systemd: add weekly and monthly scrub timers

Timers can be enabled as follows:

systemctl enable zfs-scrub-weekly@rpool.timer --now
systemctl enable zfs-scrub-monthly@datapool.timer --now

Each timer will pull in zfs-scrub@${poolname}.service, which is not
schedule-specific.

Added PERIODIC SCRUB section to zpool-scrub.8.

Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Georgy Yakovlev <gyakovlev@gentoo.org>
Closes #12193

etc/systemd/system/.gitignore

@@ -1,3 +1,4 @@
 *.service
 *.target
 *.preset
+*.timer

etc/systemd/system/Makefile.am

@@ -12,7 +12,10 @@ systemdunit_DATA = \
 	zfs-volume-wait.service \
 	zfs-import.target \
 	zfs-volumes.target \
-	zfs.target
+	zfs.target \
+	zfs-scrub-monthly@.timer \
+	zfs-scrub-weekly@.timer \
+	zfs-scrub@.service
 
 SUBSTFILES += $(systemdpreset_DATA) $(systemdunit_DATA)
 

etc/systemd/system/zfs-scrub-monthly@.timer.in

@@ -0,0 +1,12 @@
+[Unit]
+Description=Monthly zpool scrub timer for %i
+Documentation=man:zpool-scrub(8)
+
+[Timer]
+OnCalendar=monthly
+Persistent=true
+RandomizedDelaySec=1h
+Unit=zfs-scrub@%i.service
+
+[Install]
+WantedBy=timers.target

etc/systemd/system/zfs-scrub-weekly@.timer.in

@@ -0,0 +1,12 @@
+[Unit]
+Description=Weekly zpool scrub timer for %i
+Documentation=man:zpool-scrub(8)
+
+[Timer]
+OnCalendar=weekly
+Persistent=true
+RandomizedDelaySec=1h
+Unit=zfs-scrub@%i.service
+
+[Install]
+WantedBy=timers.target

etc/systemd/system/zfs-scrub@.service.in

@@ -0,0 +1,14 @@
+[Unit]
+Description=zpool scrub on %i
+Documentation=man:zpool-scrub(8)
+Requires=zfs.target
+After=zfs.target
+ConditionACPower=true
+ConditionPathIsDirectory=/sys/module/zfs
+
+[Service]
+ExecStart=/bin/sh -c '\
+if @sbindir@/zpool status %i | grep "scrub in progress"; then\
+exec @sbindir@/zpool wait -t scrub %i;\
+else exec @sbindir@/zpool scrub -w %i; fi'
+ExecStop=-/bin/sh -c '@sbindir@/zpool scrub -p %i 2>/dev/null || true'