- vSphere 4 introduced a new multipathing option for block devices (NMP Round Robin).
- This option works on both Active/Active storage platforms and Active/Passive storage platforms that support ALUA (Asymmetrical Logical Unit Access)
- NMP RR distributes IO for a device down all active paths (in the case of ALUA, it doesn’t use active-non-optimized paths), and uses a single path for a given number of IO operations, governed by a parameter (iooperationslimit)
- This value defaults to 1000, but can be changed. If you change it, there is a known issue with vSphere 4 that on ESX reboots that the value changes to a random value. (this was discussed here). This bug is expected to be fixed in vSphere 4 update 2.
So – should you change the value? In my opinion, no. Duncan did a great post on this topic here, which I commented on. He mentioned he was curious to see testing data from storage platform folks to show the results.
The conclusion (before I show all the testing) – there’s not enough benefit of changing the IOOperationLimit to change it from the default. I’m not implying that statement to others, you should defer to your particular storage vendor.
So… Here was the EMC testing results around this specific question. Read on if you’re interested. It was done with FC and IP storage.