| Q: We're upgrading a SAN with multiple operating systems (OSes). Should we run a different OS version in each fabric or upgrade both together?
A. Assuming you have separate fabrics, and thus redundant access, you want to isolate the adapter, switch, storage interface port and software from path failure. It would make sense to upgrade one fabric and ensure it's stable with the ability to revert to the previous OS. Even if the hosts usually don't notice the upgrades, there's a chance of that happening. You also need to factor in the interdependencies of the different host bus adapters, as well as interaction with the switches and storage devices.
If you upgrade one path and leave the other at the older version, try to upgrade the other path or fabric to the newer version as soon as possible. From a support and management standpoint, you'll be better off updating both.
--Greg Schulz, founder and senior analyst, StorageIO Group