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Scrub Tasks

  4 minute read.

Last Modified 2021-03-24 14:35 EDT

A “scrub” is when ZFS scans the data on a pool. Scrubs identify data integrity problems, detect silent data corruptions caused by transient hardware issues, and provide early disk failure alerts.

Default Scrub Tasks

By default, TrueNAS creates a scrub task when you create a new pool. The default schedule for a scrub is to run every Sunday at 12:00 AM. To edit the default scrub, go to Tasks > Scrub Tasks, click , and EDIT.

Creating New Scrub Tasks

A data pool must exist before creating a scrub task.

To create a scrub task for a pool, go to Tasks > Scrub Tasks and click ADD.


Scrub Task

PoolChoose a pool to scrub.
Threshold daysDays before a completed scrub is allowed to run again. This controls the task schedule. For example, scheduling a scrub to run daily and setting Threshold days to 7 means the scrub attempts to run daily. When the scrub is successful, it continues to check daily but does not run again until seven days have elapsed. Using a multiple of seven ensures the scrub always occurs on the same weekday.
DescriptionDescribe the scrub task.
ScheduleHow often to run the scrub task. Choose one of the presets or Custom to use the Advanced Scheduler.
EnabledUnset to disable the scheduled scrub without deleting it.

Tasks Advanced Scheduler

Choosing a preset fills in the rest of the fields. To customize a schedule, enter crontab values for the Minutes/Hours/Days.

These fields accept standard cron values. The simplest option is to enter a single number in the field. The task runs when the time value matches that number. For example, entering 10 means that the job runs when the time is ten minutes past the hour.

An asterisk (*) means “match all values”.

Specific time ranges are set by entering hyphenated number values. For example, entering 30-35 in the Minutes field sets the task to run at minutes 30, 31, 32, 33, 34, and 35.

Lists of values can also be entered. Enter individual values separated by a comma (,). For example, entering 1,14 in the Hours field means the task runs at 1:00 AM (0100) and 2:00 PM (1400).

A slash (/) designates a step value. For example, while entering * in Days means the task runs every day of the month, */2 means the task runs every other day.

Combining all the above examples together creates a schedule running a task each minute from 1:30-1:35 AM and 2:30-2:35 PM every other day.

There is an option to select which Months the task runs. Leaving each month unset is the same as selecting every month.

The Days of Week schedules the task to run on specific days. This is in addition to any listed Days. For example, entering 1 in Days and setting Wed for Days of Week creates a schedule that starts a task on the first day of the month and every Wednesday of the month.

The Schedule Preview shows when the current settings mean the task runs.

Examples of CRON syntax

*Every item.* (minutes) = every minute of the hour.
* (days) = every day.
*/NEvery Nth item.*/15 (minutes) = every 15th minute of the hour (every quarter hour).
*/3 (days) = every 3rd day.
*/3 (months) = every 3rd month.
Comma and hyphen/dashEach stated item (comma)
Each item in a range (hyphen/dash).
1,31 (minutes) = on the 1st and 31st minute of the hour.
1-3,31 (minutes) = on the 1st to 3rd minutes inclusive, and the 31st minute, of the hour.
mon-fri (days) = every Monday to Friday inclusive (every weekday).
mar,jun,sep,dec (months) = every March, June, September, December.

Days can be specified as days of month, or days of week.

With these options, flexible schedules can be created similar to these examples:

Desired scheduleValues to enter
3 times a day (at midnight, 08:00 and 16:00)months=*; days=*; hours=0/8 or 0,8,16; minutes=0
(Meaning: every day of every month, when hours=0/8/16 and minutes=0)
Every Monday, Wednesday and Friday, at 8.30 pmmonths=*; days=mon,wed,fri; hours=20; minutes=30
1st and 15th day of the month, during October to June, at 00:01 ammonths=oct-dec,jan-jun; days=1,15; hours=0; minutes=1
Every 15 minutes during the working week, which is 8am - 7pm (08:00 - 19:00) Monday to FridayNote that this requires two tasks to achieve:
(1) months=*; days=mon-fri; hours=8-18; minutes=*/15
(2) months=*; days=mon-fri; hours=19; minutes=0
We need the second scheduled item, to execute at 19:00, otherwise we would stop at 18:45. Another workaround would be to stop at 18:45 or 19:45 rather than 19:00.