Why do I get the error "failed unmounting /var" during shutdown?

Question

I have a fresh install of Debian 9.0.0. It has a separate 1 GB partition for /var formatted at ext4. When I shutdown from the terminal, using shutdown -h now, there is a bright-red error message stating "failed unmounting /var".

What is the fix for this error?

Solution

The problem is journald

Or rather how it is still logging to /var while systemd is trying to unmount it.

Solutions

According to this thread, there are two ways to go about it:

1. Make journald log to a volatile location in /run so it doesn't lock /var, but the tradeoff is that you lose logs at shutdown.

Edit /etc/systemd/journald.conf to change the Storage= line to

Storage=volatile

This is what I did, and I it worked for me.

2. (EDIT: this is ArchLinux-specific) Or set up a shutdown hook to unmount /var later. This won't remove the failed message as systemd will still try to unmount /var but will indeed unmount /var properly at the end of the shutdown sequence. You just won't see it. (I haven't tested that solution though)

Add a shutdown hook to mkinitcpio's HOOKS= array, e.g. :

HOOKS="base udev autodetect block keymap encrypt lvm2 filesystems usbinput fsck var shutdown"

Comments

[Marcelo Ventura]

I have two laptops with Ubuntu (one with a single HDD and one with HDD+SSD) and I got that annoying error message only on the second. In the second laptop, /var is in a different partition from / (/ is in the SSD and /var is in the HDD) and, in the first laptop, /var is within /. This might play a role in the problem.

[Marcelo Ventura]

Indeed: bbs.archlinux.org/viewtopic.php?pid=1204644#p1204644

[JanPropelforwards]

A variant of topic (1) is to leave Storage=auto in journald.conf and just remove /var/log/journal. This worked for me, too, and journald logs are done to /run/log/journal/, instead. See man journald.conf.

[telcoM]

mkinitcpio is specific to Arch, so your option 2) needs to be implemented in some other way in Debian.

[Thibault Lemaire]

@telcoM Okay thanks. I don't use Arch anymore, but I've updated the answer.