Set Latency Sensitive Threshold

When you use the HPP for your storage devices, set the latency sensitive threshold for the device, so that I/O can avoid the I/O scheduler.

By default, ESXi passes every I/O through the I/O scheduler. However, using the scheduler might create internal queuing, which is not efficient with the high-speed storage devices.

You can configure the latency sensitive threshold and enable the direct submission mechanism that helps I/O to bypass the scheduler. With this mechanism enabled, the I/O passes directly from PSA through the HPP to the device driver.

For the direct submission to work properly, the observed average I/O latency must be lower than the latency threshold you specify. If the I/O latency exceeds the latency threshold, the system stops the direct submission and temporarily reverts to using the I/O scheduler. The direct submission is resumed when the average I/O latency drops below the latency threshold again.

Procedure

1. Set the latency sensitive threshold for the device by running the following command:
   esxcli storage core device latencythreshold set --device=device name --latency-sensitive-threshold=value in milliseconds
2. Verify that the latency threshold is set:
   esxcli storage core device latencythreshold list

   Device                Latency Sensitive Threshold
   --------------------  ---------------------------
   naa.55cd2e404c1728aa               0 milliseconds
   naa.500056b34036cdfd               0 milliseconds
   naa.55cd2e404c172bd6              50 milliseconds
   

3. Monitor the status of the latency sensitive threshold. Check VMkernel logs for the following entries:
   • Latency Sensitive Gatekeeper turned on for device device. Threshold of XX msec is larger than max completion time of YYY msec
   • Latency Sensitive Gatekeeper turned off for device device. Threshold of XX msec is exceeded by command completed in YYY msec