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

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

On Tue, 2009-04-28 at 17:28 -0700, H. Peter Anvin wrote:
> Daniel Reurich wrote:
> >>>  
> >> Grub is capable of doing that IF THE FIRMWARE CAN REACH IT.
> > 
> > Well if the firmware can't find one if the disks, then it doesn't matter
> > what scheme we have.  Even a single disk won't work.
> 
> It is *quite* common that firmware can reach a subset of the disks.  If
> not when the system is set up, then when a controller is blown and the
> user has to install a new one.  I have seen this particular malfunction
> up close more times than I can count.
> 
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.

> > What's your beef. MD already reserve some space for the superblock, and
> > write-intent bitmap (which I believe is also replicated across the
> > member disks), so why not add some space to this to make it possible for
> > a bootloader as well.
> 
> My beef is that you're actively promoting an extremely dangerous
> concept, dangerous exactly because it is seductive -- "it seems so
> easy."  Most users, you included, apparently, typically will have no
> notion of the failure modes, and will pick the "easy" option.
> 
What's specifically dangerous about it?  Define the failure modes that
this scheme is unable to either cope with that it should do.

> Booting is ugly business.  I have dealt with the subtleties for almost
> two decades, and it riles me when people go and foist off bad ideas on
> users.

Then stop being so emotive about it and explain the failings of the
scheme rather than point the finger at me and say I don't know.

-- 
Daniel Reurich

Centurion Computer Technology (2005) Ltd
Ph 021 797 722