Re: PATCH: allow installing grub on the MBR if /boot is on mdraid.

[Jeremy Katz <katzj redhat com>]

On Sun, 2008-11-30 at 14:49 +0100, Hans de Goede wrote:
> Jeremy Katz wrote:
> > On Wed, 2008-11-26 at 12:06 +0100, Hans de Goede wrote:
> >> This patch removes a special case when /boot (or / without a separate /boot) is 
> >> on mdraid. For some reason we only allow installing grub to the mdX device then 
> >> instead of allowing both the mbr of the 1st disk and the device holding /boot.
> >>
> >> I've searched through the anaconda history and this special handling of mdraid 
> >> /boot has been there since before Feb. 2002, it was inherited from before booty 
> >> was introduced and it does not seem to make sense. Either we cannot have /boot 
> >> on mdraid at all (in all cases except for raid1) or we can have /boot on dmraid 
> >> (in case of raid1) and then it does not matter if the bootstrap of grub starts 
> >> on the mbr or on the first sector of a partition.
> > 
> > The reason we install to the mdX device is so that if your disk dies,
> > the bootloader install is mirrored[1] to both disks.  So while it is
> > technically possible to install to the mbr of the first disk, doing so
> > loses the redundancy of the RAID install.
> 
> Well, that is pretty useless, this could be usefull when the first disk (as 
> seen by the BIOS) dies, except that if 99% of the cases that happens the BIOS 
> will still see the disk and thus still try to boot from it and fail.

Some BIOSes actually fail over gracefully from this to the second disk
-- shocking that a BIOS would do something useful, but it occasionally
happens :-).  But even in the cases of not, pulling the disk and being
able to boot is something that customers have actually counted on.

> Software raid1 really only protects against data loss and keeps the system 
> running when a disk dies. It does not guarantee the system will still (re)boot 
> if a disk dies.
> 
> Moreover, grub (booty) does not actually install to the mdX device, it will 
> only install to the first partition of the raid set. Atleast that is how I read 
> the booty code. And this is no surprise as grub does not know anything about 
> mdraid.

grub doesn't know anything about it, but the code should be there to
install to both sides of the array.  It's a bit fugly and nowhere as
nice as LILO where this just worked thanks to dledford's work.  But it
should be functional at least since, iirc, FC6 times

Jeremy