Did you know (2) …

November 28, 2013

It’s happened again … the brain stopped working. So instead of writing something new I’m going to tell you about another little quirk of our controllers … just in case you need a bit more technical information. In reality I’m knee deep in the middle of redeveloping all my technical training slides and the brain needs a rest …

Boot order. In the bios of a Series 7 and newer controller these is an option to “Select Boot Device”. Now that sounds pretty simple and surely I can’t be blogging about that for very long? Well there is a bit more to this than meets the eye.

1. You can have multiple arrays on a controller

2. You can have raw devices (pass through) – disks that are shown straight to the OS

3. You can boot from any of the arrays, or any of the raw devices

Hmmm. The age old way of changing the boot order was to go into the raid array configuration utility (card bios) and use control + B to set the boot order. The array on the top of the list is the one that the card will boot from. Unless of course, there is a raw device (uninitialised drive) being presented to the OS … and then, of course, only if that drive has been put in in the boot order.

But wait, you can’t set raw devices using control + B as they don’t appear in the list. That is where the option “Select Boot Device” comes into it’s own. It allows you to choose raw devices (uninitialised disks) from the list and add one to be the boot device. Now what happens to your RAID array if you do that? Simply put the raw disk will precede the RAID array in the boot order.

A little trick here is that since the raw disk has no metadata we can’t store the fact that it’s a boot device on the disk … that information has to be stored in the controller. If that device fails then the boot order will revert to the first array in the list. So we have two ways of setting the boot device because each one is attached to two different types of disks (raw disks and RAID arrays).

Of course, all of this needs to be handled by your motherboard bios … and you need to set the correct device in the motherboard for your system to boot from (but they tend to handle that automatically these days).

I just wouldn’t want you getting confused :-)



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