Live Optics | Optical Prime | High Latency When I/O is Low
Summary: This article offers some observations on High Latency When I/O is Low.
Instructions
Ignore latencies that are abnormal when IOPSs are low (say below 200).
The reason is how latencies are measured by most server's Operating Systems and Subsystems. The operating systems add up a counter of the time that each I/O takes. Software like Live Optics reads those counters and divides them by the number of IOPS that the tool has calculated in a corresponding time period. Then the tool does a simple division. Avg. latency = total time/total I/Os for the sample period.
This works fine when there is a steady state of lots of I/O.
However, when there are starts and stops during idle periods, some misleading results can apply. This is partly because the collection of the latency counter and the I/Os counter are not atomic operations. Small sampling errors between the collection moments between these counters can result in overstated latencies.
Generally, unless seeing a sustained bar of bad latencies, ignore latencies where the IOPSs are small.
Additional Information
If you have any questions, please reach out to Live Optics Support at liveoptics.support@dell.com.