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What is deswizzler or deswizzling

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Applies to

  • Data ONTAP 7
  • Data ONTAP 8
  • ONTAP 9

Answer

Note: Deswizzling, deswizzle, and deswizzler are used interchanglably in this article, but all refer to the same type of wafl scan.

  • On Data ONTAP 7-mode and ONTAP 9 DP style SnapMirror destination volumes, this scan maps aggregate (physical) block locations to the volume (logical) block location, as the transfer does not include the physical disk block location.
    • Example:
      • In physical house addresses, you have a "physical" location, like 123 N. Main St, and a "logical" location, like Sunnyvale, California.
      • These two properties are tied together with a postal code.
      • It is possible for the postal service to reference together the "physical" and "logical" address parts by the postal code.
    • Note: Volume move uses the type-DP SnapMirror engine, so the destination volume will have a deswizzler scan.
How does this affect performance?
  • Deswizzling scans are minimal overhead on CPU and disk singularly.
    • Multiple scans might cause CPU or disk contention, but this is rare.
    • If a block is read from a user/host and is swizzled (deswizzle scan has not updated this block yet), there is a slight read latency increase and extra overhead.
  • The read does a slow path.
    • In systems with SSDs this is minimal, but with HDDs, it may add a few milliseconds of latency per operation.
Why is there always a deswizzling scanner running?
  • If the volume is a Data ONTAP 7-mode SnapMirror or DP type SnapMirror destination, each completed transfer induces a new snapshot needing a scan.
    • Snapshots may be cleared out to let the scan run on the Active File System (AFS).
  • Deswizzling scans may never finish due to snapshot creation restarting the scanner with the most recent to least recent snapshot.
    • If you are in this situation, please schedule a window to stop all volume snapshots and SnapMirror type jobs.
  • For more information, see Deswizzler runs constantly on a SnapMirror Destination Volume.
Does deswizzler run against SnapMirror type-XDP relationship destination volumes?
  • No. Deswizzler does not run against SnapMirror type-XDP destination volumes in ONTAP 9 and later.
Can deswizzling be aborted?
  • No. This scan is essential for performance should a SnapMirror destination be needed as a Disaster Recovery site being activated.
Does deswizzling take place on offline volumes?
  • No. Starting in Data ONTAP 7.2.4, deswizzler will restart at a checkpoint instead of restarting from the beginning when a block level update is completed in the middle of a deswizzling scan.
  • For more information, see BUG 229357. 
What if a deswizzler is running on a volume that is not a dedicated SnapMirror destination volume?
  • At some point in the volume's history, this volume was a SnapMirror destination, or a volume move destination.
    • This could go back many months or even years without observation.
    • As mentioned above, a window of no snapshots on this volume is needed to let the scan finish.
  • Another possibility is a cascade SnapMirror, where the middle node of a tertiary SnapMirror trio is the source for the end destination.
Can the deswizzling scan be disabled?
  • Yes. However, if any blocks are swizzled, there is a slow-path read penality, which on HDD systems may add a few milliseconds of read latency.
  • The command is options wafl.deswizzle.enable off  at the node shell or 7-mode shell.
    • For example, here is an example from a ONTAP 9 system:

rtp1_rr::> node run rtp1_rr-01 "options wafl.deswizzle.enable off"
rtp1_rr::> node run rtp1_rr-01 "options wafl.deswizzle.enable"   

wafl.deswizzle.enable        off       

How do I check if deswizzler is running?

Administrators might check to see if the scanner is running with the wafl scan status command:

filer> priv set advanced
filer*> wafl scan status

Volume volume1:
 Scan id                   Type of scan     progress
  226328    container block reclamation     block 1871 of 24095 (fbn 11220)
  226329             volume deswizzling     snap 157, inode 2144843 of 2172681. level 1 of normal files. Totals: Normal files: L1:10/4881 L2:0/11336 L3:0/5723 L4:0/0	Inode file: L0:0/0 L1:0/0 L2:0/0 L3:0/0 L4:0/0

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