Question: How do I discard any changes for the device which is set by blockdev option?

[Masayoshi Mizuma]

On Tue, May 19, 2020 at 01:41:08PM -0500, Eric Blake wrote:
> On 5/19/20 12:56 PM, Masayoshi Mizuma wrote:
> > Hello,
> > 
> > I would like to discard any changes while the qemu guest OS is done.
> > I can do that with snapshot and drive option.
> > However, snapshot option doesn't work for the device which set by
> > blockdev option like as:
> > 
> > $QEMU --enable-kvm \
> >        -m 1024 \
> >        -nographic \
> >        -serial mon:stdio \
> >        -blockdev driver=file,node-name=mydisk,filename=/mnt/fedora.qcow2 \
> >        -blockdev driver=qcow2,node-name=vda,file=mydisk \
> >        -device virtio-blk-pci,drive=vda,bootindex=1 \
> >        -snapshot
> > 
> > I would like to use blockdev option to set the device because
> > libvirt uses blockdev option for disk element.
> > 
> > If there's no way to do so, does that make sense to get available
> > snapshot option to blockdev as well? If that makes sense, I'll try to
> > implement that.
> > 
> > As for qcow2, I think we can do such things to use qemu-img snapshot
> > command, for example save the original image and restore the image
> > after the qemu guest OS is shutdowned. However, it may be complecated
> > for user. I would like the simple way like as snapshot/drive option...
> > 
> > If I'm missing something, let me know.
> > 
> 
> Sounds like a repeat of this thread:
> https://lists.gnu.org/archive/html/qemu-devel/2020-01/msg06144.html
> 
> where the consensus is yes, -blockdev and -snapshot are incompatible,
> libvirt has plans to use the <transient/> tag to behave the same as what
> -snapshot does (but no one has implemented it yet), and in the meantime, it
> is possible to force libvirt to avoid -blockdev if you still need to supply
> -snapshot behind libvirt's back.

Thank you for the info! I didn't notice the thread.
I got we should implement that feature for libvirt side, not qemu.

Thanks!
Masa

> 
> -- 
> Eric Blake, Principal Software Engineer
> Red Hat, Inc.           +1-919-301-3226
> Virtualization:  qemu.org | libvirt.org
>