On Fri, 22 Apr 2011 19:37:40 -0700 Dan Williams [off-list ref]
wrote:

On Sun, Apr 17, 2011 at 5:38 PM, NeilBrown [off-list ref] wrote:

quoted

As has been mentioned elsewhere, mdadm only recognised IMSM arrays on
machines with IMSM hardware.  I'm not entirely happy about this and may well
change it.

I have trouble answering the "least surprise" question in this area.

Is it more surprising to go into your BIOS, explicitly turn off raid
support and still see raid devices showing up?

Is the "RAID support has been explicitly turned off" state visible from a
running kernel? or is it indistinguishable from "platform does not have RAID
support"?

Or is it more surprising to take a raid array from a raid enabled
system to raid disabled system and wonder why things won't assemble?

For safety I think it is better if mdadm not perform operations that
might be incompatible with the platform option-rom.  But if you need
to recover to a usb attached drive, or some other
platform-incompatible configuration, you can use the environment
variable in a pinch.

There are 3 interesting cases:  create, assemble, examine.
(grow might be interesting too, but for now it would be confusing).

I am perfectly happy for 'create' to be arbitrarily hard if platform support
is not available.  One is unlikely to want to create an array in that case
anyway.
I think 'examine' should always show whatever it can, which is the case for
3.2.1.  Possibly it should also give a warning about  any difficulty that
might be experienced in assembling the array.

Assemble in the interesting case.  The law of least surprise requires it to
either work or give a good error message.  Your suggestion that it possibly
should not work in some cases seems defensible, so at least a very clear
error message would be good.
As for how to over-ride the default caution - I would prefer --force to
achieve it rather than requiring an environment variable.  I would possibly
accept --force-platform (or similar) but I think --force should be sufficient.

What think you?

Thanks,
NeilBrown
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