ZFS Scrubs

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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

Setting Value Description
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
Description string optional
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.

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