Re: Future of installing GRUB to LVM volumes (and loop devices)

[Andrei Borzenkov <arvidjaar@gmail.com>]

В Tue, 12 May 2015 12:41:19 +0200
Sebastian Pipping <sebastian@pipping.org> пишет:

> 
> >> Auto-detection of a filesystem of /dev/mapper/loop0p1 failed.
> >> Try with --recheck.
> >> If the problem persists please report this together with the output of
> >> "/usr/sbin/grub-probe --device-map="/mnt/loop-root/boot/grub/device.map"
> >> --target=fs -v /mnt/loop-root/boot/grub" to <bug-grub@gnu.org>
> >> 1
> >> # mount | fgrep /mnt/loop-root
> >> /dev/mapper/loop0p1 on /mnt/loop-root type ext4
> > 
> > There is no feasible way to map this device back to contained device,
> > short of scanning all maps, finding all partition labels on them and
> > matching maps against labels.
> 
> Why not?
> 
> lsblk manages to detect that /dev/mapper/loop0p1 is a child of /dev/loop0:
> 
> # lsblk | fgrep loop | sed 's| \{23\}||'
> loop0                 7:0    0   100M  0 loop
> └─loop0p1 (dm-9)    254:9    0    96M  0 part  /mnt/loop-root
>

That's what mean - this is direct parent-child. But consider

bor@opensuse:~/build/grub> echo 0 2048 linear /dev/loop0 0 | sudo dmsetup create whole
bor@opensuse:~/build/grub> echo 0 59392 linear /dev/loop0 2048 | sudo dmsetup create part

At this point /dev/mapper/part refers to partition of /dev/mapper/whole
but there is not direct relationships between two that you can
easily discover

bor@opensuse:~/build/grub> sudo dmsetup ls --tree
whole (254:5)
 └─ (7:0)
part (254:6)
 └─ (7:0)

and

bor@opensuse:~/build/grub> sudo ./grub-probe  -t hints_string -m device.map /mnt 
./grub-probe: warning: unknown device type loop0
.

bor@opensuse:~/build/grub> cat device.map 
(hd0) /dev/mapper/whole

but

bor@opensuse:~/build/grub> sudo ./grub-probe  -t hints_string -m device.map /mnt 
./grub-probe: warning: unknown device type loop0
.
--hint='hd0,msdos1' 
bor@opensuse:~/build/grub> cat device.map
(hd0) /dev/loop0

So in common cases it will work and is better as nothing, but there will be corner cases that may fail.

> 
> > Half working solution would be to assume that it is always child map.
> > 
> > Or you can take shortcut and assume that /dev/mapper/XXXXpY is
> > partition of /dev/mapper/XXX but you still will need to verify it.
> > 
> > Patches are welcome.
> 
> I am still wondering: how come it was working in GRUB 2.00 but now
> longer is?
> 

But by you said in previous message that HEAD works? Not sure how
though :)