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[dm-devel] Multipath on root

Hi All,

I write as I have some questions about dm-multipath, and there seems to be
a dearth of good documentation available. To make things worse the
official Wiki even contains self-conflicting information (compare:
http://christophe.varoqui.free.fr/wiki/wakka.php?wiki=FAQ """supports root
FS on multipathed SAN""" vs
http://christophe.varoqui.free.fr/wiki/wakka.php?wiki=UsageFile """DM-MP
cannot be run on either the root or boot device.""").
I figured what better place to get answers than from the experts involved
in actually developing the system :)

What I would please like to get a clear understanding of is:

1) What are the software components and dependencies involved in the bare
necessities of making dm-multipath work? I've reviewed the architectural
diagrams (which are quite good), but am looking more specifically for
answers to things like 'is there actually a hard technical necessity for
udev in order to actually make dm-multipath to work, or is its presence
more an artifact of the assumptions built-in to overall design of the

2) Why exactly can't dm-multipath be used to multipath a root filesystem?
What are the dependencies that can't be met by initrd? Why is dm-multipath
different in this regard to any other block driver, eg md or LVM?
2) What exactly, specifically is required in order to have root on a
multipath device? A high-level overview would probably be more useful than
nitty-gritty of what entry to put in what file, at this point.

I'm guessing that a solid answer to (1) above will probably answer (2) in
the process.

The reason for the request is that the seemingly desirable and obvious
option of booting from a SAN on Linux is pretty much worthless if
multipathing can't be used for root - your system is less safe than
booting from some mirrored internal disks...

Thanks in advance for any and all assistance and answers on this,

Darryl Dixon

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