For each type of server failover, you must consider storage issues.

Approaches to server failover work only if each server has access to the same storage. Because multiple servers require a lot of disk space, and because failover for the storage array complements failover for the server, SANs are usually employed in conjunction with server failover.

When you design a SAN to work in conjunction with server failover, all LUNs that are used by the clustered virtual machines must be detected by all ESX/ESXi hosts. This requirement is counterintuitive for SAN administrators, but is appropriate when using virtual machines.

Although a LUN is accessible to a host, all virtual machines on that host do not necessarily have access to all data on that LUN. A virtual machine can access only the virtual disks for which it has been configured.

Note

As a rule, when you are booting from a SAN LUN, only the host that is booting from that LUN should see the LUN.