First we must talk about ALUA, it stands for “Asymmetric Logical Unit Access”, which is a feature on some mid range storage devices, (such as a Clariion) that will allow it to emulate a higher end array running Active/Active Storage Processors. Each SP still owns the LUNS, but with ALUA, an SP can process data for the other one via the backplane in the chassis.
Keep in mind:
VMware Defaults are MRU for Active/Passive & Fixed for Active/Active
MRU never falls back automatically
Always ignore my advice and follow storage vendor best practices
I am assuming your not using a 3rd party PSP, SATP or MPP, such as EMC Powerpath/VE (those are generally the best option if available)
Active/Passive (think EMC CX3 without ALUA)
Fixed = Not a great choice but will work. Fixed will cause you to micro manage the ESX hosts to ensure that all hosts are on the same path. If a host has a path fail and causes it to fail over to the 'non-preferred' path this will cause trashing with the remaining hosts possibly leading to downtime.
MRU = Best practice and a good choice, this will allow the storage array to set all ESX hosts to the proper path and eliminate LUN trashing or trespassing of the LUNs; all hosts in the cluster should be set this way.
RR = Do not use, will cause trashing, data corruption and other issues.
Emulated Active/Active Mid Range Storage with ALUA Enabled, such as CX4 Clariion
Fixed = Decent Choice if you’re a control freak or have FC I/O bottlenecks.
MRU = Best Practice, vSphere 4 is aware of ALUA, this will allow the storage array to set all ESX hosts to the proper path and eliminate LUN trashing or trespassing of the LUNs; all hosts in the cluster should be set this way. Just make sure to balance the I/O among your SP’s.
RR = Works, but can cause excessive use of the backplane
Real Active/Active Higher End Storage such as Symmetrix
Fixed = Probably your best option, will require each host and LUN to be set to opposite paths; and will require micro-management of the storage infrastructure.
MRU = Works, but probably not your best option, does not load balance traffic, could force all traffic to one HBA
RR = Easiest option as long as the SAN is dedicated to the vCenter, if not, perhaps Fixed is your best option