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Re: update-grub fails saying it cannot find a device for /


From: Andrei Borzenkov
Subject: Re: update-grub fails saying it cannot find a device for /
Date: Sun, 14 Nov 2021 21:23:57 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.14.0

On 14.11.2021 19:52, Stefan Blachmann wrote:
> No idea why grub-probe fails finding the / disk.
> 
> To make the update-grub script work, could it be a workaround to
> replace grub-probe with a dummy script that just echoes "hd0,gpt4" or
> such?
> 

No.

> How does look a correct output when grub-probe does not fail?
> 

It depends on command options.

> 
> On 11/14/21, Stefan Blachmann <sblachmann@gmail.com> wrote:
>> Thank you for the quick reply, Andrei!
>>
>> I have mounted not only /dev, but also /sys, /proc and /run into the
>> chroot, too. Didn't mention that to keep mail short.
>>
>> Meanwhile I found a possible hint:
>> As there was no device.map file, I ran grub-mkdevicemap.
>> This did no change again, update-grub still fails.
>>
>> Then, after a lot of searching the web, I found the second answer on
>> this page might be a hint:
>> https://serverfault.com/questions/401351/update-grub2-without-hardware-access-e-g-in-a-chroot
>>
>> It basically says that I should use disk UUIDs instead of disk ids.

This is red herring.

Show output of

cat /proc/self/mountinfo

In chroot.

>> Now the problem is that grub-mkdevicemap does not make a device.map
>> using UUIDs, it uses the IDs under /dev/disk/by-id.
>>
>> And I cannot add links to UUIDs as the answer on that page recommends.
>> Because for reasons I do not understand, in /dev/disk/by-uuid I can
>> see only the UUIDs of the *mounted* file systems (eg the live CD, and
>> the ZFS boot and root filesystems), but of the drives themselves. I
>> can not see UUIDs.
>>
>> When I manually change the /dev/disk/by-id/ device.map file entries to
>> /dev/sda, sdb etc, there is no change in behaviour, update-grub still
>> fails the same way.
>>
>> This is on Debian bullseye, which I assume uses GRUB 2.
>>
>> Any idea what could be done to make update-grub succeed?
>>
>>
>>
>> On 11/14/21, Andrei Borzenkov <arvidjaar@gmail.com> wrote:
>>> On 14.11.2021 16:31, Stefan Blachmann wrote:
>>>> When I enter
>>>> # update-grub
>>>> I get an error message:
>>>> /usr/sbin/grub-probe: error: cannot find a device for / (is /dev
>>>> mounted?).
>>>>
>>>> /dev has been mounted before entering the chroot /mnt environment using
>>>> # mount --rbind /dev /mnt/dev
>>>>
>>>
>>> In chroot you need at least /sys and /proc in addition to /dev, /run may
>>> also be needed.
>>>
>>>> If I enter
>>>> # grub-probe
>>>> it answers:
>>>> No path or device is specified.
>>>>
>>>> If I enter
>>>> # grub-probe /boot
>>>> it answers:
>>>> zfs
>>>>
>>>> So, what do I need to do to make update-grub work?
>>>>
>>>
>>>
>>>
>>




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