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Re: booting btrfs
From: |
Chris Murphy |
Subject: |
Re: booting btrfs |
Date: |
Thu, 9 Jan 2014 12:29:10 -0700 |
On Jan 9, 2014, at 3:03 AM, Michael Chang <address@hidden> wrote:
> 2014/1/9 Chris Murphy <address@hidden>:
>>
>> On Jan 7, 2014, at 10:55 AM, Chris Murphy <address@hidden> wrote:
>>
>>>
>>> On Jan 1, 2014, at 10:17 PM, Michael Chang <address@hidden> wrote:
>>>>
>>>> We snapshot /boot for kernel and initrd, otherwise the rollback would
>>>> encounter problem of incompatible userland and kernel/kernel modules.
>>>> And we need the ability to rollback them in terms of usefulness.
>>>
>>> Of course, understood.
>>>
>>> core.img is only going to point to one /boot, which may not be the /boot
>>> snapshot needed for the kernel and initrd. This will be really confusing
>>> for mortal users. They'd be unlikely to figure it out and understand it,
>>> without documentation.
>>>
>>> If core.img points to the "current" /boot, which it should, that boot has
>>> the accumulated knowledge of all snapshots, and any recently updated grub
>>> modules. Choosing to boot a snapshot means using a different /boot for
>>> kernel/initramfs than what grub is using for its root. I don't off hand see
>>> a problem with this because it's literally just two files that grub loads
>>> from a different boot subvolume, found with an absolute path to that
>>> snapshot. And it also uses rootflags=subvol= to use the matching root
>>> snapshot.
>>>
>>> A separate issue that's not grub's problem is how to deal with the (now
>>> wrong) fstab entries. systemd looks at fstab and generates mount jobs from
>>> that; if taught to understand it's booting a snapshot it could second guess
>>> parts of the fstab. Based on the name of the currently booting root
>>> snapshot, which systemd definitely knows, it could mount that subvol=
>>> instead of what's in fstab. It can use name substitution to do the same
>>> thing with the other subvolume-snapshots that match the root one. Meaning
>>> all of the snapshots for a system have the same base (re)naming convention.
>>
>> Another hiccup. Maybe it's a silly use case. Consider /boot on Btrfs,
>> multiple-device, raid1 data/metadata profile, UEFI Secure Boot. A drive
>> dies, and the system needs to be rebooted before a rebuild occurs.
>>
>> This works today with /boot on raid1/10 Btrfs. Yes, I manually have to add a
>> degraded mount option as this isn't automatically done by Btrfs yet. But the
>> GRUB boot part works. Even degraded, the path to grub.cfg is valid. And the
>> file system itself keeps multiple copies so there's no work keeping it
>> current.
>>
>> With Secure Boot it's a problem. The signed grubx64.efi has a fixed prefix
>> location, valid on any computer, to search for grub.cfg, which is on the
>> ESP. So now we need to have multiple copies of grub.cfg, somehow synced, on
>> each ESP. Or another solution. If we had a Btrfs subvolumetypeGUID,
>> analogous to the GPT partitiontypeGUID, and specify that as the baked in
>> partuuid for a signed grubx64.efi to search for /boot/grub/grub.cfg.
>
> Isn't search --fs-uuid sufficient for this task? Or I'm afraid that I
> didn't understand your problem enough.
>
> cat <<EOF > /boot/efi/EFI/<DISTRIBUTION>/grub.cfg
> search --fs-uuid --set=root `grub-probe --target=fs_uuid /boot/grub`
> set prefix=(\$root)/boot/grub
> configfile \$prefix/grub.cfg
> EOF
>
> grub-mkconfig -o /boot/grub/grub.cfg
>
> This way we can avoid calling "grub-mkconfig -o
> /boot/efi/EFI/<DISTRIBUTION>/grub.cfg" and hopefully can get rid of
> the problem you have.
Yes I think that will work. Do you agree that a GUI installer permitting, e.g.
/boot on Btrfs raid1, should implement this? Or should it be a future feature
of grub-mkconfig to figure out, that two grub.cfgs are needed?
Chris Murphy