Description of problem: Please add ability to set snapshot mode for virtual harddisks. There is an excellent parameter of qemu, which can run virtual machine in mode, where all changes are stored in separate file and original image is not touched. This parameter can be well used by developers or testers, which need to test any software without reinstallation of his machine. User can install anything and then simply poweroff this virtual machine, after next boot everything will be back in original state. This can be done in qemu 2 ways: 1. add -snapshot parameter, which set snapshot mode for all harddisks (not best way) 2. add ",snapshot=on" for "-drive" parameter. By this way each partition can be set separately.
There are other options, which can be useful too, for example these: "snapshot=snapshot" snapshot is "on" or "off" and allows to enable snapshot for given drive (see -snapshot). "cache=cache" cache is "none", "writeback", or "writethrough" and controls how the host cache is used to access block data. "boot=boot" boot if "on" enables extboot for a given drive so it can be used as a boot drive.
Moving this to libvirt's upstream bug tracker - it's a common request, but there are no immediate plans to implement it (Not, cache= and boot= are already available through libvirt)
The XML is in place as of 0.9.5 (the <transient/> attribute of a <disk> in domain xml), but has not yet been wired up to qemu.
*** Bug 832194 has been marked as a duplicate of this bug. ***
the way #1 worked well until recently. The config part for it: > <qemu:commandline> > <qemu:arg value='-snapshot'/> > </qemu:commandline> But last upgrade of libvirt&qemu brought `-snapshot` option ignored by qemu. Hence would someone please re-triage?
qemu bug reported: https://bugs.launchpad.net/qemu/+bug/1860759
(In reply to ildar.mulyukov from comment #6) > qemu bug reported: https://bugs.launchpad.net/qemu/+bug/1860759 Per the qemu team, it is intentional (albeit poorly documented) that -snapshot only works with -drive, not -snapshot. Libvirt should be implementing support for the <transient/> tag per <disk> element; absent that, you may still be able to get your command-line passthrough hack of passing -snapshot by telling libvirt to use -drive instead of -blockdev. I'm not sure of the trick, but know that Peter made the choice between old or new configurable in case we encountered problems with -blockdev; this would be one such case to use the trick.
Eric, thanks for the hint. Excuse me, but Peter - who? How can we know more about the trick as we (users) are screwed in the situation when `<transient/>` isn't yet implemented and the old trick with `<qemu:arg/>` have no effect anymore? Thanks in advance!
See: https://libvirt.org/drvqemu.html#xmlnsfeatures <qemu:del capability='blockdev'/> should do the trick
Perfectly right, thanks a lot, Ján!
(In reply to ildar.mulyukov from comment #8) > Eric, thanks for the hint. Excuse me, but Peter - who? Sorry, Peter Krempa. But I see Jan answered in the meantime.
Support for <transient/> in QEMU was introduced in release 6.9.0 per https://www.redhat.com/archives/libvir-list/2020-October/msg00006.html