There was an interesting question posted recently around how you could monitor Storage I/O Control activity. Basically, how would one know if SIOC had kicked in and was actively throttling I/O queues? Well, in vSphere 5.1, there are some new performance counters that can help you with that.
By selecting an ESXi host and going to the Monitor > Performance > Advanced view, click on the Chart Options, select Datastore, and there are a number of new SIOC counters which can be selected:
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