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Re: booting a clone from a /dev/md0 disk as single ext4 disk

From: Peter Holm
Subject: Re: booting a clone from a /dev/md0 disk as single ext4 disk
Date: Sun, 23 Oct 2016 22:08:50 +0200

II don't know if this is at all related. But I had a problem  a few
days ago where  update-grub (wrapper around grub-mkconfi)  from chroot
complained about something like 'no LWM-drivers installed' ,  After
one lwhole day trying to grock that,  it turned out I had forgotten to
mount the boot-partition - which in my case was a separate subvolume.
>From chroot - ''mount -a'  solved  that problem.

2016-10-23 19:03 GMT+02:00, Andrei Borzenkov <address@hidden>:
> 23.10.2016 19:09, John Griessen пишет:
>>>> > and cannot utilize a hand written grub.cfg menu entry like this:
>>>> >
>>>> > menuentry 'Debian stretch/sid Linux 4.7.0-1-amd64' --class debian
>>>> > --class gnu-linux --class gnu --class os {
>>>> >   load_video
>>>> >   insmod gzio
>>>> >   insmod part_msdos
>>>> >   insmod diskfilter
>>>> >   insmod mdraid1x
>>>> >   insmod ext2
>>>> >   set root=UUID=87d52f42-838c-4418-a37e-f0d0a63ed8ed
>>> This line is bogus. I'm surprised code after this works at all.
>>>> >     echo 'Loading Linux 4.7.0-1-amd64 ...'
>>>> >     linux /boot/vmlinuz-4.7.0-1-amd64
>>>> > root=UUID=87d52f42-838c-4418-a37e-f0d0a63ed8ed  ro rootdelay=20
>>>> >     echo 'Loading initial ramdisk ...'
>>>> >     initrd /boot/initrd.img-4.7.0-1-amd64
>>>> > }
>>>> >
>> hints about where to read next would be helpful.
> Well, grub2 documentation
> ( could be starting
> point. This is a bit dated, but it does explain disk name syntax used by
> grub and what root variable does. It also gives some fairly complete
> command language description, although too few commands are described.
> "set root=..." sets grub2 root, and if you want to base it on UUID you
> need to use `search' command (which is what grub-mkconfig does). This
> command is even documented in the manual :)
> ...
>> Can you run
>>> dpkg-reconfigure from outside of installed root at all? I do not see any
>>> option to specify where root is located.
>> I doubt dpkg-reconfigure is ever able to do anything but affect an
>> installed debian linux system
>> from within, or with chroot from the same kernel and probably the same
>> initrd.img.
> Well, dpkg itself has `--root' option which allows you to call it off-root.
>> Any hints about where to study the boot process as far as making cloned
>> drives that started
>> with boot from /dev/md0 run as single drives will be appreciated.
> Sorry, I could not parse this sentence. What I'd do,
> 1. create target devices (presumably, Linux MD)
> 2. copy from source to target
> 3. adjust /etc/fstab, /etc/mdadm.conf and whatever else is appropriate
> (which may include recreating initrd)
> 4. chroot and run grub-install and grub-mkconfig or your distro command
> that calls them
> Note that in case of Linux MD grub2 grub-install only supports
> installation on MBR. You also need to install grub2 individually on each
> disk that belongs to MD array holding /boot/grub if you want resiliency
> against individual disk failures.
> Also note that in relation to grub2 "Linux MD" is really relevant only
> if grub2 itself (i.e. /boot/grub directory) is on MD array.
> 5. configure your distro to use new bootloader location(s) as default.
> So next time you update grub2 package it runs grub-install with correct
> parameters. In case of Debina-based distro this actually means running
> dpkg-reconfigure.
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