Re: mbr-install for Raid 1

["Zeno R.R. Davatz" <zdavatz@ywesee.com>]

Hi

I am just thinking of one more problem that turned up.

You suppose that I am running a RAID1 as you say I should install the boot-image of lilo to my root partition because Install-Mbr checks for it there. My problem: My Root-Partition is a RAID5. I would like to boot from a RAID1 and get my RAID5 going from there.

Any hints?
Zeno

On Tue, 05 Oct 2004 13:08:19 +0400
Michael Tokarev <mjt@tls.msk.ru> wrote:

> [I'm getting quite some questions similar to this
>   one -- so instead of replying to every and each,
>   I'm posting to linux-raid@, to be able to refer to
>   this post in the future... ;)]
> 
> Zeno R.R. Davatz wrote:
> > Hi!
> > 
> > I am just reading a post of yours regarding using 'install-mbr' for linux-raid.
> > 
> > Something I do not understand about the post:
> > http://lists.debian.org/debian-testing/2004/04/msg00054.html
> > 
> > is following:
> > "mark your boot raid partitions active...". What do you mean by that?
> 
> Set the partitions (single partition on every disk) where your root raid
> device resides to be active with fdisk.
> 
> > Do you mean to mark all the first partitions of all the disks /dev/sd[abcd]1 as boot with fdisk?
> 
> Not necessary first ones, but it is simpler to have your root fs on
> first partition.  Again, it is the root-raid partition that should
> be active.
> 
> > In that case I would have to do install-mbr /dev/md0 --force
> 
> No, you would have to use boot=/dev/md0 in your lilo.conf,
> and install-mbr /dev/sd[abcd]1.
> 
> > Is that correct?
> 
> Basically, the scenario is as follows.
> 
> Standard mbr (master boot record) from mbr package (note lilo also
> have it, see lilo -M) is installed into standard place (where BIOS
> will expect it to be) into all your disks once (don't forget to
> install the same mbr when you plug new disk).  The mbr code (it
> resides on the first sector of the disk) works by reading partition
> table, finding partition marked as "active" (or "boot" -- the same
> flag but different terminology), loading boot record from that
> partition and executing it.  Mbr code is stable and you don't have
> to change it -- the first sector of your disks will never change.
> In contrast, you will do eg kernel upgrades and similar stuff,
> for which lilo boot tables needs to be refreshed, and that should
> be done on all disks (to be able to boot off any disk in case
> first one fails).  For this to work, you set up lilo to write
> it's boot record into the device where your root filesystem is --
> it is raid1 array created off all active (boot) partitions of
> all your disks.  Lilo writes boot record into the beginning of
> md0, and raid code propagates that boot record into your disks,
> all of them -- remember, md0 is composed of active partitions
> on all your disks -- this is exactly the place where mbr code
> will look for the bootloader.
> 
> So, you have the same mbr code on all your disks (installed
> once when you configure each disk), and "second-stage" boot
> record, installed by lilo into md0 and again propagated to
> all disks (into active or boot partition of each), which will
> be loaded by mbr -- this boot record will be updated -- on
> all disks -- when you re-run lilo.  In case any disk fails,
> you have the same boot code and sequence on every other disk,
> so you could boot off any working, non-failed disk.
> 
> But be warned -- boot (active) partition on every disk should
> be at the exactly same place, or else file offsets written by
> lilo will be valid for one disk but not valid for other.
> 
> /mjt
> 


-- 
Mit freundlichen Grüssen / best regards

Zeno Davatz
Verkauf & Akquisition

+41 1 350 85 86

www.ywesee.com > intellectual capital connected > www.oddb.org
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