Centos 7.2 Convert single LVM to RAID Mirror Reinstall BootLoader

Clovis_Sangrail · 04-19-2016, 06:06 PM

Hello,
I am trying to convert a single-drive Centos 7.2 installation with LVM into a two-disk mdadm mirror with mrrored LVM. I was able to follow the excellent instructions at:

http://www.dgoradia.com/creating-a-r...centos-on-lvm/

and did create a two-disk mirror of both the original boot partition and the LVM partition. When I boot off a live Centos 7.2 DVD with the mirrored drives installed it does find the mirrors, and I can fsck and mount the boot and LVM filesystems in it.

Code:

[root@new-host-4 ~]# lsblk
NAME              MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINT
sda                 8:0    0 465.8G  0 disk  
├─sda1              8:1    0     1G  0 part  
│ └─md126           9:126  0  1024M  0 raid1
└─sda2              8:2    0   115G  0 part  
  └─md127           9:127  0   115G  0 raid1
    ├─centos-root 253:3    0    40G  0 lvm  
    ├─centos-swap 253:4    0     9G  0 lvm  
    ├─centos-home 253:5    0    25G  0 lvm  
    └─centos-var  253:6    0    40G  0 lvm  
sdb                 8:16   0 465.8G  0 disk  
├─sdb1              8:17   0     1G  0 part  
│ └─md126           9:126  0  1024M  0 raid1
└─sdb2              8:18   0   115G  0 part  
  └─md127           9:127  0   115G  0 raid1
    ├─centos-root 253:3    0    40G  0 lvm  
    ├─centos-swap 253:4    0     9G  0 lvm  
    ├─centos-home 253:5    0    25G  0 lvm  
    └─centos-var  253:6    0    40G  0 lvm  
sr0                11:0    1   4.4G  0 rom   /run/initramfs/live
loop0               7:0    0    20K  1 loop  
loop1               7:1    0   4.3M  1 loop  
└─live-osimg-min  253:2    0     8G  1 dm    
loop2               7:2    0     1G  1 loop  
loop3               7:3    0     8G  1 loop  
├─live-rw         253:0    0     8G  0 dm    /
├─live-base       253:1    0     8G  1 dm    
└─live-osimg-min  253:2    0     8G  1 dm    
loop4               7:4    0   512M  0 loop  
└─live-rw         253:0    0     8G  0 dm    /
[root@new-host-4 ~]#

However these instructions are specific to Centos 6, and the section for installing a bootloader on the new mirror completely does not apply in Centos 7. In Centos 7.2 there does not seem to be any commandline utility that provides a "grub>" command prompt, ant the grub binary itself is replaced by a bunch of "grub2-*" executables.

Can anyone explain how to reinstall the bootloader on the new mirror? The Centos 6 instructions say it must be installed separately on sda1 and sdb1.

frostschutz · 04-20-2016, 05:03 PM

You have to chroot into the installed system, make sure everything relevant is mounted (/ obviously, /boot, as well as bind-mounts for /proc /sys /dev /run ...)

Within the proper chroot environment, it should be a simple matter of updating the grub config and grub2-install /dev/sda, grub2-install /dev/sdb.

If that fails you will hopefully get an error message that will give some clue as to what else has to be done.

Quote:

The Centos 6 instructions say it must be installed separately on sda1 and sdb1.

should be sda and sdb, you install bootloaders to disks, not individual partitions (even if the bootloader may use partitions in the end, but ...)

You should also show your partition table (parted /dev/disk unit s print free), /proc/mdstat (and perhaps mdadm --detail /dev/md*, mdadm --examine /dev/sd*), and tell us whether your system should boot UEFI or traditional BIOS style...?

Clovis_Sangrail · 04-26-2016, 08:18 PM

Quote:

You have to chroot into the installed system, make sure everything relevant is mounted (/ obviously, /boot, as well as bind-mounts for /proc /sys /dev /run ...)

Hello frostshutz, thanks for your reply. I did not even know what a bind-mount was.

Unfortunately I had to use that machine on a higher-priority project, and while I was able to save that pair of hard drives I haven't even had a chance to put them in the machine and boot off the DVD since last week.

When I can I will try to set up the chroot environment that you describe, though at this point I don't know what I have to do to the grub configuration. I suppose reading the Redhat grub2 docs should be on my list of things to do.

The bios is set to boot traditionally rather than UEFI. I can set it either way, but the production machine which I need to upgrade to a mirror is traditional and not UEFI.