BNILfm7AMoBNO0jQXhT-g&sig2=uuw_TsdiLNXQmhQJvD3vDQ
I hope this helps a bit.
Cheers,
Mike
http://VirtuallyMikeBrown.com
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Message was edited by: VirtuallyMikeB