Take these steps before upgrading to vSphere 4.1
Upgrade vCenter. You can't add vSphere 4.1 hosts to vCenter unless you're running vCenter 4.1 but you can add older vSphere hosts to vCenter 4.1. As such, your first step in an upgrade scenario is to upgrade vCenter to the latest release available. There are actually multiple places where I've seen people run across this problem.
- Caveat. vCenter 4.1 is a 64-bit only affair. Older versions of vCenter/VirtualCenter support 32-bit architectures. If you're simply upgrading an existing 64-bit vCenter server, you can simply install the update once you take a backup for safety. If, however, you're running a 32-bit vCenter server, you will need to build a new vCenter server and migrate the database.
- Caveat. Watch out for this potential ODBC gotcha.
- Consider a transition to ESXi now. Although not required, if possible, take the 4.1 installation as an opportunity to move from ESX to ESXi. vSphere 4.1 is the last significant release that will support ESX so this is a step that you need to consider at some point anyway.
- Read the product release notes. VMware provides a very comprehensive set of release notes (ESX/ESXi) that help you to identify potential upgrade issues. If you have any problems after you upgrade, refer to these notes to see if you're experiencing a known issue.
- Verify hardware compatibility. Double-check that your platform is still fully supported by VMware. If necessary, wait until VMware certifies your hardware before undertaking the upgrade even if you have a fully supported older version of ESX or ESXi.