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Re: booting btrfs

From: Chris Murphy
Subject: Re: booting btrfs
Date: Tue, 15 Oct 2013 21:37:23 -0600

On Oct 15, 2013, at 8:50 PM, Andrey Borzenkov <address@hidden> wrote:

> В Mon, 14 Oct 2013 14:20:14 -0600
> Chris Murphy <address@hidden> пишет:
>>> Is there a way to detect that mountinfo gives garbage and somehow get
>>> where the real root points?
>> I don't know. I've asked on address@hidden Instead of rebooting, I merely 
>> tried mounting without options after changing the default subvolume to a 
>> nested subvolume (one attempt subvolume in a subvolume, another a subvolume 
>> in a directory): in both cases /proc/self/mountinfo reports / as the root, 
>> not the full path or ID of the subvolume actually mounted.
>> Somehow it seems like the mountinfo root field should return a block device 
>> and full path to the mounted subvolume or its ID. Currently it seems like a 
>> problem.
> To quote one of btrfs developer (I had unrelated discussion on openSUSE
> list):
> --><--
> This is a known problem, on my todo list, with a few non-working
> solutions.
> If you mount via subvol=/subvol then /proc/self/mountinfo will show
> 'subvol' as the mounted subvolume (4th column)
> 4 19 0:17 /subvol /mnt/ rw,relatime - btrfs /dev/sda15 rw,space_cache
> but if the subvolume is set-default and then implicitly mounted,
> mountinfo will not show that (that's the bug).

If mounted with subvolid= the same thing happens. Mountinfo doesn't show the 
subvolume name, it's just /. It's also a problem understanding from mountinfo 
all of the devices that make up a btrfs volume. This can be learned from 
> --><--
> That said, information can be obtained also using different means
> (btrfs utility or directly btrfs IOCTL). The question is to which
> extent we want to depend on existence of btrfsprogs.

Yeah at the moment if I use subvolid= to mount, I then have no idea how to find 
out what subvolume is mounted. As far as I know btrfsprogs doesn't a way to 
determine what subvols are mounted. It must be inferred (by mountpoint or by 
contents of the mountpoint).

Anyway, as for support for subvolid in GRUB, I still think it would be nice as 
it's shorter than full paths. But this is not an enhancement hill I'm willing 
to die on by any means.

Chris Murphy

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