Storage I/O Control (SIOC) is a feature of VMware vSphere Enterprise Plus that provides I/O prioritization of virtual machines running on a group of VMware vSphere hosts that have access to the same datastore. If the latencies of a datastore come above a certain threshold, Storage I/O Control will throttle the I/O bandwidth of the virtual machines that use this datastore (according to their respective share value). The default value for the latency threshold is 30 milliseconds. You should keep this default value unless you have a good reason to change it. One good reason can be the use of SSDs, where a latency threshold of 10 - 15 milliseconds is recommended.
You can enable or disable Storage I/O Control using the -StorageIOControlEnabled parameter of the Set-Datastore cmdlet. You can also specify a value for the threshold with the -CongestionThresholdMillisecond parameter. The syntax of the Set-datastore cmdlet is as follows:
Set-Datastore [-Datastore] <...