Having recently moved into an enviroment where the storage is a little alien to me, I thought would be helpful to buff up on some storage knowledge and thought it might help some readers too.
Here is a diagram of a midrange san:
(Thanks Virtualgeek for this picture)
See the two items list as "Data Processor(head) A" and "Data Processor(head) B"?
Traditionally if you are using Active/Active Processor array you should use "Fixed" as the Multipathing method and In an Active/Passive array use "MRU".
However this changed with:
ALUA:Symmetric Logical Unit Access
Essentially in midrange san enviroments (EMC Clariion etc), this allows an unoptimized and an optimized path to a lun through different heads.
ESX(4) the HBA is aware of optimized and unoptimized paths as it knows which head has control of the LUN!
Suddenly we can use MRU with Active/Active heads.
MRU Most recently used:Use the Optimized Path unless it is not avalible then use the Unoptimized path (ESX 4.0/vSphere only)
Fixed: Always use this LUN unless it is unavalible.
NMP:Native MultiPath Driver:
MMP:Multipath Plugin (EMC Powerpath)
Round Robin: Within ESX server's iSCSI HBA it sends 4000 IO blocks down one path then moves to the next path.
Use the following commmand to tweak the iSCSI HBA
esxcfg-mpath --lun vmhba32:0:8 --policy custom --custom-hba-policy any --custom-max-blocks 1024 --custom-max-commands 50 --custom-target-policy any