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So I was running into an issue where our Ubuntu machines would not allow live external snapshots via libvirt virsh snapshot-create-as "" --diskspec vda,file= --disk-only. *Note the lack of the “–atomic” option, which is certainly recommended in a mission-critical environment, as it ensures the snapshot completely correctly and dies if it does not — this is due to running an older version of libvirt (0.9.8) because we’re using Precise Pangolin.

agoree@computing-node-3:/var/lib/libvirt/images$ virsh snapshot-create-as fedora-guest2 snap1-fedora-guest2 "1st virsh snap of fedora-guest2" --diskspec vda,file=/var/lib/libvirt/images/snap2-fedora-guest2.img --disk-only
error: internal error unable to execute QEMU command 'blockdev-snapshot-sync': An undefined error has ocurred

The error lead me to this bug report, along with these helpful blog posts.

I ended up simply creating this file:

agoree@computing-node-3:~$ cat /etc/apparmor.d/local/abstraction-libvirt-storage 
"/var/lib/libvirt/images/*" rw,

Then added an include for it in /etc/apparmor.d/libvirt/TEMPLATE, like so:

agoree@computing-node-3:~$ cat /etc/apparmor.d/libvirt/TEMPLATE 
#
# This profile is for the domain whose UUID matches this file.
#

#include <tunables/global>

profile LIBVIRT_TEMPLATE {
  #include <abstractions/libvirt-qemu>
  #include <local/abstraction-libvirt-storage>
}

After a reboot, all worked fine. Next up, using a newer libvirt version on Precise Pangolin for better live snapshot support (namely, “–atomic” option and “virsh blockpull” command to consolidate imgs/snaps).

At my current workplace we use libvirt and qemu/kvm running on the latest Ubuntu LTS release for our cloud platform. Recently, I wanted to use the awesome tool to simplify the provisioning of guest images. The configuration is for Fedora, but it can easily be done on Ubuntu.

First install the prerequisite packages:

apt-get install bash sed qemu-img virtinst cloud-utils util-linux genisoimage python-guestfs perl libguestfs-tools libguestfs-perl cloud-utils openssl util-linux

Then clone the git repo:

cd /opt
sudo git clone git://github.com/lzap/snap-guest.git
sudo ln -s /opt/snap-guest/snap-guest /usr/local/bin/snap-guest

You’re now good to go! From the snap-guest link (https://github.com/lzap/snap-guest):
“It’s recommended to use “base” string in the guest name (e.g. fedora-10-base or rhel4-base) to differentiate those files (snap-guest lists them using -l option), but it is not mandatory (option -a lists them all).”

I’d done this previously anyways, so was very easy in my case.

./snap-guest --list
./snap-guest -p /mnt/data/images --list-all

Visit the aforementioned link for further details.