14.04 - Replaced drive in Software RAID 1 - How to make bootable?

Question

I have an Ubuntu 14.04 x64 server with 2 500GB drives (sda and sdb) in a software RAID 1.

I just had to replace sdb. Adding it back to the array with mdadm and resyncing was a snap. The problem I'm running into is making sure it can boot if sda happens to fail in the future.

Note: all commands below were run as root

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1st try

Most walkthroughs I've found told me to run:

grub-install /dev/sda /dev/sdb

Which is the exact same command I've seen the Ubuntu installer run a hundred times. However, when I ran that, I got:

grub-install: error: More than one install device?.

This has always worked for me. Maybe something changed with 14.04?

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2nd try

Some walkthroughs recommended setting up grub for the new hard drive manually. The grub prompt scares me, but I was willing to try these steps:

# grub --device-map=/boot/grub/device.map
grub> root (hd0,0)
grub> setup (hd0)
grub> root (hd1,0)
grub> setup (hd1)
grub> quit

Only problem? After the first command, I get:

The program 'grub' is currently not installed. You can install it by typing:
apt-get install grub

???

So I tried installing grub.

# apt-get install grub
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Suggested packages:
  grub-legacy-doc
The following packages will be REMOVED:
  grub-gfxpayload-lists grub-pc grub2-common
The following NEW packages will be installed:
  grub
0 upgraded, 1 newly installed, 3 to remove and 0 not upgraded.
Need to get 913 kB of archives.
After this operation, 438 kB of additional disk space will be used.
Do you want to continue? [Y/n] 

Needless to say, I answered no.

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3rd try

Getting desparate, I tried something I was pretty sure was wrong. The official server guide recommends (at the bottom of the "Raid Maintenance" section):

grub-install /dev/md0

This was the output:

# grub-install /dev/md0
Installing for i386-pc platform.
grub-install: warning: Couldn't find physical volume `(null)'. Some modules may be missing from core image..
grub-install: warning: Couldn't find physical volume `(null)'. Some modules may be missing from core image..
grub-install: warning: File system `ext2' doesn't support embedding.
grub-install: warning: Embedding is not possible.  GRUB can only be installed in this setup by using blocklists.  However, blocklists are UNRELIABLE and their use is discouraged..
grub-install: error: will not proceed with blocklists.

Scary.

---

It looks like grub has been superceded somehow in 14.04, but I can't find any information about how things have changed. I do have a /boot/grub/ directory, so I know some form of grub is installed.

---

Update:

I was able to run the following commands independently:

grub-install /dev/sda
grub-install /dev/sdb

However, I'm unsure if sda will boot after installing grub on sdb and vice versa. Am I ok?

Solution

I found the answer. The way to make sure GRUB is properly installed on and recognizes both drives from each is to run:

sudo dpkg-reconfigure grub-pc

When prompted which drives to install on, make sure all boot drives are selected. In my case, this was /dev/sda and /dev/sdb. Do not select /dev/md0 or any of the other md devices.

Comments

[MGodby]

You could remove one drive at a time and seeing if they boot normally.

[Andrew Ensley]

At the moment, I'm not sure either drive will boot (see my last update). I can't just take this server down to test it and (potentially) reload it if it is hosed. There has to be a proper way to do this. I know I'm not the first person who has replaced a bootable RAID1 drive in Ubuntu 14.04 (maybe just the most ignorant).

[aSteve]

[Apologies if this doesn't help] When I configured my Ubuntu raid-1 server I found I couldn't put my boot partitions on on the raid volume. I assumed this was because I couldn't access software raid until the kernel is loaded - i.e. post boot. I created two small boot partitions - one on each drive - and duplicated the boot partition between the physical disks using dd. I can now boot from either drive... as they have identical boot partitions.

[αғsнιη]

Your are Ok just now mount your /dev/sdb partition and run update-grub in it. this answer might help you askubuntu.com/a/88432/283843