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Determining whether to modify the caching policy of Flash Pool aggregates

You can assign cache-retention policies to volumes in Flash Pool aggregates to determine how long the volume data remains in the Flash Pool cache. However, in some cases changing the cache-retention policy might not impact the amount of time the volume's data remains in the cache.

About this task

If your data meets any of the following conditions, changing your cache-retention policy might not have an impact:

  • Your workload is sequential.

  • Your workload does not reread the random blocks cached in the solid state drives (SSDs).

  • The cache size of the volume is too small.

The following steps check for these conditions. The task must be done in advanced privilege mode.

  1. View the workload volume:statistics start -object workload_volume
  2. Determine the workload pattern of the volume: statistics show -object workload_volume -instance volume-workload -counter sequential_reads
  3. Determine the hit rate of the volume: statistics show -object wafl_hya_vvol -instance volume -counter read_ops_replaced_pwercent|wc_write_blks_overwritten_percent
  4. Determine the Cacheable Read and Project Cache Alloc of the volume: system node run -node node_name wafl awa start aggr_name
  5. Display the AWA summary: system node run -node node_name wafl awa print aggr_name
  6. Compare the volume's hit rate to the Cacheable Read.

    If the hit rate of the volume is greater than the Cacheable Read, then your workload does not reread random blocks cached in the SSDs.
  7. Compare the volume's current cache size to the Project Cache Alloc.

    If the current cache size of the volume is greater than the Project Cache Alloc, then the size of your volume cache is too small.
  • Modifying caching policies of Flash Pool aggregates
    You should modify the caching policy of a volume only if a different caching policy is expected to provide better performance. You can modify the caching policy of a volume on a Flash Pool aggregate.
  • Setting the cache-retention policy for Flash Pool aggregates
    Beginning with ONTAP 9.4, you can assign cache-retention policies to volumes in Flash Pool aggregates. Data in volumes with a high cache-retention policy remains in cache longer and data in volumes with a low cache-retention policy is removed sooner. This increases performance of your critical workloads by making high priority information accessible at a faster rate for a longer period of time.