Storage Path Selection Policy Choices

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

Great place to find recommendations of what to use on your SAN

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

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