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

["M.A. Williamson" <maw48@xxxxxxxxx>]

> > 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.
That's the physical device file, so you won't need to actually mount the 
drive. You will need to have the disk in the drive before you start the 
domain / block-attach it, otherwise the size probing may go wrong :-(
> 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.
It is a bit "unconventional", to say the least!

Just to clarify, it's perfectly valid to export as a whole device (e.g. 
'file:/var/lib/xen/images/file.img,hda,w ') as you would on other virtual 
machine systems. The ability to export as a partition (e.g. 
'file:/var/lib/xen/images/file.img,hda1,w ') is provided for ease of 
management: you don't really *need* to have a partition table in a paravirt 
guest, and it's nice to simply be able to work with individual guest 
filesystems when configuring from dom0, rather than having to deal with 
accessing subranges of a guest disk file.
> > 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.
I'm afraid you're not going to be able to burn and optical disks from 
either paravirt or HVM domains for the time being :-(
You *could* add a second IDE controller card to your system, export that to 
a guest via the PCI passthrough and then use that to drive a CD writer, but 
this would be rather heavyweight and imply trust of the guest OS.
When USB virtualisation is working (it may already work in HVM) it should 
be possible to pass a USB burner through to the guest, and that would 
enable this kind of use.
> > 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.
Afraid so, sorry :-( For many uses (e.g. just grabbing some files from the 
disk), it should work OK. But you can't just boot off a CD-ROM in a PV 
domain, nor can you place CD music, etc.
> Thanks. How are you downloading the unstable source?
I get mine using mercurial:
hg clone http://xenbits.xensource.com/xen-unstable.hg

I'd generally recommend using a release version for production systems, 
3.0.4 being the latest and (hopefull) greatest. 
http://xenbits2.xensource.com/xen-3.0.4-testing.hg should contain 3.0.4 + 
bugfixes that are being tested to be rolled up into the next 3.0.4.x 
update.
There will be tarballs of 3.0.4 linked off one of the Xen homepages, once 
they have been updated. Building your own Xen is reasonably straightforward 
once you know your way around the components, and there should be various 
HOWTOs hanging around.
Hope that helps. If you still have problems with the CD-ROM we can try and 
find a workaround for you.
Cheers,
Mark

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