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What is ownblocks_calc scanner and how does it work?

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ontap-9
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core
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Applies to

  • Data ONTAP 8
  • ONTAP 9

Answer

  • The snapshot is safely written to disk on the first consistency point after a snapshot command is issued.
  • The size etc are computed after the face by a slow ownblocks_calc scan that is deliberately throttled.
  • We run the ownblocks_calc when a snap is created, or blocks used completes (kicked off after a snap delete) or if a list command is executed.
  • Further, since we only invalidate the values when blks used runs, it is possible that the values become quite stale until blks used finishes.
  • And blks used can get restarted every time a new delete occurs so it might take a long time for the ownblocks to get a chance to run & update the values, but there is no data integrity issue as far as the snapshot itself is concerned.
  • Running wafl scan ownblocks_calc from nodeshell forces us to recalculate own blocks values through the scanner even if they are valid/scan has not been kicked off through blocks used which is why you see updated values once this is executed.