Re: md extension to support booting from raid whole disks.

[Daniel Reurich <daniel@centurion.net.nz>]

> 
> v1.x metadata has explicit knowledge of where the start of the data
> is, so it is quite possible to leave the first few (dozen) sectors
> unused (let's not talk about cylinders this century - OK?).
> So mdadm could grow a --grub flag to use with --create which arranged
> for data/bitmap to not use the first (say) 512 sectors of any device.
> (1.1 and 1.2 would still use reserved blocks for the superblock).
> [I can cut you a patch to experiment with if you like]

That would be nice.  The 1.1 superblock is no good as the bootsector
goes in the same place, and 1.2 is where I expect grub would be writing
data too.  I'll check this though.  Grub still needs patching to
understand v1.X superblocks, so that could include blacklisting the
location of a v1.2 superblock location and some for the write-intent
bitmap. (Is there a way to determine where the w-i bitmap gets located
and how big it is from the super block.)

I'd say put lets reserve the first 64K (2 cylinders) for boot and
superblock.


> grub could then write whatever it wants to write to any of these
> sectors.
> 
> That only leaves the question of what happens when a spare is added to
> the array - how does the grub data get written to the space on the
> spare.
> I would rather that grub were responsible for this, than for md to
> treat that unused space as RAID1.

Fair enough for the short term, but I imagine in the long run it would
be a better way then calling an external application.  Isn't this
already done for the w-i bitmap anyway?

> We already have a notification system based on "mdadm --monitor" to
> process events.  We could possibly plug grub in to that somehow so
> that it gets told to re-write all it's special blocks every time
> something significant changes in the array.

If mdmon can call external commands, it should only need to call the
appropriate grub-install [/dev/md/dX | "(md0)", and it would rewrite the
superblock on all the devices.

-- 
Daniel Reurich

Centurion Computer Technology (2005) Ltd
Ph 021 797 722