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Why is a workload's latency high when the IOPS are low?

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Category:
ontap-9
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perf
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Applies to

  • ONTAP 9
  • Clustered Data ONTAP 8
  • Data ONTAP 7-mode

Answer

  • ONTAP will respond to requests as they come in, and a workload that has few requests will appear to be higher but be responding perfectly fine
  • Low IOP workloads (ie., 5 IOPs and 32kB/s) will:
  • To put this another way: low IOP workloads are not a problem in the absense of other symptoms (errors, application not responding, network issues, etc.)

Additional Information

  • Definitions:
    • mean: average, or the sum of all instance values divided by number of instances
    • median: the instance value in the middle when values are ordered from smallest to largest
    • mode: the instance value occurring most often
  • In the statistics branch of math, you need to use mean, median, and mode to help calculate that.
 
Example 1: Latency observed across 3 instances in a period (say 3 ops in a minute): 1 ms, 100 ms, 1 ms
  • mean: (1+100+1)/3=34 ms
  • median: 1 ms
  • mode: 1 ms
  • ONTAP will often give average latency, but in this case, the median and mode show that latency is actually really good.

Example 2: Latency observed across 20 instances (7 ops/second): 1ms, 1ms, 1ms, 1ms, 100ms, 1ms, 1ms...1ms (19 @ 1 ms, 1@100 ms)

  • mean: (19+100) /20=5.95ms
  • median: 1 ms
  • mode: 1 ms
  • In this case, average latency is more accurate than the prior example because we have enough data to have better confidence in the numbers

How to identify a client, network, or ONTAP problem calculating concurrency