Re: md extension to support booting from raid whole disks.

[Rudy Zijlstra <rudy@grumpydevil.homelinux.org>]

Op vrijdag 01-05-2009 om 23:10 uur [tijdzone +0200], schreef Goswin von
Brederlow:
> Gabor Gombas <gombasg@sztaki.hu> writes:
> 
> > On Wed, Apr 29, 2009 at 12:43:51PM +1200, Daniel Reurich wrote:
> >
> >> In which case your probably using a hardware raid controller anyway so
> >> not our problem.  Otherwise if the array is broken by a failed
> >> controller we probably shouldn't boot of it anyway.
> >
> > I have set up a box with 8 SATA disks attached to 2 on-board
> > controllers. The BIOS can boot from any of the controllers, but then it
> > can only see the disks that are attached to the selected controller.
> > Which is quite reasonable if the BIOS handles the controller selection
> > by redirecting INT 13h (I have not checked).
> >
> > With "/" on RAID1, I can boot in any failure scenarios (I've actually
> > tested that anno). With your setup, the box would never boot, since it
> > could never access enough disks in a RAID5/6 array, even if all the
> > disks/controllers are perfectly fine.
> 
> So you have lost nothing. It doesn't boot now from raid5 (accross all
> disks) and it still doesn't boot from raid5 with the proposal. Your
> hardware just doesn't support it.
> 
> >> What's specifically dangerous about it?  Define the failure modes that
> >> this scheme is unable to either cope with that it should do.
> >
> > There is no need for a failure mode. Your scheme does not work even when
> > everything is fine.
> >
> > Gabor
> 
> It would only be a problem if with the proposal things would get
> worse.
> 

It would get worse, as in many situations the installer would succeed,
and the boot would fail. Many raid 5/6 configurations will spread over
controllers, and not BIOS supports booting over several controllers. 

I really prefer the current situation, where you need to explicitly
configure a RAID1. This makes clear what is happening, and reduces
confusion. This propposal would make debug of a failed boot just so much
more difficult. 

Cheers,

Rudy