Re: Root Drive Mirroring and LVM.

[Neil Brown <neilb@cse.unsw.edu.au>]

On Tuesday January 27, atossava@cc.helsinki.fi wrote:
> Sorry about the crossposting.
>
> I wrote on the Yellow Dog Linux list when somebody asked about software
> RAID on YDL about my experiences with it:
>
> >> The one really big gotcha is that the Macintosh partitioning scheme
> >> can't tell the Linux kernel that certain partitions are to be
> >> considered "Linux RAID autodetect" (as in x86 using the DOS partition
> >> table type 0xfd). This means that you can't boot a Mac Linux system
> >> directly from RAID because the kernel won't be able to autostart the
> >> RAID devices. You have to work around this by creating an initial RAM
> >> disk that uses the raidstart command to start your metadevices, then
> >> swaps the initrd out of the way and proceeds to start the real system.
>

This is not entirely true.  Certainly an initial-ram-disk is one
solution and is (I think) the preferred long-term solution. However
you can also boot from raid with kernel-parameters like:

   md=0,/dev/hda1,/dev/hdc1 boot=/dev/md0

where '0' indicated which md device (md0 in this case), and the
remaining words are the devices to assemble it from.


> to which Tim Seufert replied on the same list:
>
> > Hmmm.  That would seem to be a lack in the Linux RAID code, since the
> > Macintosh partition table has a vastly more flexible partition type
> > field than DOS: instead of a single byte it's a string.  It would mean
> > breaking from the convention of using the "Apple_SVR2_UNIX" type for
> > Linux partitions, but that really is just a convention as far as I know.
>
> Perhaps the PPC Linux developers and the Linux RAID developers should
> get together on this and make some decisions so as to make it happen.
>

I personally think auto-detect is the wrong approach and have no
desire to extend it to other partition types (I cannot remove it from
DOS partitions as that breaks back-compatability).
Just use "md=..."

NeilBrown

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