[SOLVED] Handling of /etc/fstab

[FGD-Garuda]

Hello!

This tool is really cool, makes my grub menu always updated after my snapshots so I should be able to easily boot from them. I guess that's the idea.

I noticed grub-btrfs does not modify the /etc/fstab file from within the snapshot. This is my fstab for one of my SSD within one snapshot, it still uses the original subvolume references it got snapshoted from:

LABEL=SSD1 / btrfs defaults,ssd,noatime,subvol=@ 0 1
LABEL=SSD1 /home btrfs defaults,ssd,noatime,subvol=@home 0 2
LABEL=SSD1 /.snapshots btrfs defaults,ssd,noatime,subvol=@snapshots 0 2
LABEL=SSD1 /var/cache btrfs defaults,ssd,noatime,subvol=@varcache 0 2
LABEL=SSD1 /tmp btrfs defaults,ssd,noatime,subvol=@tmp 0 2

If I'm booting from a snapshot of "@" (let's say snapshot is called "snapshot_@20201219") that has a different subvolume name than "@", it has to be addressed here, otherwise "@" will be mounted as root (/) but I need the snapshot to be mounted as root.
Before I try booting from one of the snapshot I want to make sure subvolume "@" will not be mounted to "/", but instead it would be "snapshot@_20201219".

What should I be expecting?

On the same type of question, how does grub-btrfs know which /home snapshot to mount alongside root?
I have snapshots of both @ and @home, done at the same time. So in /etc/fstab the snapshots names should replace "@" and "@home".

Am I trying to use grub-btrfs for the right purpose?

Thanks!

[Kr1ss-XD]

The fstab entry for the / mountpoint doesn't have any effect, you could also remove it from the file.
/ is mounted during the boot process, at a point before the kernel even has access to fstab (that file can't be read before the subvolume where it's located is mounted). Which partition/subvolume to mount as / is defined in the kernel boot parameters, grub-btrfs takes care of this for snapshots. See the entries starting with linux inside grub-btrfs.cfg in your boot partition.

As for /home - since you are doing snapshots anyways at the same time when / is snapshotted, maybe consider not having a @home subvolume at all, so that your snapshots automatically include the home tree ?

Yes, I guess you are trying to use the script for the right purpose, I'm just not sure if there could be any way how grub-btrfs could guess which /home snapshot to mount in combination with a certain / snapshot, since we can't assume contemporaneous snapshots on every system it runs on (and not even a certain subvolume layout, by the way).

[FGD-Garuda]

You're absolutely right about fstab, didn't think about that timing situation during boot, it's darn obvious when you think about it! :)

And yes that's also what I thought about @home, maybe that's what most btrfs people do anyway, I started only a year ago on this file system and was using ext4 and xfs before. Will never go back, though. I guess I tried to replicate the file system structure but in a way it doesn't make sense as I have to maintain 2 snapshots every time with different names and I don't take advantage of whatever advantages there is to segregate both @ and @home since I always keep the pair together, snapshot them together and mount/boot them together. They act as just one big subvolume, actually.

Since having my snapshots in a grub submenu has very important uses to me, I will modify my btrfs tree and then all will be good. :) Your tool will save me a lot of time and provide me more fun!

Thanks for the very quick reply, man!
Keep up the good work and Happy Holidays!

[Kr1ss-XD]

😄 No, I'm not the author, I'm just a user of this script. Nevertheless I'm glad if this was helpful. Happy holidays to you !