Re: R: Re: Subvolumes and /proc/self/mountinfo

Goffredo Baroncelli
2012-06-20T16:34:35Z

On 06/20/2012 05:37 PM, H. Peter Anvin wrote:
> On 06/20/2012 05:02 AM, Goffredo Baroncelli <kreijack@libero.it>
wrote:
>>
>> If I swap (via a rename) __active and __rollback, in the next boot my
system
>> uses a "good" copy of the root filesystem. This is a simple
way to swap
>> two subvolumes, without involving the boot logic
>>
>> Instead if I had tracked the subvolume-id, to swap the root filesystem
I
>> would have update the boot logic.
>>
>> I suspect that could exists other cases where it is preferable to track
the
>> subvolume-id instead the path. However what I would highlight it is the
two
>> ways aren''t equal.
>>
>
> Yes.  The question here is what makes sense for the low-level part of a
> bootloader (Syslinux in this case) to use, and it sounds like you have
> some experience here that would be highly useful to have.
>
> The thing to keep in mind here is that the low level bootloader code
> *must* match what is installed in the boot block (functionally another
> part of the bootloader), or all hell will break loose.  I think that
> means that relying on the subvolume ID makes more sense.  To upgrade the
> bootloader, invoke the bootloader installer at the end of the update;
> that will repoint *everything*, which is rather nice.


At the first I tough that having the /boot separate could be a good
thing. Unfortunately /boot contains both the bootloader code and the
kernel image. The kernel image should be in sync with the contents of
/lib/modules/....

This is the tricky point. If I handle /boot inside the filesystem
submodule a de-sync between the bootloader code and the boot sector
could happens. In I handle /boot as separate subvolume/filesystem a
de-sync between the kernel image and the modules could happens.

Anyway, from a bootloader POV I think that /boot should be handle
separately (or as filesystem or as subvolume identified by specific ID).
The best could be move the kernel in the same subvolume as /lib/modules,
so a switch of the subvolume as root filesystem would be coherent.


GB
>
> 	-hpa