Several clients are accessing an ONTAP SnapMirror DP destination volume. Some clients frequently access the newest SnapMirror volume dat a. Accessing…

When data is transferred to a SnapMirror destination volume, it is initially written in a non-optimized format to accelerate the transfer speed. After the transfer completes, a background WAFL (Write Anywhere File Layout) scanner process called "volume deswizzling" runs on the destination. This process reorganizes the newly written data blocks into a read-optimized layout. If clients access this new data before or during the deswizzling process, they will experience significantly slower read performance. The wafl scan status command would show the "deswizzle" scanner as active, which is the primary symptom of this performance issue.

A. blocks used summary update: This scanner updates volume space metadata. While it is a background process, it does not directly cause slow read performance for newly transferred data blocks.

B. active bitmap rearrangement: This process optimizes the file system's active block map for better write performance and space management, not the read layout of new data.

C. container block reclamation: This is a space-saving process related to aggregate-level storage efficiency (FabricPool) and is not the cause of slow reads on a specific SnapMirror destination volume.

1. NetApp Technical Report TR-4015: SnapMirror Configuration and Best Practices Guide for ONTAP 9.8

Section 4.3

"Post-transfer operations": This section explains that after a SnapMirror transfer

background operations run on the destination to optimize the data layout for subsequent reads. It describes the deswizzling process as essential for read performance on the destination volume.

2. NetApp ONTAP 9 Commands: Manual Page Reference

volume snapshot compute-checksum command page: In the description

it notes that the command might run slowly if the "deswizzle" background scanner is active on a SnapMirror destination volume

directly linking this scanner to performance.

wafl scan status command page: This page documents the command used to view the status of background WAFL scanners

including "deswizzle

" confirming it is the correct tool to identify the process.

3. NetApp Knowledge Base Article 1030086: "What is the deswizzling process and how does it affect SnapMirror performance?"

This article explicitly states: "The deswizzling process is a background WAFL scanner that runs on SnapMirror destination volumes after a transfer... Reading from a SnapMirror destination volume while the deswizzling process is running can result in poor read performance." This directly supports the answer.