Xen users - Jun 2008 - how xen recognizes fibre channel based storage resources

Hello

I''m a newbie here so please don''t laugh at me...

I plan to setup several (4) physical servers to run xen virtualized 
OS-es (in example web servers). What I need is that those virtualized 
OS-es must have simultaneous access to disk storage physically created 
on FC array.

I read that this could be done by OCFS or Lustre file system but my 
question is: how virtualized OS (dom>0) recognizes FC resources.

If I understand well dom0 sees all as local disks: scsi disk in 
physicall server and connected via HBA fibre channel resource. And if I 
create dom1 partially (system part) located on local scsi disk and 
partially (shared part) on FC array - then how other domu''s located on 
other physical machines will see this shared ocfs-ed volume?

Is it an issue of HBA driver to make it possible?

I would be glad to get some kind of answer about this.


PS

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On Fri, Jun 6, 2008 at 3:17 PM, Piotr Szałamacha <piotr@horyzont.net>
wrote:
> If I understand well dom0 sees all as local disks: scsi disk in physicall
> server and connected via HBA fibre channel resource. And if I create dom1
> partially (system part) located on local scsi disk and partially (shared
> part) on FC array - then how other domu's located on other physical
machines
> will see this shared ocfs-ed volume?
you can share the same block device to several DomUs, just add 'w!' to
the options to say: "yes, write access to more than one".  if you use
only 'w', Xen would complain at the second DomU that tries to use it,
which is a reasonable safeguard when you don't use cluster-aware
filesystems.

-- 
Javier

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Hello
> you can share the same block device to several DomUs, just add
''w!'' to
> the options to say: "yes, write access to more than one".  if you
use
> only ''w'', Xen would complain at the second DomU that
tries to use it,
> which is a reasonable safeguard when you don''t use cluster-aware
> filesystems.
Thanks for your answer but if I understand well you assume in your 
answer that domu''s sharing block device are located on the same
physical
server while they are not.

I would like to create on four physical servers virtual machines that 
must to have simultaneous access to the same FC volume (blck device) - 
therefore no communication between domus0 will be established to 
synchronize writes as you suggests: "yes, write access to more than
one".

Is it possible to do something I''d like to do?

Thanks for your help.

Piotr
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On Saturday 07 June 2008, Piotr Szałamacha wrote:
> Hello
>
> > you can share the same block device to several DomUs, just add
''w!'' to
> > the options to say: "yes, write access to more than one". 
if you use
> > only ''w'', Xen would complain at the second DomU that
tries to use it,
> > which is a reasonable safeguard when you don''t use
cluster-aware
> > filesystems.
>
> Thanks for your answer but if I understand well you assume in your
> answer that domu''s sharing block device are located on the same
physical
> server while they are not.
sure, you mentioned FC, that usually means several boxes.

but that''s no problem, just do it.  the DomU''s would see the
block devices,
they won''t need FC drivers; but OCFS doesn''t need them, just
that the same
block device is seen by all machines (virtual or real) at the same time.

of course the heartbeat and lock propagation can go via the same block device, 
or (more common) via the network, but that''s part of setting OCFS, not
Xen
problem.

in short: Xen won''t get in your way, just publish the same block device
to
several DomU''s (on several boxes) and go!


-- 
Javier


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