Re: Multiple btrfs root subvolumes?

[Justin Vallon]

On 4/9/21 1:55 PM, Andrei Borzenkov wrote:
> On 09.04.2021 08:02, Justin Vallon wrote:
> > I am wondering whether Grub2 has any btrfs support for multiple root
> > subvolumes.  It appears it might be able to handle writing a grub.cfg
> > that would reboot into the current root partition if the partition is
> > btrfs (though, not sure).
>
> grub does not care about filesystem content at all. It only needs to
> know location of kernel (and possibly initrd) and kernel options. How
> kernel finds its root is entirely up to this kernel.

I was under the impression that "grub" included os-prober.  From the 
point of view of doing the actual boot-loading and loading the kernel, 
sure, btrfs is working.  I am talking about generating the grub.cfg.

I am on Debian (10).

> > The grub-mkconfig code seems confused by btrfs:
> >
> > 1) It assumes a device has a single filesystem
>
> btrfs *is* single filesystem.

It is a single filesystem that can be mounted multiple times (at 
subvolumes).  So, mount shows the same device at multiple mount points.

> > 2) It steps through devices, then maps device to mount point
>
> I am not sure what you mean here, but grub needs to resolve file
> location on btrfs to the absolute path so it can access it during boot.
> No mount points are present during bootloader phase.

Again, talking about os-prober.  It sees /dev/vg/btr, and tries to 
figure out whether the device is a bootable partition.  However, it is 
really device + subvol that is bootable.

> > 3) 30_os-prober has some code for btrfs from os-prober output, but I
> > don't see where os-prober will generate such entries
>
> os-prober is not part of grub. It was mistake to carry it in grub
> sources in the first place.
>
> The code you talk about was added 8 years ago, probably to support some
> os-prober patch available in some distribution at this time. Those
> distributions that attempt to support boot from subvolumes include
> custom, often incompatible, patches to os-prober. There is no real
> upstream for os-prober anyway and for all I can tell it is failed
> attempt to multiboot linux.
>
> > It would be nice if it would be able to generate a grub.cfg that could
> > boot into any subvolume that was bootable:
>
> Define "bootable subvolume".

I can have btrfs subvol=@debian9 subvol=@debian10 subvol=@debian11, each 
with its own kernel, etc.

> > 1) Recognize btrfs filesystem
> > 2) Temp mount subvol=. if not already mounted
> > 3) Run os-detector on each subvol
>
> That is more or less what os-prober in openSUSE does.

I came to grub to ask about why os-prober is broken before I realized 
os-prober is not part of grub.

> > Does this not work?
>
> I am not sure what you are asking. Does it work now? It depends on what
> your distribution provides. Does it work with upstream os-prober? There
> is no real upstream for os-prober, the closest is Debian and as you
> probably have seen this version does not handle subvolumes. Is it
> possible to implement it? Probably, the proof of pudding is in eating.

So, this is really a debian os-prober question.  It appears openSUSE is 
using 1.77, and debian is 1.77, so at least they might be related and I 
can pull patches from openSUSE.  And, maybe push to debian.

I found debian git at 
https://salsa.debian.org/installer-team/os-prober.git.  Now, off to look 
for opensuse repo.

--
-Justin
JustinVallon@gmail.com