Xen devel - Nov 2004 - which (pseudo)-device can be used as VBD in xenU ?

I saw examples of VBD using hdXY and sdXY inside virtual machines, like 
this : disk=phy:hda5,hda1,w

And I would like to do this : disk=phy:md8,md1,w

But it failed (the "real" md driver tried to access something that 
didn''t exist, of course, and rootfs couldn''t be mounted). Did
I do
something wrong, or is the frontend driver only able to "fake" hdXY
and
sdXY devices ?

FYI, my motivation is to supply mdX devices to ease migration from/to 
real servers.


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On Mon, Nov 29, 2004 at 05:59:41PM +0100, J?r?me Petazzoni
wrote:
> And I would like to do this : disk=phy:md8,md1,w
> 
> But it failed (the "real" md driver tried to access something
that
> didn''t exist, of course, and rootfs couldn''t be mounted).
Did I do
> something wrong, or is the frontend driver only able to "fake"
hdXY and
> sdXY devices ?
I think that''s about right.  Certainly there''s quite a lot
more
infrastructure that would need to be faked for an md device.
> FYI, my motivation is to supply mdX devices to ease migration from/to 
> real servers.
I would think that what you''d need to do here is to pass the
virtualized partitions (or whole disks?) to the guest and let md in the
guest assemble them into the RAID set.  I think that might work,
anyway...

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Jérôme Petazzoni wrote:
> I saw examples of VBD using hdXY and sdXY inside virtual machines, like 
> this : disk=phy:hda5,hda1,w
> 
> And I would like to do this : disk=phy:md8,md1,w
> 
> But it failed (the "real" md driver tried to access something
that
> didn''t exist, of course, and rootfs couldn''t be mounted).
Did I do
> something wrong, or is the frontend driver only able to "fake"
hdXY and
> sdXY devices ?
I guess that you can export _ALL_ the block devices that make your md8 
and create the md *in* the guest.

Exporting a "normal" block device (md8, from the host) is the same as 
exporting another "normal" block device, like sda2.
> FYI, my motivation is to supply mdX devices to ease migration from/to 
> real servers.
IMHO one of the biggest advantages of virtualization is hardware 
independence. You''ll loose (part of) that if you create your md inside 
the guest... At least it''ll be a pain to mantain...

Regards,
Nuno Silva


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> I saw examples of VBD using hdXY and sdXY inside virtual machines, like 
> this : disk=phy:hda5,hda1,w
> 
> And I would like to do this : disk=phy:md8,md1,w
> 
> But it failed (the "real" md driver tried to access something
that
> didn''t exist, of course, and rootfs couldn''t be mounted).
Did I do
> something wrong, or is the frontend driver only able to "fake"
hdXY and
> sdXY devices ?
> 
> FYI, my motivation is to supply mdX devices to ease migration from/to 
> real servers.
You should be able to export md1 just fine, but the blkfront
virtual driver won''t currently register itself as anything other
than sdX or hdX.

You should be able to do "phy:md8,sda1,w".

Is this good enough?

Ian


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>>And I would like to do this : disk=phy:md8,md1,w
> You should be able to export md1 just fine, but the blkfront
> virtual driver won''t currently register itself as anything other
> than sdX or hdX.
I''m having a look at the blkfront driver ; I guess I should be able to 
hack it easily to support registering with md major number. That might 
do the trick (even if it''s not very nice).

Question : can a non-privilegied host manage a SCSI HBA (thru PCI 
exclusions) or a IDE controller, and then directly mount SCSI or IDE 
disks ? Or would blkfront "shadow" those device numbers and prevent 
direct access to "real" disks ?


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> 
> >>And I would like to do this : disk=phy:md8,md1,w
> 
> > You should be able to export md1 just fine, but the blkfront
> > virtual driver won''t currently register itself as anything
other
> > than sdX or hdX.
> 
> I''m having a look at the blkfront driver ; I guess I should be
able to
> hack it easily to support registering with md major number. That might 
> do the trick (even if it''s not very nice).
> 
> Question : can a non-privilegied host manage a SCSI HBA (thru PCI 
> exclusions) or a IDE controller, and then directly mount SCSI or IDE 
> disks ? 
Yep, if its a separate PCI device it should work fine.
You''ll obviously need the kernel you''re using to have the
appropriate driver and e.g. scsi stack. The -xen0 kernel will
probably have what you want.

(this will be an io-privileged domain)
> Or would blkfront "shadow" those device numbers and prevent 
> direct access to "real" disks ?
The real device should get in before blkfront has a
chance. Hmm, I guess there could be a problem if you were using
drivers loaded by an initrd...

Ian


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