To start multiple KVM guests from one qcow2 image with transient disk option

[Peter Krempa]

On Mon, Jan 04, 2021 at 15:30:19 -0500, Masayoshi Mizuma wrote:
> On Sat, Dec 19, 2020 at 11:30:39PM -0500, Masayoshi Mizuma wrote:

[...]

> I think following qemu command line options and QMP commands work for sharing
> the qcow2 disks. The following uses disk hotplug instead of snapshot overlay.
> Does that make sense for libvirt...?
> 
> qemu command line options:

So you are proposing to ...

> 
>   qemu-system-x86_64 \
> 	-M q35,accel=kvm,usb=off,vmport=off,smm=on,dump-guest-core=off \
> 	-smp 1 \
> 	-m 4096 \
> 	-blockdev '{"driver":"file","filename":"/home/mmizuma/debug/guest.qcow2","node-name":"storage1","auto-read-only":true,"discard":"unmap"}' \
> 	-blockdev '{"node-name":"format1","read-only":true,"driver":"qcow2","file":"storage1","backing":null}' \

... start with the disk already in 'read-only' mode _and_ skip addition
of the disk ...

> 	-nographic \
> 	-nodefaults \
> 	-no-user-config \
> 	-serial telnet::10000,server,nowait \
> 	-qmp tcp::10001,server,nowait \
> 	-S \
> 	-device pcie-root-port,id=pci.1
> 
> QMP commands:
> 
>   {"execute":"qmp_capabilities"}
>   {"execute":"blockdev-add","arguments":{"driver":"file","filename":"/var/lib/libvirt/images/guest.TRANSIENT","node-name":"storage2","auto-read-only":true,"discard":"unmap"}}
>   {"execute":"blockdev-create","arguments":{"job-id":"create","options":{"driver":"qcow2","file":"storage2","size":4294967296,"cluster-size":65536,"backing-file":"/var/lib/libvirt/images/guest.TRANSIENT","backing-fmt":"qcow2"}}}
>   {"execute":"blockdev-add","arguments":{"node-name":"format2","read-only":false,"driver":"qcow2","file":"storage2"}}

... and then add a writable overlay ...

>   {"execute":"device_add","arguments":{"driver":"virtio-blk-pci","drive":"format2","id":"transient-disk","bootindex":1,"bus":"pci.1","addr":0}}

... and hotplug the disk.
>   {"execute":"cont"}

So that is a no-go. Some disk bus-es such as IDE don't support hotplug:

https://gitlab.com/libvirt/libvirt/-/blob/master/src/qemu/qemu_hotplug.c#L1074

You could try to just instantiate the backend of the disk as read-only,
and then create a writable overlay. You just need to make sure that the
disk will be writable and that it works even for IDE/SATA which doesn't
support read-only:

https://gitlab.com/libvirt/libvirt/-/blob/master/src/qemu/qemu_validate.c#L2634