Grub, RAID 1 and USB sticks
We recently took to installing our Linux servers from a USB stick. This is much more efficient than downloading four of five cd iso’s time and again, since Linux distros like Fedora and CentOS keep spawning new releases constantly. So we just write the rescue cd iso to a usb stick, and proceed with a network install. This works like a charm, except that for a few times running, after completing the installation, instead of being met with the grub boot menu, we were dropped to the Grub shell brutally. This had us stmied for a while, until we realized that for some dumb reason or other, Grub insists on recognizing the USB stick we install from as device hd0. This means that during installation, Grub writes itself a nice config file containing the line
which points to the second drive, in our case our first hard disk, /dev/sda. After installation – without the USB stick, Grub sees /dev/sda as the first device, hd0, and proceeds to boot the hd1 device, which now is /dev/sdb instead of /dev/sda. Since the MBR (master boot record) was only written to /dev/sda, there’s no way Grub can boot from /dev/sdb.
In order to solve all this, from within the Grub shell, do the following:
root (hd0,0)
setup (hd0)
device (hd0) /dev/sdb
root (hd0,0)
setup (hd0)
reboot
This will instruct Grub to see both the hard disks as hd0 – since in case the first hard disk fails, /dev/sdb is effectively the first device, and install the MBR and Grub on them. After this, reboot and do not forget to edit your grub.conf. Change the line that reads
to
This way, Grub will once again boot from its first device as it was meant to.
Installing Grub and the MBR on the second device of your RAID 1 array should be done in any case, even if you don’t experience USB stick troubles. If you do not, in case of a failure of your first hard disk, your system will be unable to boot until you install the MBR and Grub on your second hard disk.
