Re: [libvirt-users] How to burn a CD using libvirt?
On Wed, Mar 25, 2015 at 05:02:33PM -0500, Steve Ross wrote:
Hi,
My first e-mail... my apologies if this is not the appropriate e-mail list.
You did, sorry for the super-late response :)
My goal is to *write* from the guest OS to a CD or DVD reader/writer
on
my host system. Currently, the write operation is failing. I believe
it is because the virtual CD-ROM device is set to be "readonly". Is it
possible to write to a QEMU CDROM device and, if so, how? (The host
machine's CD-ROM drive does support writing/burning.)
I'm using the "virt-manager" GUI to start the virtual machine. Once
started, the "qemu" command line from my virtual machine process
includes the following, where the host's built-in CD reader/writer is
"/dev/sr0".
/usr/bin/qemu-system-x86_64 ... -drive
file=/dev/sr0,if=none,id=drive-ide0-0-1,readonly=on,format=raw
This obviously says that the device is "readonly".
A snippet of XML from my virtual machine from the CDROM is:
<domain type='kvm'>
...
<devices>
<disk type='block' device='cdrom'>
<driver name='qemu' type='raw'/>
<source dev='/dev/sr0'/>
<target dev='hdb' bus='ide'/>
<readonly/>
<address type='drive' controller='0' bus='0'
target='0' unit='1'/>
</disk>
Libvirt adds that automatically to cdrom devices.
I've tried removing the <readonly/> tag from the above VM definition,
and then running the VM, but the tag magically reappears in the VM
definition when it runs.
Also, the "virt-manager" GUI shows the CDROM device as having the option
of "Readonly" being checked (and the GUI does not allow me to uncheck it.)
And this is for the same reason -- libvirt adds it by default.
On the root cause as to why even libvirt does that -- I was thinking
about few scenarios and I came to only one unclear conclusion. There
might have been historical causes etc., but I think there's a safety
feature smell behind this. If you pass it like this, it won't be
opened exclusively and you might get to a problem where your burned CD
is not working, although this should be handled by the device itself.
Anyway, after thinking about how to fix it I realized there is a
workaround for this. You just need to pass-through the CD writer as a
device. QEMU will have exclusive access and all the ioctl()s and
fcntl()s will for sure work in the guest.
Hope that helps at least a bit.
Have a nice day,
Martin
Other details:
* qemu version 2.1.3 on Fedora 21.
* libvirt version 1.2.0.2
Thanks for any help or pointers you can provide,
-- Steve
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