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Storage → ZFS Scrubs allows you to schedule and manage scrubs on a ZFS volume. Performing a ZFS scrub on a regular basis helps to identify data integrity problems, detects silent data corruptions caused by transient hardware issues, and provides early alerts to disk failures. If you have consumer-quality drives, consider a weekly scrubbing schedule. If you have datacenter-quality drives, consider a monthly scrubbing schedule.
Depending upon the amount of data, a scrub can take a long time. Scrubs are I/O intensive and can negatively impact performance. They should be scheduled for evenings or weekends to minimize the impact to users.
A ZFS scrub only checks used disk space. To check unused disk space, schedule a S.M.A.R.T. Test Type of Long Self-Test to run once or twice a month.
When you create a volume that is formatted with ZFS, a ZFS scrub is automatically scheduled for you. An entry of the same volume name is added to Storage → ZFS Scrubs and a summary of this entry can be viewed in Storage → ZFS Scrubs → View ZFS Scrubs. Figure 6.4a displays the default Edit settings for the volume named volume1. Table 6.4a summarizes the options in this screen.
Figure 6.4a: Viewing a Volume's Default Scrub Settings
Table 6.4a: ZFS Scrub Options
|Volume||drop-down menu||select ZFS volume to scrub|
|Threshold days||integer||number of days since the last scrub completed before the next scrub can occur, regardless of the calendar schedule; the default is a multiple of 7 which should ensure that the scrub always occurs on the same day of the week|
|Minute||slider or minute selections||if use the slider, scrub occurs every N minutes; if use minute selections, scrub starts at the highlighted minutes|
|Hour||slider or hour selections||if use the slider, scrub occurs every N hours; if use hour selections, scrub occurs at the highlighted hours|
|Day of Month||slider or month selections||if use the slider, scrub occurs every N days; if use month selections, scrub occurs on the highlighted days of the selected months|
|Month||checkboxes||scrub occurs on the selected months|
|Day of week||checkboxes||scrub occurs on the selected days; default is Sunday to least impact users|
|Enabled||checkbox||uncheck to disable the scheduled scrub without deleting it|
You should review the default selections and, if necessary, modify them to meet the needs of your environment.
While a delete button is provided, deleting a scrub is not recommended as a scrub provides an early indication of disk issues that could lead to a disk failure. If you find that a scrub is too intensive for your hardware, consider disabling the scrub as a temporary measure until the hardware can be upgraded.
If you do delete a scrub, you can create a new scrub task by clicking Storage → Volumes → ZFS Scrubs → Add ZFS Scrub.