Storage I/O Control (SIOC) extends the constructs of shares and limits to handle storage I/O resources. SIOC is a proportional share scheduler that, under contention, throttles IOPS. You can control the amount of storage I/O that is allocated to VMs during periods of I/O congestion. Controlling storage I/O ensures that more important VMs get preference over less important VMs for I/O resource allocation.

There are two thresholds: one for standalone SIOC and one for Storage DRS. For Storage DRS, latency statistics are gathered by SIOC for an ESXi host and sent to the vCenter Server and stored in the vCenter Server database. With these statistics, Storage DRS can make the decision on whether a VM should be migrated to another datastore.

The default latency threshold of SIOC is 30 milliseconds. The default setting might be acceptable for some storage devices, but other devices might reach their latency threshold well before or after the default setting is reached. For example, Solid State Disks (SSD) typically reach their contention point sooner than the default setting protects against. Not all devices are created equally.