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Booting from Raid5 whole disk array.

From: Daniel Reurich
Subject: Booting from Raid5 whole disk array.
Date: Sat, 25 Apr 2009 01:08:13 +1200


I have been playing with booting from Raid5 and managed to hack together
a fix for the grub-setup to make grub install correctly on Raid5 disk
sets.  It's not pretty yet but it worked for installing onto a raid5
whole disk set and having it boot.  I will start creating a proper set
of patches to submit soon. 

This boots fine except for the partition table that the kernel sees on
1st disk is for the size of the whole raid volume, and complains about
trying to access beyond the end of the device until the kernels raid
driver starts up and assembles the array.

Another problem with booting of a raid 5 diskset is that if the first
disk is not available then we can't boot.  This is because the only
bootsector and partition table is on the first disk in the set.  (One
can recover it from the parity disk once we've got the degraded array
assembled, but we can't boot unless we can read the bootsector and
assemble the raid array.)

I have proposed the below solution to the linux-raid mailing list as a
solution for booting from software raid levels 4/5/6/10 using grub2.

The first cylinder which needs to be mirrored across all the devices,
would be for the Volume/Master Boot record + Boot Sector Code.
Grub2 bootsector + core.img should fit in here at (or least enough of it
to bring grub up with the appropriate raid drivers required to assemble
the array.)

We could include a dummy partition table with the whole disk in the 1st
partition labeled as something like linux-raid (0xfd) or Non-FS data

The second cylinder has the md superblock and write intent bitmap, and
the raid volume starts at the beginning of the 3rd cylinder, starting
with a normal mbr/partition table for this raid volume.

Let me know what you think of this scheme.

Daniel Reurich

Centurion Computer Technology (2005) Ltd
Ph 021 797 722

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