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Re: [Xen-users] using SAN tape library in domU?

To: xen-users@xxxxxxxxxxxxxxxxxxx, Sebastian Reitenbach <sebastia@xxxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-users] using SAN tape library in domU?
From: Mark Williamson <mark.williamson@xxxxxxxxxxxx>
Date: Wed, 12 Dec 2007 15:03:33 +0000
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> I've several virtual PVM domains, running on Linux. I have a san attached
> to the dom0, and the tape is presented there, to not stress the network so
> much.
> I found this faq entry:
> http://www.cyberciti.biz/faq/xen-vps-guest-use-tape-device/
> There is no date mentioned, so I'd like to ask whether this is still true?
> Well, I assume yes, but just wanted to make sure.
> In case I cannot use it, is there a recommendation to somehow work around
> that limitation?

The only way you could share the tape library between multiple domUs is to 
export it from dom0 over the virtual network.

If you have a specific domU you want to access the drive then you could pass 
the entire SAN adaptor through to it using PCI passthrough but then only that 
domU would have access to the SAN (you can share a PCI device between 
domains).  Also, unless you have IOMMU capable hardware (most people don't) 
then you would have to trust that domU as much as dom0 and the domU would 
have to be PV.  Sorry for the complicated restrictions, it'll get less 
confusing as IOMMU hardware gets more popular.

In principle, you could put multiple SAN adaptors in the machine and dedicate 
one to each domU, I suppose.  It'd still be subject to trusting the PV domUs 
equally to dom0.

There's some work underway to pass SCSI devices through from dom0 to domUs, 
which perhaps could eventually do want you want.  In the meantime, I guess 
I'd recommend for simplicity that you look at a network-based approach as you 
would do if the dom0 and domUs were separate physical hosts.


Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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