[libvirt] qemu and <transient/> disks
Partly following up this message:
https://www.redhat.com/archives/libvirt-users/2011-October/msg00142.html
> And what about qemu's option "snapshot=on" ?
Unreliable. It won't work with SELinux (since qemu tries to create the
snapshot on /tmp), and it makes your guest unmigratable. I think that
it is easier to have libvirt use qemu-img to create a qcow2 wrapper
prior to booting the guest, at which point the solution is easier to
control from an sVirt perspective, and is more likely to allow us to
figure out a way to make things work with migration (at least, I'm
hoping I can figure out how to migrate a guest with a transient disk).
Lack of this feature is pretty annoying, particularly since
(a) libguestfs needs it and (b) it's a trivial patch to add
snapshot=on to the qemu driver. So to concentrate on the two
objections above:
- Why is SELinux concerned about qemu creating and using a file in
/tmp? Obviously it should stop qemu opening and reading random
files from /tmp but that would be a different rule surely?
- The documentation actually states that using <transient/> may make
your guest unable to migrate, and in any case we don't care.
But if you want to use qemu's snapshot=on in the meantime, you
can
use the <qemu:commandline> namespace XML to add it.
Is there an example how to do this? It seems like it might be
possible using a global qemu '-set device.<id>.snapshot=on' parameter,
but how can I know what <id> libvirt will give to a '-drive'
parameter? (I found from experimentation that it's something like
"drive-virtio-disk0", but I can't track down the bit of code that
produces this string yet ...)
Rich.
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming blog: http://rwmj.wordpress.com
Fedora now supports 80 OCaml packages (the OPEN alternative to F#)
http://cocan.org/getting_started_with_ocaml_on_red_hat_and_fedora