I am currently doing some test to simulate the real update requirement for my production VMware Farm to be upgraded from ESX 3.5 to vSphere 4. To minimize the impact to our production system, we may want to do this with as minimal impact as possible. Due to the virtual hardware version and tools upgrade require, is pretty tough for us to perform the entire upgrade at 1 time. Therefore, our plan is get the host upgraded and follow by each individual virtual machine to be upgraded follow the suitable timing for different business unit. HA and DRS will need to be disable temp during the ESX upgrade.

I am able to get the ESX 3.5 to be manage by the latest vcenter. A vmotion from ESX 3.5 to vsphere 4 had been successes too, but the latest version of virtual machine which built from vSphere 4 might not compatible to vmotion back to the ESX 3.5 hosts. At the same time, if you have different processors chipset in the environment and require EVC to be turned on, it may be a little challenge to do so. You may want to ensure the EVC to be done with no down time.  You may need to refer to my previous post about how to enable EVC with no down time.

This trick is workable in the case. The new cluster created in vSphere 4 environment, may temporally disable the HA and DRS due to the incompatibility within ESX 3.5 and vSphere 4 for VMotion, which may happen on the latest virtual hardware version which provided by vSphere 4. Anyway, my test will still going on to further analyze the best upgrade method to be used to our environment due to the challenge of different region, time zone and business function we currently serve with our VMware environment.