Several clients are accessing an ONTAP SnapMirror DP destination volume. Some clients frequently access the newest SnapMirror volume data. Accessing this data is much slower when compared to clients that directly access data on the source. You use the wafl scan status command against the destination.
What is a symptom of the problem in this scenario?
Volume deswizzling is the correct answer. In SnapMirror DP destination volumes, deswizzling is the process where the system maps aggregate (physical) block locations to the volume block location because the SnapMirror transfer does not include the physical disk block location. If deswizzling has not updated a block yet, accessing that block from a client can result in a slight read latency increase and extra overhead, compared to accessing blocks that are already deswizzled or directly from the source. This causes the observed slower access times for clients frequently accessing new SnapMirror volume data.
From reading all the possible answers only D: volume deswizzling made sense. 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. Answer D:
https://kb.netapp.com/on-prem/ontap/DP/SnapMirror/SnapMirror-KBs/What_is_deswizzler_or_deswizzling
deswizzling: On Data ONTAP 7-mode and ONTAP 9 DP style SnapMirror destination volumes, this scan maps aggregate (physical) block locations to the volume block location, as the transfer does not include the physical disk block location.
https://community.netapp.com/t5/ONTAP-Discussions/Disks-always-busy-WAFL-scans-neverending/td-p/106978