An Explanation of Storage I/O Control vs. Storage Adapter Load Balancing

During a recent conversation I was stumped when asked what's the difference between VMware Storage I/O and Round Robin Multipathing.

1. Storage I/O balances storage resources among VM's according to allocated shares per VM when contention exists for storage resources.Therefore, storage I/O's are prioritized based on VM share level and balances the available storage I/O's among VM's on an Storage I/O enabled disk volume. This feature is a solution to storage resource contention among VM's sharing the same datastore.
   
   
2. Round Robin Multpathing alternates paths between the ESXi/ESX host and the datastore upon exceeding a specified threshold of I/O blocks. It continues to do this and ping-pong back and forth as well as provide fail over should one the used path become unavailable. This method is a solution to path availability and congestion.
   

Both can be described, or debated, as a method of load balancing but each are solutions to different problems. For Multipath, check your SAN vendor before enabling because the wrong MP policy can cause thrashing on the SAN.

Enabling Storage I/O or multipathing is easy. From the ESX/ESXi host or datacenter, RIGHT-CLICK the storage, choose properties, the select ENABLED uner the Storage I/O Control.


To enable Multipath, click the MANAGE PATHS button lower right and you get the following window: