[CentOS] Software RAID + LVM + Grub

Matthew Topper topperm9 at gmail.com
Sat Sep 18 11:13:08 EDT 2010

I'm playing with software RAID and LVM in some virtual machines and
I've run into an issue that I can't find a good answer to in the docs.

I have the following RAID setup:

md0: sda1 and sdb1, RAID 1.  This is /boot

md1: sda2 and sdb2, RAID 1.  This is a PV for LVM.

VolGroup00, this is the volume group and md1 is the only PV in it.

LogVol00 is swap
LogVol01 is /
LogVol02 is /home

So, I tested to see what happens if I disable sdb in virtualbox.
Machine booted find and I was able to see that part of the raid array
was gone.

I reattached the disk and rebuilt the array
mdam --add /dev/md0 /dev/sdb1
mdam --add /dev/md1 /dev/sdb2

The array rebuilt without issue.  But now, if I remove sda, the machine
doesn't boot and grub complains.  The question I have is how does one
reinstall the boot sector at this point?  I'm a little confused as far
as to what device to install it on and what to specify as the root.

This is grub.conf if it helps:

title CentOS (2.6.18-194.e15)
	root (hd0,0)
	kernel /vmlinuz-2.6.18-194.e15 ro root=/dev/VolGroup00/LogVol01
	initrd /initrd-2.6.18-194.e15.img

Grub has always been a little confusing for me, so I guess I don't
understand.  What do the two root entries specify?  And again, my main
question is how do I reinstall grub to the repaired disk?  Sorry for
the lengthy post, I'm trying to provide as much information as possible.

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