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Does ONTAP apply efficiency to blocks that only exist in Snapshots?

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ontap-9
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Applies to

  • ONTAP 9
  • Active Filesystem (AFS)
  • Snapshot
  • Efficiency
  • Deduplication
  • Compression
  • Compaction

Answer

No, ONTAP does only deduplicate  blocks that exist in active filesystem (AFS).

Additional Information

  • Once a block got deleted from AFS but still exists in a snapshot, subsequent efficiency operations do not consider that block for deduplication anymore.
  • There is parameter -snapshot-blocks true under volume efficiency start when run with  set advanced privileges. This does not run deduplication but compression only against snapshot-only blocks that do not exist in AFS anymore.

Example:

  1. File or Block was written to a volume
  2. Inline efficiency was NOT applied
    • FAS with no inline configured
    • AFF under high write workload (inline is best effort)
  3. Written block is tracked in volume efficiency changelog, to be picked up by next efficiency background job
  4. Background efficiency job schedule did NOT yet trigger
  5. Snapshot snap1 is created
  6. File or Block written in 1. is deleted from AFS and so remains trapped in a Snapshot snap1 only
  7. Background efficiency schedule triggers
  8. File or Block written in 1 remains in WAFL (snap1) without deduplication applied until Snapshot snap1 is deleted
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