Tux Tip: Grub+md mbr

Scenario: System with two hard drives (sda, sdb) mirrored for redundancy.

The 1st drive in the BIOS is sda and the 2nd drive is sdb. Grub is installed on the MBR of sda during the O/S installation. We need it installed on the MBR of sdb as well in case the 1st drive fails and the system reboots.

The trick is the grub device command issued before the root and setup commands.

While the system is booted up with both drives operational type the following commands as root:

[root@myhost]# grub
grub>   device (hd0) /dev/sdb   # Set (hd0) to be the 2nd drive for subsequent commands
grub>   root (hd0,0)            # Root device (with /boot/grub) is 1st partition of 2nd drive
grub>   setup (hd0)             # Write the MBR on the 2nd drive
grub>   quit

That's all.

Here's what my session looked like doing the same for hda,hdb mirrors, where the root command in /boot/grub/grub.conf is "root (hd0,0)":

grub> device (hd0) /dev/hdb
                                                                                
grub> root (hd0,0)
 Filesystem type is ext2fs, partition type 0xfd
                                                                                
grub> setup (hd0)
 Checking if "/boot/grub/stage1" exists... no
 Checking if "/grub/stage1" exists... yes
 Checking if "/grub/stage2" exists... yes
 Checking if "/grub/e2fs_stage1_5" exists... yes
 Running "embed /grub/e2fs_stage1_5 (hd0)"...  16 sectors are embedded.
succeeded
 Running "install /grub/stage1 (hd0) (hd0)1+16 p (hd0,0)/grub/stage2 /grub/grub
.conf"... succeeded
Done.
                                                                                
grub> quit