Is it ok if the OS fails to unmount a directory during shutdown?

I installed the GNU Guix package manager and have a rather weird mounting layout. Here’s my fstab:

/dev/sda1		/boot		vfat		defaults,noatime			0 2
/dev/sda2		/		btrfs		compress=zstd,noatime,ssd,subvol=@	0 1
/dev/sda2		/home		btrfs		compress=zstd,noatime,ssd,subvol=@home	0 1

/dev/sdb10		/home/user/hdd	btrfs	compress=zstd,noatime			0 1
/dev/sdb6		/home/user/mount	btrfs	compress=zstd,noatime			0 0
/home/user/mount/atvar	/var		none	defaults,bind				0 0
/home/user/mount/attemp 	/tmp		none 	defaults,bind				0 0

/home/user/mount/guixs/gnu	/gnu	none		defaults,bind				0 0
UUID=eefb060f-e70b-4024-aff6-e91b064bbc4c   none    swap    sw  0 0

Basically Guix normally creates a read-only mount on /gnu/store. I have a directory mounted on my hdd (/dev/sdb6), and have /gnu bind mounted to that directory. Here’s
the output of mount | grep gnu:

/dev/sdb6 on /gnu type btrfs (rw,noatime,compress=zstd:3,space_cache,subvolid=5,subvol=/)
/dev/sdb6 on /gnu/store type btrfs (ro,noatime,compress=zstd:3,space_cache,subvolid=5,subvol=/)
/dev/sdb6 on /home/user/mount/guixs/gnu/store type btrfs (rw,noatime,compress=zstd:3,space_cache,subvolid=5,subvol=/)

I have to be honest that output kind of scares me. Is the OS redundantly mounting the same thing three times?

Also everytime I shutdown systemd throws an error saying it couldn’t unmount /gnu/store, this doesn’t cause any problems but I was wondering if it could cause any hardware related issues?

I do not think that mounting something to several mount points can cause you problems.

I am no expert on btrfs but what makes me wonder is this mounting to /gnu and /gnu/store.
You should check in which order you unmount the drives as well. When you unmount /gnu before /gnu/store that may throw the error.
Since the mount is read-only I do not expect you should loose any data. But for me this looks like undefined behaviour.

Which logs have that information?

I do not think it is in a log since you cannot log it on the drive when you unmount it. :thinking:
I is written on the screen during shutdown so if you can read very fast you can catch it.

edit: I just checked my journalctl and it is written there just before last -- Boot <some-id> -- marker. There is whole shutdown output.
How does it write to unmouted drive? That looks like witchcraft to me. :rofl: