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What is Adaptive QoS and how does it work?

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Applies to

ONTAP 9.3 or newer

Answer

  • Adaptive QoS uses QoS Throughput Floors and Ceilings (Minimum and Maximum throttle limits) to set individual Volume limits.
  • Floors are used to guarantee nearly-idle workloads do not interfere with busier workloads.
  • Adaptive QoS is dynamic based upon volume size, meaning a 10GB volume would have a different Floor and Ceiling than a 10 TB volume.
  • Terms
    • Adaptive QoS: Dynamic QoS Ceilings and Floors which grow or shrink based upon volume size used or allocated
    • Throughput Floor (Minimum): The amount of IOPs required to not hit deadline scheduler and obtain optimal latency.
    • Throughput Ceiling (Maximum): A hard limit of how many IOPs a Volume is allocated.
    • Expected IOPs: The Throughput Floor value in IOPs per terabyte.
    • Absolute minimum IOPs: An IOP Throughput Floor, used when Expected IOPs becomes too low and overrides Expected IOPs or Peak
    • Example: A 10GB Volume with a default Adaptive QoS "value" policy group will have a Floor of 75 IOPs, not 1.28 IOPs Expected.
    • Peak IOPs allocation: Adaptive QoS values can be assigned based upon used space or total space in the Volume.
       
  • By default, three buckets are created:
policy-group expected-iops peak-iops absolute-min-iops peak-iops-allocation
extreme 6144IOPS/TB 12288IOPS/TB 1000IOPS used-space
performance 2048IOPS/TB 4096IOPS/TB 500IOPS used-space
value 128IOPS/TB 512IOPS/TB 75IOPS used-space
 
 
How does Adaptive QoS decide when to throttle?
  1. In ONTAP 9.6 and earlier, Volumes not in an adaptive policy will be put into a deadline scheduler called Best Effort if CPU Headroom is over 100%
  2. In ONTAP 9.7 and later, Volumes not in an adaptive policy will be put into a deadline scheduler called Best Effort.
  3. Volumes in an adaptive policy with IOPs are below the Throughput Floor will be put into the deadline scheduler Best Effort.
  4. Volumes in an adaptive policy with IOPs above the Throughput Floor will not be throttled by QoS.
  5. Volumes in an adaptive policy with IOPs at the Throughput Ceiling will be throttled at that value.

Additional Information

Release notes for ONTAP: Storage QoS features by release

Documentation on Adaptive QoS.

Documentation on Throughput Floors

Here is some additional examples of space usage and throttling levels for comparison:

  • Expected IOPS = 128/TB
  • Peak IOPS = 512/TB
  • Absolute Min IOPS = 75
The minimum and maximum IOPS will be calculated as follows for used-space allocation:

Volume Size

Data Stored

QoS Min IOPS

QoS Max IOPS

1 GB

0GB

75

75

1 TB

0 TB

128

128

1 TB

.1 TB

128

128

1 TB

.2 TB

128

128

1 TB

.3 TB

128

154

1 TB

.5 TB

128

256

1 TB

1 TB

128

512

2 TB

2 TB

256

1024