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Why does the volume latency or IOPS not match the aggregate in Active IQ Unified Manager or ONTAP?

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Applies to

  • OnCommand Unified Manager (OCUM)
  • Active IQ Unified Manager (AIQUM)
  • ONTAP 9

Answer

How do background workloads impact disk IOPS?
  • There are background operations which run on disk but not counted as part of the volume IOPS (frontend) and may elevate disk IOPS/transfers
    • These include things such as:
      • WAFL scanners
      • Deduplication (inline or scheduled)
      • Anything snapshot related
      • Tiering/FabricPool
    • Examples:
      • The tiering scanner may have 60,000 aggregate IOPS while the busiest volume has 2,000 IOPS
      • A DR or backup filer with minimal frontend will use all available CPU and disk I/O bandwidth to process SnapMirror/backup workloads as quickly in the absense of frontend work
    • These background workloads yield to frontend as workload from clients increases
What other factors may affect aggregates having more or less IOPS than all the volumes added up?
  • Reads are prefetched through ONTAP's readahead engine
    • Readahead reduces latencies as readahead has been optimized for years and is very efficient at predicting accurately what is needed
    • By prefetching, the reads are in cache (RAM) as the IOP comes in through the network
  • Reads are also cached in RAM, and may be cached using Flash Cache or Flash Pool technology with lower latency
  • Writes are cached in RAM until written asynchronously to disk in a consistency point, delivering low latency on writes
  • Other IOPS may not require going to disk as metadata structures are also cached in RAM as needed

Additional Information

 

  • Example: The first volume listed on the left has a latency of 0.569 ms/op, while aggregate average latency is approximately 10 ms

 

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