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.
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.

Procedure

  1. Browse to the datastore in the vSphere Web Client navigator.
  2. Click the Configure tab and click Settings.
  3. Click General.
  4. Click Edit for Datastore Capabilities.
  5. 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.
  6. (Optional) Adjust the Congestion Threshold.
    • Select a value from the Percentage of peak throughput drop-down menu.
    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 Manual 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.
  7. (Optional) Click Reset to defaults to restore the congestion threshold setting to the default value (30ms).
  8. Click OK.