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

[Daniel Reurich <daniel@centurion.net.nz>]

On Mon, 2009-05-04 at 22:57 +0200, Rudy Zijlstra wrote:
> Op maandag 04-05-2009 om 18:55 uur [tijdzone +0200], schreef Goswin von
> Brederlow:
> > Rudy Zijlstra <rudy@grumpydevil.homelinux.org> writes:
> > 
> > > Op zaterdag 02-05-2009 om 00:49 uur [tijdzone +0200], schreef Goswin von
> > > Brederlow:
> > >> Rudy Zijlstra <rudy@grumpydevil.homelinux.org> writes:
> > >> 
> > >> > 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
> > >> 
> > >> As said in another mail the same problem is there with raid1. The
> > >> proposal should allow creating a raid1 over sda/b without partitioning
> > >> and any replacement drive automatically becoming bootable without you
> > >> having to manually reinstall the bootloader to the new disk.
> > >> 
> > >> Wouldn't that be a huge plus?
> > >> 
> > >> MfG
> > >>         Goswin
> > >
> > > Agreed to the automatic bootable of replacement disk, I still prefer the
> > > partitioning though, as it makes clear what is happening. There are two
> > > aspects here:
> > > 1/ ease of installation
> > 
> > Which isn't true. Installing thebootloader on every component device
> > is currently a pain and easy to forget when changing disks.
> 
> which means it is an important aspect... And i agree at the moment easy
> to forget. So far i do not see improvement in this aspect from current
> proposal. 
> 
> > 
> > > 2/ ease of debug
> > >
> > > The latter is very important in boot situations. It gets worse with any
> > > implied action. Please take a look at what you are specifying: an
> > > implicit RAID1 over 2 "special" disks, within a RAIDX device... Now you
> > > expect a user to debug that, in case it fails?? I have had too often
> > > trouble to get my systems to boot the way i wanted to boot them, to
> > > trust any BIOS to do the expected :(
> > >
> > > Cheers,
> > >
> > > Rudy
> > 
> > Not over 2 "special" disks. Plain across all disks.
> 
> Which is even worse, as i have not seen *any* BIOS able to handle
> that... and i have a intel based board wich according to this email
> thread should do that (and linux did not recognise the raid it had
> configured, so let us forget about booting from it)
> 
I agree it shouldn't be the bioses problem to figure out how to boot
from software raid.  The Bios should not have to handle anything other
than reading the bootsector of the first disk it finds and executing the
bootcode it finds there.  It is then the responsibility of the boot
loader be that grub or linux kexec or whatever to use int13h calls to
load the enough of itself to get proper disk access working and then do
whatever it needs to do to boot the OS proper.  Grub2 is supposed to
have scsi/ata support so that it can access devices the bios doesn't
make available via int13, and does have support for assembling raid and
acessing LVM volumes.  Linux kexec based solutions will have full
support for accessing the disks, raid and LVM also.  So the BIOS only
needs to find a valid boot sector on one of the attached disks and then
hand control over to the boot loader which should assemble any raid and
boot the chosen OS.

Whether implicit volumes or explicit partitions for the boot volume,
whatever the solution it should still replicate the bootsector code, and
leave the assembly of raid to the bootloader.

I'd like to someone tell me how to get a partitioned scenario where the
partition that starts right at the start of the disk and includes the
bootsector, so that we raid1 that partition and have the bootsector
replicated as well?
> 
> 
> Cheers,
> 
> Rudy
> 
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-- 
Daniel Reurich

Centurion Computer Technology (2005) Ltd
Ph 021 797 722