As a general rule, active/passive luns connected directly to and ESX server should be set to MRU.
You should not see path thrashing issues with MRU as the LUN's will 'stick' to the most recently used path. The reason path thrashing occurs is that a fixed policy will try to 'pull' a the path back tot he prefered path and and MRU policy will try and 'pull' the path back to to the most recently used path.
I believe we'd treat the SVC as active/active (you may want to confirm this with IBM)
Normally we recommend Fixed for active/active arrays. Just be sure that it's set to Fixed across all hosts.
The VMware ESX Server VMkernel has a built-in proprietary implementation of multipathing that establishes and manages SAN fabric path connections. Once paths are detected, VMkernel establishes which path is active, in standby, or no longer connected (a dead path). When multipathing detects a dead path, it provides failover to alternate paths. Because of this multipathing mechanism, you can leverage all available paths to manually load balance I/O work-oad.
By default, ESX Server only uses one path, regardless of available alternate paths.
For active/active type arrays, you must manually assign a LUN to a specific path. For active/passive type arrays, ESX Server automatically picks one path for I/O workloads; you cannot load balance to an alternate path. This is a limitation of the active/passive array types as only one path can be active at one time.
For best performance, each virtual machine should reside on its own LUN. In this way, the volume can be specified a preferred path to the SAN fabric. Before doing this, however, you must determine which virtual machine or volume (if multiple virtual machines are sharing this one volume or LUN) has the most intensive I/O workload. Note that the level of I/O traffic varies for different applications at different times of the day. To determine the maximum level of I/O traffic for a virtual machine (or for a specific volume), monitor actual performance at intervals of a few hours apart (or shorter time duration) during a typical day. Monitor performance first by measuring the IOPS generated by the application. Second, measure the bandwidth availability between the associated volume and the SAN fabric path.
Balancing loads among available paths also improves performance. You can set up your virtual machines to use different paths by changing the preferred path for the different HBAs. This is possible only for active/active storage processors (SP) and requires that you have path policy set to Fixed.
If a path fails, the surviving paths carry all the traffic. Path failover should occur within a minute (or within two minutes for a very busy fabric), as the fabric might converge with a new topology to try to restore service. A delay is necessary to allow the SAN fabric to stabilize its configuration after topology changes or other fabric events.
