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 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.
Verify that Storage I/O Control is enabled.
- Browse to the datastore in the vSphere Client.
- Click the Configure tab and click Settings.
- Click General.
- Click Edit for Datastore Capabilities.
- Select the Enable Storage I/O Control check box.
Storage I/O Control automatically sets the latency threshold that corresponds to the estimated latency when the datastore is operating at 90% of its peak throughput.
- (Optional) Adjust the Congestion Threshold.
The percentage of peak throughput value indicates the estimated latency threshold when the datastore is using that percentage of its estimated peak throughput.
- Select a value from the Percentage of peak throughput drop-down menu.
The value must be between 5ms and 100ms. Setting improper congestion threshold values can be detrimental to the performance of the virtual machines on the datastore.
- Select a value from the Manual drop-down menu.
- (Optional) Click Reset to defaults to restore the congestion threshold setting to the default value (30ms).
- Click OK.