[SOLVED] GRUB2 question

baddah · 02-01-2021, 12:59 AM

Hi,

I have a centos 7 server running software RAID1. One of the drives failed and i'm getting ready to replace it. Already removed the failed drive from array.

Code:

cat /proc/mdstat
Personalities : [raid1] 
md128 : active raid1 sda4[2]
      1771962368 blocks super 1.2 [2/1] [U_]
      bitmap: 5/14 pages [20KB], 65536KB chunk

md125 : active raid1 sda1[2]
      49151872 blocks super 1.2 [2/1] [U_]
      
md126 : active raid1 sda2[3]
      1023936 blocks super 1.0 [2/1] [U_]
      
md127 : active raid1 sda3[2]
      102399872 blocks super 1.2 [2/1] [U_]
      bitmap: 1/1 pages [4KB], 65536KB chunk

Wanted to make sure system will boot from remaining drive so I ran grub2-install /dev/sda but I got the following back.

Code:

grub2-install /dev/sda
Installing for i386-pc platform.
grub2-install: warning: Couldn't find physical volume ‘(null)’. Some modules may be missing from core image..
grub2-install: warning: Couldn't find physical volume ‘(null)’. Some modules may be missing from core image..

My /boot partition is part of the md array. (md126)

I just want to make sure the system will boot when I switch it off to replace the drive. Any pointers on what I can do to be 100% sure it will boot and even suggestions on what to do if it does not.

As a side note - I also have a 3rd drive in this server that is not part of the raid array and just have a normal ext4 filesystem I use to save some extra files. Can i install grub2 to this drive as well?

pmv · 02-04-2021, 08:49 AM

I know this commands:

Code:

sudo mount /dev/sda6 /mnt
(or if BtrFS: sudo mount -o subvol=@ /dev/sdXY /mnt)
sudo mount -o bind /dev /mnt/dev
sudo mount -o bind /sys /mnt/sys
sudo mount -t proc /proc /mnt/proc
sudo cp /proc/mounts /mnt/etc/mtab
sudo chroot /mnt /bin/bash

Code:

sudo grub-mkconfig --output=/boot/grub/my.cfg (instead of grub.cfg)

colorpurple21859 · 02-04-2021, 09:30 AM

Quote:

I also have a 3rd drive in this server that is not part of the raid array and just have a normal ext4 filesystem I use to save some extra files. Can i install grub2 to this drive as well?

Yes, if something goes wrong, you should be able to boot the third drive from the bios boot menu.
Would suggest to make a /boot/grub directory on the drive and then

Code:

grub2-install --boot-directory=<mount point of 3rd drive>/boot /dev/<third drive>
grub2-mkconfig -o /<mount point of 3rd drive>/boot/grub/grub.cfg

mrmazda · 02-04-2021, 10:50 AM

Quote:

Originally Posted by pmv

I know this commands:

Code:

sudo mount /dev/sda6 /mnt
(or if BtrFS: sudo mount -o subvol=@ /dev/sdXY /mnt)
sudo mount -o bind /dev /mnt/dev
sudo mount -o bind /sys /mnt/sys
sudo mount -t proc /proc /mnt/proc

I've always used mount -o bind /proc /mnt/proc.

Quote:

Code:

sudo cp /proc/mounts /mnt/etc/mtab
sudo chroot /mnt /bin/bash

At this point also sudo mount -a can be needed before proceeding if using btrfs.

Quote:

Code:

sudo grub-mkconfig --output=/boot/grub/my.cfg (instead of grub.cfg)

baddah · 02-08-2021, 12:23 AM

So after going through some scenarios in a test environment I have found that those warnings are related to the degraded array and grub will work fine on the remaining drive even with those warnings. So one can safely switch the server off to replace the drive with these warnings.

It looks like the warnings only go away after you have re synced the drive + rebooted it. Not sure why the reboot is needed as well.

Thanks