When using Storage IO Control (SIOC), you have to set a congestion threshold. The congestion threshold determines when SIOC kicks in to regulate the storage queues. Regulating these queues results in a more optimal overall storage performance for your VMs (although it can result in lesser performance for some VMs).
The congestion threshold is 30 ms by default The question is; what is the optimal value?
The optimal value depends on the type of disks used in your environment. After a little research I have found the following values:
- Fibre Channel: 20-30 ms
- SAS: 20-30 ms
- SATA: 30-50 ms
- SSD: 15-20 ms
The conclusion is: the slower the disks, the higher the accepted latency. The above values can guide you set the optimal value, although you always have to monitor the environment. A different value might be configured depending on the used disks and storage solution!