Re: qemu-img snapshots with backing stores?
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On 06/06/2012 10:55 AM, Sean Abbott wrote: > So, I was attempting to use qemu snapshots with backing stores. The > QEMU docs (http://wiki.qemu.org/Documentation/CreateSnapshot) make it > sound like you simply point your qemu at the snapshot after it's > creation, and you're golden. > > When attempting this with libvirt, though, it fails. Libvirt definitely supports this, as I use it for my guests, so let's figure out where you went wrong. By the way, libvirt can create qcow2 files itself, rather than forcing you to hand-create it with qemu-img, although support for this could probably be improved with more APIs and documentation. Patches welcome. > > I created a snapshot using the above tutorial. the resulting file is > disk.0, and a qmeu-img info on it returns: > > image: disk.0 > file format: qcow2 > virtual size: 29G (31457280000 bytes) > disk size: 140K > cluster_size: 65536 > backing file: /var/lib/one/public/lin_client_current.qcow2 (actual path: > /var/lib/one/public/lin_client_current.qcow2) > > So that all looks groovy, right? Unfortunately, 'qemu-img info' output doesn't say whether you properly populated the backing_fmt property, but I will assume that is not your issue (do note, however, that failure to use the backing_fmt property is a security hole - it means libvirt and/or qemu will autoprobe the format from the backing file itself, but if the backing file is supposed to be raw, the guest can manipulate the backing file into looking non-raw, and cause your host to hand over control of files to the guest that should not normally be accessible to the guest). > > Then, I created (via opennebula) an xml deployment file like so: > http://paste.ubuntu.com/1027145/ which included: <disk type='file' device='disk'> <source file='/var/lib/one/vm/56/images/disk.0'/> <target dev='hda' bus='virtio'/> <driver name='qemu' type='qcow2' cache='none'/> and that looked correct to me. > > When I attempt to do a virsh create, I get the following errors: > > virsh # create deployment.0 > error: Failed to create domain from deployment.0 > error: internal error process exited while connecting to monitor: > file=/var/lib/one/vm/56/images/disk.0,if=none,id=drive-virtio-disk0,boot=on,format=qcow2,cache=none > qemu-kvm: boot=on|off is deprecated and will be ignored. Future versions > will reject this parameter. Please update your scripts. This warning is not the real problem, but a patch to libvirt to avoid it might be nice, if it hasn't already been patched in newer libvirt. > qemu-system-x86_64: -drive > file=/var/lib/one/vm/56/images/disk.0,if=none,id=drive-virtio-disk0,boot=on,format=qcow2,cache=none,boot=on: > could not open disk image /var/lib/one/vm/56/images/disk.0: Invalid argument You mentioned Ubuntu - do you have appArmor running? This could be a case of the apparmor settings on your machine preventing qemu from opening the backing file. I don't have Ubuntu experience myself to tell you how to resolve it (I tend to work with SELinux on Fedora as my security mechanism), but suspect that it might be a failure along the lines of an over-strict security policy. > > So...something isn't working. Is it possible to do this, or should I > give up on this path? Libvirt definitely supports what you want to do, but I don't know what to suggest to help you get further. -- Eric Blake eblake@xxxxxxxxxx +1-919-301-3266 Libvirt virtualization library http://libvirt.org
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