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Scrub Tasks

  5 minute read.

Last Modified 2022-08-25 16:43 EDT

When TrueNAS performs a scrub, 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

TrueNAS generates a default scrub task when you create a new pool and sets it to run every Sunday at 12:00 AM.

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Adjust Scrub/Resilver Priority

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Resilver Priority

NameDescription
EnabledSet to run resilver tasks between the configured times.
BeginChoose the hour and minute when a resilver process can run at a higher priority.
EndChoose the hour and minute after which a resilver process must return to running at a lower priority. A resilver process running after this time will likely take much longer to complete due to running at a lower priority compared to other disk and CPU activities, such as replications, SMB transfers, NFS transfers, Rsync transfers, S.M.A.R.T. tests, pool scrubs, user activity, etc.
Days of the WeekSelect the days to run resilver tasks.

Creating New Scrub Tasks

TrueNAS needs at least one data pool to create scrub task.

To create a scrub task for a pool, go to Data Protection and click ADD in the Scrub Tasks window.

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Scrub Task

NameDescription
PoolChoose a pool to scrub.
Threshold daysControls the task schedule by setting how many days must pass before a completed scrub can run again. If you schedule a scrub to run daily and set Threshold days to 7, the scrub attempts to run daily. If the scrub succeeds, it will check but won’t run again until seven days pass. Using a multiple of seven ensures the scrub runs 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.

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Choosing a Presets option populatess 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.

You can set specific time ranges 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.

You can also enter lists of values. 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, entering * in Days runs the task every day of the month. Entering */2 runs it every other day.

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

TrueNAS has 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 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 dipslays when the current settings mean the task runs.

Examples of CRON syntax

SyntaxMeaningExamples
*Every item.* (minutes) = every minute of the hour.
* (days) = every day.
*/NEvery Nth item.*/15 (minutes) = every 15th minute of the 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.

You can specify days of the month or days of the week.

TrueNAS lets users create flexible schedules using the available options. The table below has some 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/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.

Editing Scrub Tasks

To edit a scrub, go to Data Protection and click the scrub task you want to edit.

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