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Re: [Xen-users] Using CD drives with Xen 3.0.3 on CentOS 4.4

To: mark.williamson@xxxxxxxxxxxx
Subject: Re: [Xen-users] Using CD drives with Xen 3.0.3 on CentOS 4.4
From: Nico Kadel-Garcia <nkadel@xxxxxxxxx>
Date: Thu, 28 Dec 2006 15:49:49 +0000
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M.A. Williamson wrote:
I'd like to be able to use a CD drive on the Dom0 to be accessible on the DomU systems. I do see some notes involving cdrom drives in the Xen 3.1 Enterprise notes, and in the VT notes, but not for systems without that on-board virtualization feature. And the syntax of the cdrom command options are documented nowhere in 3.0.3.

You can export the CD-ROM drive to the guest by using the file: directive, as you would for other disks. This will work for iso files and for /dev/cdrom

e.g. something like

disk = [ 'file:/dev/cdrom,hdd,r', <your entries for other disks go here> ]
Hmm. That seems only fully workable for already mounted drives, but I'll check it out.

I am amused and confused by the use of 'file:/var/lib/xen/images/file.img,hda,w ' as opposed to 'file:/var/lib/xen/images/file.img,hda1,w' and the fascinating partition adventures involved. It could also use some documentation. Having to deduce that sort of thing from scratch is asking for pain and work out the tradeoffs for yourself is asking for pain.
will give the domain access to the contents of the real physical CD-ROM drive. Note that you'll want to have the CD-ROM you want to use in the drive when the domain boots, otherwise it'll confuse the guest's probing of disk size. Using an ISO file is probably more straightfoward and convenient. If you want to change ISO file you should be able to use xm block-detach and xm block-attach to add a new ISO to the guest. If you change the CD-ROM in an exported physical drive, I'd suggest doing xm block-detach, then changing the CD, then xm block-attach, so that the guest knows something has changed.
ISO files are great, but wind up taking space on your Dom0 to build. And creating them from DVD's becomes even more burdensome, and *burning* CD's or DVD's from the guest domain would be even more awkward.
Note that the Xen paravirtualised block drive doesn't provide full CD-ROM emulation: you should be able to mount the filesystem, but you won't be able to treat it entirely like a CD drive because some of the special operations will not be supported (eject springs to mind, but there are other CD-ROM specific operations).
Yeah, that's why I hadn't even considered the file: approach. I may have to use it after all.


Hopefully the above method will be sufficient from your needs. I run my development machine on CentOS 4.4 (albeit running xen-unstable built from source for my purposes) so can try to replicate and resolve any specific problems you run into.

Hope that helps some. You're quite right that this sort of thing should be documented more thoroughly. Since the next release of RHEL will include Xen, hopefully CentOS (5) integration will be an improvement - sorry you had problems with the existing packages.

Thanks. How are you downloading the unstable source?

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