The congestion threshold value for a datastore is the upper limit of latency that is allowed for a datastore before Storage I/O Control begins to assign importance to the virtual machine workloads according to their shares.
Caution: Storage I/O Control will not function correctly if you share the same spindles on two different datastores.
If you change the congestion threshold setting, set the value based on the following considerations.
- A higher value typically results in higher aggregate throughput and weaker isolation. Throttling will not occur unless the overall average latency is higher than the threshold.
- If throughput is more critical than latency, do not set the value too low. For example, for Fibre Channel disks, a value below 20ms could lower peak disk throughput. A very high value (above 50ms) might allow very high latency without any significant gain in overall throughput.
- A lower value will result in lower device latency and stronger virtual machine I/O performance isolation. Stronger isolation means that the shares controls are enforced more often. Lower device latency translates into lower I/O latency for the virtual machines with the highest shares, at the cost of higher I/O latency experienced by the virtual machines with fewer shares.
- A very low value (lower than 20ms) will result in lower device latency and isolation among I/Os at the potential cost of a decrease in aggregate datastore throughput.
- Setting the value extremely high or extremely lowly results in poor isolation.
Prerequisites
Verify that Storage I/O Control is enabled.