When using ESXi in conjunction with a SAN, you must follow specific guidelines to avoid SAN problems.
You should observe these tips for avoiding problems with your SAN configuration:
Place only one VMFS datastore on each LUN. Multiple VMFS datastores on one LUN is not recommended.
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 configuration, access control, storage, switch, server and iSCSI 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 iSCSI HBAs are installed in the correct slots in the ESXi 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 ESXi performance charts, Ethernet switch statistics, and storage performance statistics.
Be cautious when changing IDs of the LUNs that have VMFS datastores being used by your host. If you change the ID, virtual machines running on the VMFS datastore will fail.
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.
If you need to change the default iSCSI name of your iSCSI adapter, make sure the name you enter is worldwide unique and properly formatted. To avoid storage access problems, never assign the same iSCSI name to different adapters, even on different hosts.