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.
You do not need to adjust the threshold setting in most environments.
Storage I/O Control will not function correctly unless all datatores that share the same spindles on the array have the same congestion threshold.
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 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.