Re: To start multiple KVM guests from one qcow2 image with transient disk option
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#...
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...