How Data ONTAP uses continuous media scrubbing to prevent media errors

The purpose of the continuous media scrub is to detect and correct media errors in order to minimize the chance of storage system disruption due to a media error while a storage system is in degraded or reconstruction mode.

By default, Data ONTAP runs continuous background media scrubbing for media errors on all storage system disks. If a media error is found, Data ONTAP uses RAID to reconstruct the data and repairs the error.

Media scrubbing is a continuous background process. Therefore, you might observe disk LEDs blinking on an apparently idle storage system. You might also observe some CPU activity even when no user workload is present.

Note: You can disable continuous media scrubbing for disks in use in aggregates by using the raid.media_scrub.enable option. In addition, you can disable continuous media scrubbing for spare disks by using the raid.media_scrub.spares.enable option. However, you are advised not to disable continuous media scrubbing, especially for ATA disks and disks used in RAID4 aggregates.

For more information about the raid.media_scrub options, see the na_options(1) man page.

How continuous media scrub impacts system performance
Because continuous media scrubbing searches only for media errors, the impact on system performance is negligible. In addition, the media scrub attempts to exploit idle disk bandwidth and free CPU cycles to make faster progress. However, any client workload results in aggressive throttling of the media scrub resource.
Why continuous media scrubbing should not replace scheduled RAID-level disk scrubs
Because the continuous media scrub process scrubs only media errors, you should continue to run the storage system’s scheduled complete RAID-level scrub operation. The RAID-level scrub finds and corrects parity and checksum errors as well as media errors.

Parent topic: How Data ONTAP monitors disk performance and health