How to start a new instance of QEMU based on the same image and snapshot

kvm-virtualizationlibvirtqemu

I have a QEMU image (qcow2) with a snapshot stored in it. Right now I'm using libvirt to start it.

However, I want to be able to run more than one instance of the same image snapshot.

I guess I can do that by cloning the virtual-hd and installing/creating a new domain (virsh) and then running revert from snapshot. But I want to be able to do that pretty much "on-the-fly" with as little as possible latency from the time I decide I need to run another instance of image X to the time that instance is running from the stored snapshot. (I wan't to avoid writing to the hard-drive as possible)

Anyone did anything like that? I started thinking maybe libvirt is not low-level enough for this?

Best Answer

Libvirt supports this very well. Create one disk image that has everything you want to share between your domains. This will be your read-only backing file. When creating volumes for new domains, set backingStore to refer to it. The new volume you're creating for each domain will start out taking zero bytes but grow as you make changes.

Related Solutions

Libvirt and qemu/kvm -snapshot option

The only way to add command-line switches that libvirt doesn't support yet is to create a wrapper script and change your VM's configuration to use it instead. For example,

# cat >/usr/local/bin/qemu-snapshot <<'END'
#!/bin/sh
exec /usr/bin/qemu "$@" -snapshot
END
# chmod +x /usr/local/bin/qemu-snapshot
# virsh -c qemu:///system edit my_vm
change
    <emulator>/usr/bin/qemu</emulator>
to
    <emulator>/usr/local/bin/qemu-snapshot</emulator>

(It might be /usr/bin/kvm or something like that for you.)

Linux – mount qcow2 snapshots

You should apply the snapshot and mount the image like always. It's better to create a snapshot right before applying if you want to turn back to the original disk contents before you do these operations. After you apply the snapshot you just mount the image with qemu-nbd as always:

# modprobe nbd max_part=8
$ qemu-img snapshot -c backup image.qcow2
$ qemu-img snapshot -a old_snapshot image.qcow2
$ qemu-nbd --connect=/dev/nbd0 image.qcow2
# mount /dev/nbd0p1 /mnt

You might want to save snapshot separately and mount it as another disk drive. For this you should use convert comand for qemu-img.

$ qemu-img convert -s old_snapshot image.qcow2 old_snapshot.qcow2
$ qemu-nbd --connect=/dev/nbd0 old_snapshot.qcow2
# mount /dev/nbd0p1 /mnt

See man qemu-img(1) and qemu-nbd(1) for more details.

Best Answer

Related Solutions

Libvirt and qemu/kvm -snapshot option

Linux – mount qcow2 snapshots

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