GRUB requires re-installation whenever the Master Boot Record (MBR) on the boot device is damaged or overwritten by other boot loaders. It is also required in situations where the boot device changes post-installation like when the boot from SAN configuration is enabled after the server has been installed or when a single-pathed SAN boot device is reconfigured to become multipathed.
To view the current multipath configuration, you may run the following command from in a rescue environment:
# multipath -ll
Re-Install GRUB to a multipath device
1. Boot the server in rescue mode with multipathing enabled using the “linux rescue mpath” boot option.
2. Edit /etc/fstab to use multipath device names (/dev/mapper/mpathX) instead of plain names (/dev/sdY) to Mount /boot. We recommend using the UUID of /boot to make the device identification more reliable.
3. Add the multipath device containing /boot to grub and install the boot loader using:
# grub grub> device (hd0) /dev/mapper/mpatha grub> root (hd0,0) grub> setup (hd0) grub> quit
4. Re-generate the initial ramdisk as per the below post.