Arch Linux

noctavian

Member

Registered: 2013-07-11

Posts: 21

Can I ignore remounting errors for Btrfs snapshots + overlay

I'm using Btrfs snapshots with Snapper (for /, /home, and my data directory) and grub-btrfs as part of my back-up strategy. It worked pretty well when I need to restore my system (/) due to issues. The biggest shortcoming with my setup was that I couldn't boot into a snapshot and log into GNOME to check that everything worked before restoring the snapshot because various GNOME services need write access and the snapshot is read-only.

To fix this, a couple of weeks ago, I updated the configuration so that an overlay filesystem would be mounted on top of a snapshot when booting into it (added sd-volatile to the HOOKS array in /etc/mkinitcpio.conf, added GRUB_BTRFS_SNAPSHOT_KERNEL_PARAMETERS="systemd.volatile=overlay" to /etc/default/grub-btrfs/config). That fixed the issue with logging into GNOME and it also unlocked the possibility of trying out various things in a snapshot sandbox like upgrading/downgrading packages or configuration tweaks. This configuration update seems to work ok, except that when booting into a snapshot with overlay I'm getting an error from systemd-remount-fs.service:

Oct 28 13:33:17 takina systemd[1]: Starting Remount Root and Kernel File Systems...
Oct 28 13:33:17 takina systemd-remount-fs[703]: mount: /: fsconfig() failed: overlay: No changes allowed in reconfigure.
Oct 28 13:33:17 takina systemd-remount-fs[703]:        dmesg(1) may have more information after failed mount system call.
Oct 28 13:33:17 takina systemd-remount-fs[702]: /usr/bin/mount for / exited with exit status 32.
Oct 28 13:33:17 takina systemd[1]: systemd-remount-fs.service: Main process exited, code=exited, status=1/FAILURE
Oct 28 13:33:17 takina systemd[1]: systemd-remount-fs.service: Failed with result 'exit-code'.
Oct 28 13:33:17 takina systemd[1]: Failed to start Remount Root and Kernel File Systems.

It's not clear to me if this is an actual issue given my use case (boot into a read-only Btrfs snapshot with overlay on top to check stuff) and I need to fix it, or if I can safely ignore it. I'm leaning towards ignoring it since I can't see any obvious problems, but I want to make sure that I'm not making a mistake since the back-up has to be reliable.

topcat01

Member

Registered: 2019-09-17

Posts: 355

Re: Can I ignore remounting errors for Btrfs snapshots + overlay

overlay does not support the reconfigure operation, so the remount will not work. This is expected. When using overlay the error is benign, and everything is ro anyway. However, you might consider moving all the mount options for / (e.g. subvol, compress, etc.) to the kernel commandline and comment out the fstab line for /. This will eliminate the error.