[X4U] running OS from software RAID

[David Ledger]

>From: Stroller <macmonster at myrealbox.com>
>On 22 Mar 2008, at 07:34, David Ledger wrote:
>>  I run my 544MHz G4 tower booted from a pair of raid 1 (mirrored)
>>  40GB drives (the original and a second almost as old) using Apple's
>>  s/w raid. ...
>
>Out of curiosity, will the system boot if one of the drives fails?
>(or if one of the drives is simply unplugged?)

Havn't had the time to to do the test that makes the whole thing 
worthwhile. Hopefully I'll get a chance in the next few weeks. 
There's no complex boot-time disc setup as far as I can see, so I'm 
expecting each drive to be a boot drive. HP-UX mirrored boot drives 
do this, and they have an intermediate layer. Solaris has a middle 
layer, but you have to mess about to boot off half a mirror.

>Software RAID under Linux is supposed to be very good these days, but
>for a while i have been wondering - you put GRUB on /dev/sda and
>then / on a RAID of /dev/sda and /dev/sdb. What happens if /dev/sda
>dies - presumably one puts GRUB and a /boot partition also on /dev/
>sdb against this eventuality. Presumably Apple do something similar
>all transparently & seamlessly, so that the user doesn't even notice?
>Stroller.

I'm assuming that when a Mac boots it starts off booting from the 
PRAM selected half-mirror drive as set by System Prefs., and when 
it's booted far enough it picks up the second half. I'm then assuming 
that if that half mirror is faulty at a low level it just won't get 
offered for boot. And if it starts to boot and fails, you just select 
the other one manually. If they've made it more complex they've 
hidden it well.

When I get a chance I'll experiment.

David


-- 
David Ledger - Freelance Unix Sysadmin in the UK.
HP-UX specialist of hpUG technical user group (www.hpug.org.uk)
david.ledger at ivdcs.co.uk
www.ivdcs.co.uk