Replacing the top-level root

Replacing the top-level root

[C Anthony Risinger]

On Mon, Oct 25, 2010 at 2:58 PM, Chris Mason <chris.mason@oracle.com> wrote:
>
> Oh, and it shouldn't work on the root of the FS either ;)
>
> -chris

This is not 100% related but...

Will removing [replacing] the top-level root be something that
can/will be supported?  I've asked in the past about this regarding an
initramfs hook i maintain implementing system rollbacks, but I never
really got a solid answer.

For example, right now extlinux support booting btrfs, but _only_ from
the top-level root.  if i just had a way to "swap" the top-level root
with a different subvol, i could overcome several problems i have with
users all at once:

) users install their system to the top-level root, which means it is
no longer manageable by snapshot scripts [currently]
) if the top-level root could be swapped, extlinux could then boot my
snapshot? (i'm probably wrong here)

"set-default" is not enough; i'm looking for a way to gain control
over the system state when the system has been installed into the
top-level root.  currently, i have no way to manipulate/move/change
it, because the top-level is essentially "immutable", or so it seems.

thus it's not possible to support kernel rollbacks without manually
syncing <snapshot>/boot to /boot.

is there a solution to this that i'm missing?

C Anthony

Re: Replacing the top-level root

[David Brown]

On Mon, Oct 25, 2010 at 03:20:58PM -0500, C Anthony Risinger wrote:

>For example, right now extlinux support booting btrfs, but _only_ from
>the top-level root.  if i just had a way to "swap" the top-level root
>with a different subvol, i could overcome several problems i have with
>users all at once:
>
>) users install their system to the top-level root, which means it is
>no longer manageable by snapshot scripts [currently]
>) if the top-level root could be swapped, extlinux could then boot my
>snapshot? (i'm probably wrong here)

I don't think this is a solution to the extlinux problem, but I've
moved roots into new subvolumes, basically something like this.

Root is mounted as /, I've also mounted the volume on /mounted in this
example.

   # btrfs subvolume snapshot /mounted /mounted/newrootname

Now reboot, adding the subvol option to use the newrootname.  

Go into /mounted and make sure files touced there don't show up in '/'
(we really are mounting the submount).

Then just use rm -rf to remove everything that isn't a subvol.  I
don't know of an easy way to do that, and be careful.

This doesn't really change the default root, but by making a snapshot
of it, can move all of the data elsewhere.

David

Re: Replacing the top-level root

[C Anthony Risinger]

On Wed, Oct 27, 2010 at 12:53 AM, David Brown <btrfs@davidb.org> wrote:
> On Mon, Oct 25, 2010 at 03:20:58PM -0500, C Anthony Risinger wrote:
>
>> For example, right now extlinux support booting btrfs, but _only_ fr=
om
>> the top-level root. =A0if i just had a way to "swap" the top-level r=
oot
>> with a different subvol, i could overcome several problems i have wi=
th
>> users all at once:
>>
>> ) users install their system to the top-level root, which means it i=
s
>> no longer manageable by snapshot scripts [currently]
>> ) if the top-level root could be swapped, extlinux could then boot m=
y
>> snapshot? (i'm probably wrong here)
>
> I don't think this is a solution to the extlinux problem, but I've
> moved roots into new subvolumes, basically something like this.
>
> Root is mounted as /, I've also mounted the volume on /mounted in thi=
s
> example.
>
> =A0# btrfs subvolume snapshot /mounted /mounted/newrootname
>
> Now reboot, adding the subvol option to use the newrootname.
> Go into /mounted and make sure files touced there don't show up in '/=
'
> (we really are mounting the submount).
>
> Then just use rm -rf to remove everything that isn't a subvol. =A0I
> don't know of an easy way to do that, and be careful.

yeah, this is precisely what i do currently...

the problem is i have to tell the user to do this themselves, as there
isn't a way to safely accomplish it automatically.  i expect this will
become a more common problem, esp. for those trying to integrate
advanced btrfs handlers into their distro; like myself.  basically, i
have ultimate control over all subvols, except top-level, and i want
to why/how to address this.

imo, if there really is not a proper solution to this issue because of
some technical reason, i think mkfs.btrfs should automatically create
a subvol, and mark it default... because from the feedback i'm getting
or lack thereof, the top-level subvol is dangerous because it's not
manageable.

this should be done immediately imo, but at the very least, once
grub2/extlinux support booting from subvols.

thoughts?

C Anthony
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