booting RO snapshots on EndeavourOS systems with Dracut instead of mkinitcpio?

[jfabernathy]

Since Endeavour OS switched to Dracut, I can't boot Read-only snapshots because I don't know how to configure the solution explained in one of the Read.md files. I had this working back when mkinitcpio was used.

I need some help as the explanation for Dracut systems doesn't make sense to me. Not sure where to modify the kernel parameters, or where to set the variable GRUB_BTRFS_SNAPSHOT_KERNEL_PARAMETERS

[Schievel1]

First off all I guess you need to tell dracut first, that you need to add the dracut modules for overlayfs. You can search in /usr/lib/dracut/modules.d for the overlayfs module, on gentoo it's called 90overlayfs.
Add it to dracut using this: https://wiki.gentoo.org/wiki/Dracut#Adding_modules
Then you need to regenerate the initramfs of the kernel you want to boot the readonly snapshot with later. To regenerate every initramfs use sudo dracut --regenerate-all -f -H

I would suggest you first try passing this in Grub. So in Grub you go to the snapshot you want to boot, press e to edit the grub menu, navigate to where it says something like linux /boot/vmlinuz-linux-6.2.11something root=UUID=somelongnonsese rw quiet splash and add the rd.live.overlay.overlayfs=1 (rd.live.overlay.readonly=1) to that line.
Then press F10 to boot.

However this is not permanent, it's only valid for this boot. And the nextime you boot you would have to type this in manually again. This is were the variable GRUB_BTRFS_SNAPSHOT_KERNEL_PARAMETERS in /etc/grub.d/41_snapshots-btrfs comes into play. This is simply a string that gets appended to every kernel command line of every snapshot by grub-btrfs. Set your read only argument there and regenerate your Grub menu.

With all that said, I haven't got this done myself. I always wanted to test this out but I haven't found the time yet.

[jfabernathy]

Thanks for the information. The version of grub-btrfs that is on the Archlinux repo is not up to date as far as I can tell. I built grub-btrfs from source on one system and it's man grub-btrfs showed the GRUB_BTRFS_SNAPSHOT_KERNEL_PARAMETERS variable, but it didn't on my VM under test which was loaded from the repo.

So I going to start over with a fresh VM and install grub-btrfs from the github version.

I did confirm that the GRUB_BTRFS_SNAPSHOT_KERNEL_PARAMETERS variable get added to the /etc/default/grub-btrfs/config file before sudo /etc/grub.d/41_snapshot-btrfs script gets run. The current readme implies this is all you have to do.

I'll test from scratch again.

[Schievel1]

Yes they added this just recently, I think in dracut 58. See dracutdevs/dracut#1934

But older dracut still supports overlayfs, ist just that this is in it's own module now. Before this was a part of the dmsquash-live module I think.

I think when you do the regenerate-all thing, skip the -H argument that I suggested. This is kind of a gentooism, it lets dracut build a minimal initramfs that only includes the things the host needs. I think dracut fails to find a overlayfs module than since your current host is not using overlayfs.

It is very strange to me how KDE-plasma on EOS can boot into snapshots without issue and does none of these things.

So, again: you can boot into a overlayfs snapshot and it resets to the original state when rebooted into that snapshot?
All that with an initramfs generated by dracut?

[jfabernathy]

Just checked the package for Archlinux for dracut and it's 056-2 and flagged out of date.

As to EOS KDE: Yes, I can pick a pre snapshot in the list on grub and boot it, login, and test to see the the application is missing as expected. I can also use btrfs-assistant and restore the root and home snapshot to the very snapshot I'm actually booted to. When I reboot to the normal kernel it's running the system without the app I had previously installed.

So it works just as I'd want with KDE and nothing special done to accommodate overlays parameters, etc.

I'll open a question on EOS forum about this strange behavior on KDE that works great.

[Schievel1]

Well but that means dracut works just fine for you. I don't thing the newer version with the overlayfs module will help with Cinnamon.

Regarding that, you have to have some directories in /var as their own subvolume for some desktop environments. Or the whole /var directory.

[jfabernathy]

I do have @cache on /var/cache, @log on /var/log and I think @tmp on /var/tmp

So somebody smart working on KDE must have put in the logic to deal with the RO snapshot without any of the grub-btrfs recommended modification.

I'd rather just stick with KDE but I'm on a 4K UHD display and the software I use for DVR playback has a few issues on Plasma. I'm doing a lot of testing with Distros, DE, and snapper vs. timeshift.

Timeshift works very well with grub-btrfs, but there is no equivalent to snap-pac that properly does a pre-post snapshot that is properly labeled with what the snapshot was for. Just generic pre-post with no description. timeshift-autosnap isn't as good as snap-pac

[Schievel1]

At least for gnome you needed /var/lib/gdm and /var/lib/account service as well. I don't know how this is with cinnamon though.

[jfabernathy]

I have a solution at least for Endeavour OS (Archlinux). Copying some dracut modules from Garuda solve the issue. You don't need any of the grub-btrfs variable or kernel cmd line options. So you can use the pacman repo version (4.12-2) of grub-btrfs instead of the github master version here.

see eos forum
for the details.

[Schievel1]

Thanks I will try this out as well.
Maybe we can include that in grub-btrfs in the future or something. It seems more written for the purpose than the original module of dracut, which is rather thought for live disks.

How does it determine if your want to boot into a overlayfs or boot for you though? When you boot your “real” system and not a snapshot, is it also overlayfs or normal persistent read-write?

[jfabernathy]

I'm too old to offer any read on code unless it's Fortran, but as I see the function mount_snapshot_overlay, it checks to see if the newly mounted root is type btrfs and it's read-only, then they mount a ram overlay. Not sure about all that but clearly they are testing for a BTRFS RO file system being booted.

[Schievel1]

Yes, but your snapshots are not marked ro, are they?

[jfabernathy]

Default setup for snapper is RO snapshots. When I look at btrfs-assistant table of snapshots I see all automatic ones as RO.

[jfabernathy]