-----Original Message-----
From: Phillip Susi [mailto:psusi@ubuntu.com]
Sent: Tuesday, January 15, 2013 3:57 PM
To: Patelczyk, Maciej
Cc: Tomczak, Marcin; neilb@suse.de; linux-raid@vger.kernel.org; Dorau,
Lukasz
Subject: Re: [PATCH] imsm: Forbid spanning between multiple
controllers.

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On 1/15/2013 5:31 AM, Patelczyk, Maciej wrote:

quoted

Hi Phillip,

Mdadm does not care about the controller unless you created IMSM
based RAID. Basically you can create that type of RAID *only* on
Intel based platforms with OROM enabled. It's Intel solution, we
support it and we maintain it. It's very specific type of metadata.
We provide this functionality in Linux, Windows and OROM/uEFI. It
must be compatible between all three environments. If you have dual
boot machine you can safely boot into Windows and then into Linux
and your RAID Volume will be in proper state.

quoted

Dmraid cannot boot from RAID Volume, OROM can boot from IMSM RAID
Volume directly. You don't need separate partition, hard drive or
anything else. If you run Intel platform you can boot directly
from supported RAID Volume. This is because OROM supports it.
Mdadm respects OROM restrictions when creating IMSM based RAIDs.

I understand that you will not be able to boot directly from the array
without the OROM, but sometimes you just need to access your data any
way you can, and mdadm should not refuse to mount the array based on
the absence of the OROM.  Think disaster recovery.  You should always
be able to connect the disks to another machine that may not have an
Intel controller and rescue your data.

Additionally the ability to create and use arrays without the OROM
allows for simulation and testing, which is something the mdadm test
suite should be doing.

I agree. That is why you have IMSM_NO_PLATFORM flag.
Just do:
$export IMSM_NO_PLATFORM=1
$mdadm ...

And it should work as you mentioned.
Right now this option is described in manual.

quoted

Spanning in Linux is something obvious. I know that is simply
works because of Linux architecture. However spanned RAID Volumes
are not supported in OROM and in Windows. If you allowed for
spanned RAID Volumes in Linux we open the Pandora's box. In the
worst case you will lose you data when entering to OROM (OROM will
see only one set of disks attached to one controller and can mark
RAID Volume as Failed) or if you boot to Windows (Windows driver
will see two failed RAID Volumes in the worst case). In other case
you will have RAID Volume marked as Degraded and rebuild will
start. And what if you create 'bootable' RAID Volume? Well you may
not be able to boot from it when it's spanned.

Warnings about potentially troublesome situations are good, but
outright refusal is not.  Yes, I realize it would be a problem for
Windows due to the poor way the driver has to be implemented ( why
can't the OROM see other disks on other controllers? ), but sometimes
you don't care about that.  For instance, if you are setting up the
array on one machine where you can not connect all of the drives to
the same controller ( and do not care about booting from the array on
this machine ), but you are planning on moving them to a machine where
they will be.  This is just one example of many situations where you
need to be able to say "I know what I'm doing, go ahead anyway".

Of course I agree in general. This is standard Unix way to do it.
I'm root and I take full responsibility. I think that IMSM_NO_PLATFORM
should handle spanning too. I will discuss it internally.

Errr..., "I know what I'm doing, go ahead anyway" - well not quite so...
Do you really know the target platform OROM's capabilities? Do you?

When creating or assembling IMSM RAID mdadm looks for OROM and tries to 
determine its capabilities. You can look at the code (like platform-intel.h).
When you create RAID Volume directly on target machine you're safe. Mdadm
will prevent you from creating OROM incompatible RAID. If you create the
RAID Volume with IMSM_NO_PLAFTORM flag or with different OROM you may be
in situation is which some capabilities in one OROM will not be present in
other OROM and then RAID Volume will not be assembled. Target OROM may not
support given RAID Level, Strip Size or some other capability. Yes I know that is
not nice, but this is how it works. There are many platforms, some changed by
Vendors and we don't control all of it. If you create RAID with IMSM_NO_PLATFORM
it's even worse ;-)
So it's not so obvious with this 'go ahead' method.
Of course you can always run mdadm --detail-platform on target platform and check
if you're creating OROM compatible RAID Volume. 


maciej