Re: how does GRUB read from /boot on software-RAID partition?
On Tue, 24 Feb 2009 15:21:23 -0500
"Barclay, Daniel" <daniel@fgm.com> wrote:
> Eduardo M KALINOWSKI wrote:
> > Barclay, Daniel wrote:
> ...
> >> Since GRUB hasn't loaded the kernel file yet, GRUB can't be using
> >> the kernel and its md driver, and therefore can't be reading the
> >> partition _as_a_RAID_ _volume_ (/dev/mdX), right?
> >>
> >>
> >> So is GRUB just reading the partition directly to get to the file
> >> system?
> >>
> >
> > GRUB does not know anything about RAID, so I assume this is true.
>
> That's what I have been thinking, but I just found the message at
> http://www.mail-archive.com/linux-raid@vger.kernel.org/msg09712.html
> that says::
>
> ... once grub2 has determined that the intended boot partition is
> a raid partition, the raid code takes over ... and it scans for
> all the other members of the raid array and utilizes whichever drives
> it needs to in order to complete the boot process. ... [I]t doesn't
> need any member of a raid1 array to be perfect[;] it will attempt
> a round robin read on all the sectors and only fail if all drives
> return an error for a given read.
>
> Is that _just_ for GRUB2 and or does the current GRUB (0.97) in Lenny
> also do that?
>
>
>
> >> ... is GRUB taking advantage of the fact that the RAID metadata is
> >> written at the end of a partition ...
> ...
> >> If so, how reliable is that?
> >>
> >> Should one put /boot on a plain, non-RAID partition on one disk and
> >> somehow (manually or automatically) maintain a backup /boot
> >> partition on
> >> the second disk, or is it fine to put /boot on a mirrored
> >> partition (so maintaining redundancy is automatic) and let GRUB
> >> read the partition directly?
> >>
> >
> > Again, while I haven't tried, I've seen several reports that this
> > works. ... So why make things more complicated and not automatic?
>
> I don't get why you're asking that. I _am_ trying to avoid the
> complicated and non-automatic solution (trying to check whether the
> simpler solution is reliable).
>
>
>
> Daniel
Hi, Daniel et al
The following is an outline of my setup on a couple of the system disks.
Roughly I have two md devices for the host system, md0 & md1:
jack@host:~$ df -m
Filesystem 1M-blocks Used Available Use% Mounted on
/dev/md0 9389 7157 1755 81% /
tmpfs 4005 1 4005 1% /lib/init/rw
udev 10 1 10 2% /dev
tmpfs 4005 0 4005 0% /dev/shm
/dev/dm-0 40318 11628 26642 31% /home
/dev/dm-4 19686 15356 3331 83% /home/jack/XP_VDI
/dev/dm-6 30238 7453 21250 26% /home/jack/suse
/dev/dm-2 3024 70 2802 3% /tmp
/dev/dm-1 8064 3426 4229 45% /var
/dev/hda 90 90 0 100% /media/cdrom0
The only concern I have is that / is marginally small. It's
expandable tho.
/boot is just on /mdo. I have run without incident for over a year.
Provides sufficient isolation from my bumbling around... 8-)
FWIW.
Jack
.
--
Reply to: