When using ESXi in conjunction with a Fibre Channel SAN, you must follow specific guidelines to avoid SAN problems.
You should observe these tips for preventing problems with your SAN configuration:
Place only one VMFS datastore on each LUN.
Do not change the path policy the system sets for you unless you understand the implications of making such a change.
Document everything. Include information about zoning, access control, storage, switch, server and FC HBA configuration, software and firmware versions, and storage cable plan.
Plan for failure:
Make several copies of your topology maps. For each element, consider what happens to your SAN if the element fails.
Cross off different links, switches, HBAs and other elements to ensure you did not miss a critical failure point in your design.
Ensure that the Fibre Channel HBAs are installed in the correct slots in the host, based on slot and bus speed. Balance PCI bus load among the available busses in the server.
Become familiar with the various monitor points in your storage network, at all visibility points, including host's performance charts, FC switch statistics, and storage performance statistics.
Be cautious when changing IDs of the LUNs that have VMFS datastores being used by your ESXi host. If you change the ID, the datastore becomes inactive and its virtual machines fail. You can resignature the datastore to make it active again. See Managing Duplicate VMFS Datastores.
If there are no running virtual machines on the VMFS datastore, after you change the ID of the LUN, you must use rescan to reset the ID on your host. For information on using rescan, see Storage Refresh and Rescan Operations.