AakashJ,
Thanks for the input but I'm not sure I understand what you mean by "Donot remove ESXi 4.1 from inventory of vCenter 4.1 since that would mean you cannot add the ESXi back as the VMs are running in EVC mode. - recommend to shutdown vCenter 4.1 during upgrade of vCenter 5.1 VM"?
If I use the same database for vCenter 5.1 and the ESX 4.1 host are available and can be managed by vCenter 5.1 since it's backwards compatible in this area, why would I not want to remove the ESX 4.1 hosts after I've moved the VM's into the new ESXi 5.1 hosts? I won't be adding the ESX 4.1 host servers back as these are being replaced by the new ESXi 5.1 host servers. I'm planning on moving VMs off old host to new host and removing these older hosts because I'm going to need the Fibre channel cables to connect another new host to my SAN and I'll then be upgrading the license from 4.1 to 5.1 for these so that I can apply them to the new hosts. So, basically I'm migrating one host at a time from old to new environment to have zero downtime.
Remember these are ESX not ESXi and it's the cluster that runs in EVC mode which controls compatibility for CPU family differences on the hosts not the VMs.
In regards to recommending that I shutdown vCenter 4.1 during upgrade, ok that seems fine since that's a separate server but not sure its necessary based on what the KB article says?
As for "Donot upgrade your existing datastores to vmfs5". I wasn't planning on doing this...my existing datastores running VMFS-3 are staying. I'm creating new VMFS-5 datastores to move the VMs into. Once all is well and running on new VMFS-5 then I'll remove the older VMFS-3 datastores from my SAN. I'm definitely taking a backup of the DB before doing the migration if I'm going to use the existing databases just to be safe, but to clarify your question...the SQL cluster does not need to be broken during this migration/upgrade. The existing databases are already running with vCenter 4.1, so according to the KB article all I have to do is create the new vCenter 5.1 and select the option to "use existing databases".
Again, even if I couldn't do this and had to start from scratch with these databases, I'm ok with that...I have a small environment and it's fairly easy for me to add and configure my hosts & clusters, etc.