[Asterisk-Users] Re: What is the best Dell Machine for Asterisk?

[Mike McMullen]

> I am talking about SW Raid also.  Now, what happens when your BIOS fails 
> to read the MBR from the first mirror and doesn't try the second because 
> it sees the first?
> 
> I see this problem, and I do not know how to fix it without either 
> replacing the BIOS or installing a PCI disk controller that handles this 
> scenario properly.
> 
> Let me repeat, if your BIOS doesn't look hard enough for a MBR, you will 
> never get to the bootloader.
> 
> I'm looking for solutions, do you understand the problem I am 
> describing?  I hope so, because otherwise you are just denying a valid 
> failure case.
> 
> Solutions wanted.
> 
> Mike

The fallback option of GRUB may help you in that scenario. If one boot fails
you "fallback" to the specified boot option listed in your grub.conf. I run multiple
servers using s/w raid 1 for our business and have simulated various types of
failures. PATA devices in most not all devices maintain drive 0 and drive 1 
locations even if drive 0 fails. In that case, the fallback option works perfectly
if the fallback is set to an image on drive 1. 

You also need to make certain your boot device order is set in your BIOS.
I have yet to find a BIOS that doesn't give you at least three boot device
designations.

Almost all on-board SATA controllers I've used work a little different. 
If drive 0 fails, they assign drive 1 to the drive 0 designation and your
grub.conf boots correctly anyway. This is assuming you have /boot as
separate partition and it is mirrored.

In this scenario fallback isn't required because the SATA controller essentially
does a h/w fallback for you.

In the scenario you mention, I've yet to run across a situation where the mbr was found
on a bad drive but /boot was gone and the system hung. Not that it can't happen, I just
haven't seen it yet.

In that scenario you will have to manually intervene I would assume and force
a boot from the 2nd drive if fallback doesn't handle that case. Again, I've not 
personally tested this scenario.

The GRUB home page has several examples including one interesting one that
dynamically changes the fallback option to handle many different situations. Perhaps
an interation of it will cover your case.

Mike M (another Mike)