Mailinglist Archive: opensuse (818 mails)

[Joachim Schrod <jschrod@xxxxxxx>]

James Knott wrote:
> Joachim Schrod wrote:
> > James Knott wrote:
> > > Per Jessen wrote:
> > > > John Andersen wrote:
> > > >       
> > > > > At one time there was a problem having /boot on raid, and its been a
> > > > > while since I had to reconfigure a fresh box, so I don't know if this
> > > > > is still the case.
> > > > >
> > > > >          
> > > > With lilo it works fine, but I don't know about grub.
> > > >        
> > > It doesn't work with grub.  With RAID and also LVM, /boot has to be on a
> > > regular partition.
> > >      
> > Here LVM is the problem, not RAID-1.
> >
> > With RAID-1 via md and no LVM, /boot on a mirrored ext3/ext4
> > filesystem definitively works with grub. We use it since several
> > years, and had enough disk errors to prove it working. One needs to
> > take care that grub bootloader is installed on both disks, though,
> > openSUSE doesn't do this itself during installation. Extra
> > robustness comes with stanzas that name both disks, in case hd0 is
> > not visible to the BIOS any more and hd1 suddenly gets hd0.
> >
> >    
> I was wondering about that.  Also, the boot menu points to a specific
> kernal.  How does that get updated with RAID 1?

/boot is mirrored, so any change (any new kernel and any boot menu)
is written to both disks.

E.g., let's assume /boot is on /dev/md0 that is a mirrored device
made up from /dev/sda1 and /dev/sdb1. Now let's have two stanzas:

title Desktop -- openSUSE 11.4 1st disk - 2.6.37.6-0.7
    root (hd0,0)
    kernel /vmlinuz-2.6.37.6-0.7-desktop root=/dev/md2 noresume
quiet showopts nowatchdog vga=0x317 ipv6.disable=1
    initrd /initrd-2.6.37.6-0.7-desktop

title Desktop -- openSUSE 11.4 2nd disk - 2.6.37.6-0.7
    root (hd1,0)
    kernel /vmlinuz-2.6.37.6-0.7-desktop root=/dev/md2 noresume
quiet showopts nowatchdog vga=0x317 ipv6.disable=1
    initrd /initrd-2.6.37.6-0.7-desktop

When one does a grub setup one both disks (I usually install it in
the MBR), grub's bootloader, named stage 1 and stage 1.5, is
installed on both disks. I.e., one calls

  # umount /boot
  # grub
  grub> root (hd0,0)    # this is /boot on /dev/sda1
  grub> setup (hd0)     # MBR-Setup on 1st disk
  grub> root (hd1,0)    # this is /boot on /dev/sdb1
  grub> setup (hd1)     # MBR-Setup on 2nd disk
  grub> quit
  # mount /boot

During boot from either disks, it will look at the first partition
of the respective disk, will use its build-in filesystem driver,
and will access /boot/grub/menu.lst on that filesystem. (That's the
reason why you need a symlink boot -> . on /boot.) Then it gets
kernel and initrd from the first partition (accessed as ext3/ext4
filesystem, not as a mirrored device!!) and loads that. initrd
contains the md kernel module and assembles the raid devices. At
this point during boot, /dev/md? is available. Therefore we can now
mount /dev/md2 as root and later /dev/md0 on /boot.

This works because software raid (md, don't know if that works with
dmraid as well) doesn't create a special layout on its partition if
the md partition is just mirrored and not striped. Beneath it all,
there is still a straight-forward filesystem on that partition, and
grub knows how to handle that filesystem. I.e., after unmounting
/boot and stopping RAID on /boot (mdadm --stop md0), one *can*
mount both filesystems in the regular way, no problem, mount -t
ext4 /dev/sda1 /boot works. This md feature is the base for grub
booting from mirrored filesystems.

HTH,
        Joachim

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Joachim Schrod                          Email: jschrod@xxxxxxx
Roedermark, Germany

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