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How snapshot copy retention interval affects capacity usage in ONTAP？

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Applies to

ONTAP 9

Answer

  • Snapshot capacity usage fundamentals in ONTAP
    • ONTAP Snapshot copies are point in time (PIT) images.
    • Snapshot creation does not copy active data blocks.
    • Snapshot copies store metadata references to existing blocks.
    • When data changes after a Snapshot copy is created:
      • Pre change blocks are preserved for Snapshot access.
      • New data is written to new blocks by WAFL.
    • A data block remains allocated while referenced by one or more Snapshot copies.
    • Note: Snapshot capacity usage depends on the amount of changed data since the previous Snapshot copy, not on the total dataset size.
  • Capacity comparison: hourly versus daily Snapshot copies
    • Example assumptions
      • Data change rate: 100 MB per hour
      • Total daily change: 2.4 GB
      • Each data change overwrites unique blocks
    • Hourly Snapshot copies (24 hour retention)
      • One Snapshot copy created per hour
      • Each Snapshot copy references approximately 100 MB of changed blocks
      • 24 Snapshot copies retained
      • Approximate Snapshot capacity usage:100 MB × 24 ≈ 2.4 GB
    • Daily Snapshot copies (1 day retention)
      • One Snapshot copy created per day
      • Snapshot copy references all changes from the previous 24 hour
      • Approximate Snapshot capacity usage:≈ 2.4 GB
    • Result:When block changes do not overlap, hourly and daily Snapshot copies consume approximately the same capacity.
  • Why Snapshot capacity usage differs in real environments
    • Snapshot generation count
      • Hourly schedules retain more Snapshot generations.
      • More generations extend the lifetime of referenced blocks.
      • Extended block retention increases Snapshot space usage.
    • Overwrite heavy workloads
      • Frequent overwrites cause multiple versions of the same blocks to be retained.
      • Shorter Snapshot intervals retain more historical block versions.
    • Result:Longer Snapshot intervals typically consume less Snapshot capacity for overwrite heavy workloads.
  • Impact of long retention with frequent Snapshot copies
    • Example:
      • Hourly Snapshot copies retained for 14 days
      • Total Snapshot generations:24 × 14 = 336
    • Impact:
      • Each Snapshot copy can reference unique or repeatedly changed blocks.
      • Blocks cannot be freed until the referencing Snapshot copies expire.
      • Snapshot capacity usage increases as:
        • Snapshot frequency increases
        • Retention duration increases
  • Effect of increasing the Snapshot interval
    • Increasing the Snapshot interval (for example, from hourly to every 2 or 3 hours) can:
      • Reduce the total number of Snapshot copies
      • Shorten the reference lifetime of old blocks
      • Reduce Snapshot capacity usage
    • Note: Actual capacity savings vary based on workload type and data change patterns.

Additional Information

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