Is it safe to kill mdadm --wait-clean ?

[mrubioroy]

I see there is a /lib/systemd/system-shutdown/mdadm.shutdown file that executes mdadm --wait-clean --scan right before actual shutdown.

When using nut-server, a /lib/systemd/system-shutdown/nutshutdown file exists that waits for a few seconds (ups.delay.shutdown=20 seconds by default on my system) before asking the UPS to cut power in case a forced shutdown is underway.

My question is: if the md array had an operation going on, is it safe to kill it? systemd will wait for all processes to finish before the actual shutdown, but nut could potentially bypass it by commanding the UPS to cut power. mdadm could still be running at that point?

Debian 13, systemd 257.8, mdadm 4.4, nut-server 2.8.1

[mtkaczyk]

Hi @mrubioroy
It is depending on the raid level you are using. In general it will cause resync to happen so nothing serious (if you have not enabled write-ident bitmap to avoid it).

With raid456 it may cause dirty-degraded problem, known as "Raid Write Hole" if after next boot one of the raid members will not appear. Please familiarize yourself with RWH problem if you have this risk. There are journaling drive or PPL features that can be used to avoid it.

[mrubioroy]

I see. Because the whole goal of using a UPS is to avoid unclean shutdowns, wouldn't it be better to have a systemd service on final.target so that whenever we get to systemd-shutdown, mdadm is done and actual shutdown work can be done ?

final.target: A special target unit that is used during the shutdown logic and may be used to pull in late services after all normal services are already terminated and all mounts unmounted.

It was added to systemd 258 but from what I've seen it has been backported to many current distributions.

Could a service like this do the job?:

[Unit]
Description=Finalize MD RAID after unmounts (late shutdown)
DefaultDependencies=no
Wants=final.target
After=final.target
Before=systemd-poweroff.service systemd-reboot.service systemd-halt.service

[Service]
Type=oneshot
ExecStart=/usr/sbin/mdadm --wait-clean --scan

[Install]
WantedBy=final.target

[mtkaczyk]

As you said it is pretty fresh, mdadm must work in many, many environments, so we would keep both ways and after 3-5 years we can drop the legacy way. I'm not against it but the changes here takes time :)

[mtkaczyk]

oh, about the service proposed, I don't know. If it is working in your env then it gives us some confidence but it still requires wide testing.

[mrubioroy]

Not tested yet. I’ll let you know whenever I do.