The importance of RAID

Sun, 27 Apr 2014 15:05:12 GMT

Backups are important, right? Really important. Computers die. Hard drives die. If you don't have backups, data may be lost for ever.

But making backups is a nuisance, it's a chore, one of those things that you feel virtuous for doing a few times and then get distracted or bored and stop doing. Especially with home systems, it's easy to be slack.

About a week ago, I had a hard drive suddenly die in my home server. And I had no backups. Oops.

Fortunately, I did have RAID.

Although one drive had died, the second hard drive in the RAID array was okay, with a complete copy of all my data, including a working operating system, and my server just kept going. After a couple of days, I got a new hard drive, moved furniture around so I could actually get to the server, replaced the hard drive (and the long-dead DVD drive as well), moved everything back, and ... the damn server wouldn't boot.

Hmmmm.

As far as I am concerned, RAID is fantastic. It's not really practical in a laptop, but in a desktop or server, I couldn't do without it. RAID isn't really designed as a backup system, but it behaves as a poor man's backup. Or perhaps a slack person's backup. It lets you keep going even in the face of an otherwise catastrophic hard drive failure. But it does have one horrible flaw: the boot loader isn't included in the RAIDed partition. So I had a situation like this:

Before the disk died:

    +--------+-----------------------------+
hda |  GRUB  |      RAIDed Partitions      |  <== Boots from this drive.
    +--------+-----------------------------+

    +--------+-----------------------------+
hdb | blank  |      RAIDed Partitions      |
    +--------+-----------------------------+


After the disk was replaced:

    +--------+-----------------------------+
hda | blank  |      RAIDed Partitions      |  <== The former hdb, moved.
    +--------+-----------------------------+

    +--------+-----------------------------+
hdb | blank  |      blank                  |  <== The replacement drive.
    +--------+-----------------------------+

So here I was with two good disks and no working computers (all my desktops mount their home from the server via NFS). Since neither disk had GRUB installed, there was no way to boot from either of them. After making an attempt to fix the situation with the Centos recovery system, I soon decided that this was beyond my level of expertise. (I might administer my own system, but I have no illusions that I'm a system administrator. A man's got to know his limitations.) Fortunately I was able to get one of the sys admins that I work with to re-install GRUB (thanks David!), this time on both hard drives, and configure RAID for the new drive.

The moral of this story:

Backups are important.

RAID makes a nifty backup for slackers.

But when you configure RAID, your Linux installer probably won't install GRUB on both drives. You need to do it yourself.

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The importance of RAID