One of the many features/improvements/enhancements introduced in vSphere 4.1 is Storage I/O Control. Similar to shares and limits available on CPU and memory, you can now set individual virtual machines to a certain number of I/O per second (IOPS) to their underlying datastore, or use shares to set a &quot;relative priority&quot;. As mentioned below in &quot;UPDATE II&quot;, Shares and their relative priority to the storage resource will kick in only under contention, whereas limits are ALWAYS enforced.

Also, disk shares specified without SIOC enabled still apply, but only at a local-ESX host level. With SIOC enabled, disk shares are enforced across ALL hosts that access that datastore.

<!--more--> The mechanism used to determine &quot;contention&quot; is average latency between the host and the datastore I/O control is enabled on. By default this threshold is 30ms, and VMware states that for most environments this should be sufficient. However, Advanced settings allow the user to set a custom threshold between 10 and 100ms (lower for SSD is recommended...10-15ms)

UPDATE: The default threshold is in fact 30 ms, but is 30ms of delay for 4 seconds-- so it needs to be a sustained delay, and not just a one-off spike.

UPDATE II: As mentioned in the comments, shares kick in under contention, whereas IOPS limits do not-- in fact, according to VMware documentation, Limits are &quot;always enforced regardless of whether or not SIOC is enabled&quot;.