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.Dd July 25, 2021
.Dt ZPOOL-SCRUB 8
.Os
.
.Sh NAME
.Nm zpool-scrub
.Nd begin or resume scrub of ZFS storage pools
.Sh SYNOPSIS
.Nm zpool
.Cm scrub
.Op Fl s Ns | Ns Fl p
.Op Fl w
.Ar pool Ns …
.
.Sh DESCRIPTION
Begins a scrub or resumes a paused scrub.
The scrub examines all data in the specified pools to verify that it checksums
correctly.
For replicated
.Pq mirror, raidz, or draid
devices, ZFS automatically repairs any damage discovered during the scrub.
The
.Nm zpool Cm status
command reports the progress of the scrub and summarizes the results of the
scrub upon completion.
.Pp
Scrubbing and resilvering are very similar operations.
The difference is that resilvering only examines data that ZFS knows to be out
of date
.Po
for example, when attaching a new device to a mirror or replacing an existing
device
.Pc ,
whereas scrubbing examines all data to discover silent errors due to hardware
faults or disk failure.
.Pp
Because scrubbing and resilvering are I/O-intensive operations, ZFS only allows
one at a time.
.Pp
A scrub is split into two parts: metadata scanning and block scrubbing.
The metadata scanning sorts blocks into large sequential ranges which can then
be read much more efficiently from disk when issuing the scrub I/O.
.Pp
If a scrub is paused, the
.Nm zpool Cm scrub
resumes it.
If a resilver is in progress, ZFS does not allow a scrub to be started until the
resilver completes.
.Pp
Note that, due to changes in pool data on a live system, it is possible for
scrubs to progress slightly beyond 100% completion.
During this period, no completion time estimate will be provided.
.
.Sh OPTIONS
.Bl -tag -width "-s"
.It Fl s
Stop scrubbing.
.It Fl p
Pause scrubbing.
Scrub pause state and progress are periodically synced to disk.
If the system is restarted or pool is exported during a paused scrub,
even after import, scrub will remain paused until it is resumed.
Once resumed the scrub will pick up from the place where it was last
checkpointed to disk.
To resume a paused scrub issue
.Nm zpool Cm scrub
again.
.It Fl w
Wait until scrub has completed before returning.
.El
.Sh EXAMPLES
.Bl -tag -width "Exam"
.It Sy Example 1 : Status of pool with ongoing scrub:
Output:
.Bd -literal -compact -offset Ds
.No # Nm zpool Cm status
  ...
  scan: scrub in progress since Sun Jul 25 16:07:49 2021
        403M scanned at 100M/s, 68.4M issued at 10.0M/s, 405M total
        0B repaired, 16.91% done, 00:00:04 to go
  ...
.Ed
Where:
.Bl -dash -offset indent
.It
Metadata which references 403M of file data has been
scanned at 100M/s, and 68.4M of that file data has been
scrubbed sequentially at 10.0M/s.
.El
.El
.Sh PERIODIC SCRUB
On machines using systemd, scrub timers can be enabled on per-pool basis.
.Nm weekly
and
.Nm monthly
timer units are provided.
.Bl -tag -width Ds
.It Xo
.Xc
.Nm systemctl
.Cm enable
.Cm zfs-scrub-\fIweekly\fB@\fIrpool\fB.timer
.Cm --now
.It Xo
.Xc
.Nm systemctl
.Cm enable
.Cm zfs-scrub-\fImonthly\fB@\fIotherpool\fB.timer
.Cm --now
.El
.
.Sh SEE ALSO
.Xr systemd.timer 5 ,
.Xr zpool-iostat 8 ,
.Xr zpool-resilver 8 ,
.Xr zpool-status 8