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.
- 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 20 ms could lower peak disk throughput. A very high value (above 50 ms) 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.
- If latency is more important, a very low value (lower than 20 ms) will result in lower device latency and better isolation among I/Os at the potential cost of a decrease in aggregate datastore throughput.
Launch the vSphere Client and log in to a vCenter Server system.
Verify that Storage I/O Control is enabled.
- In the vSphere Client inventory, select a datastore and click the Configuration tab.
- Click Properties.
- Under Storage I/O Control, select the Enabled check box.
- (Optional) Click Advanced to edit the congestion threshold value for the datastore.
The value must be between 10 ms and 100 ms.
- (Optional) Click Reset to restore the congestion threshold setting to the default value (30 ms).
- Click OK and click Close.