I’ve been using my laptop for development for a while now (Fedora, of course, and we’re at 19 now), but the trouble is – several times without paying much attetention I let battery run dry on it while VMs are running. In KDE it would normally result in automatic graceful shutdown, under LXDE however all you get is a notice and then whatever happens – happens. First time I’ve learned to disable caching on my “images” partition:
$ grep libvirt /etc/fstab
/dev/mapper/vg_delliquent-libvirtfs /var/lib/libvirt ext4 defaults,sync,dirsync 1 2
That kind of helped with me using SSD and other things – I have eliminated double-caching of FS transactions and now my images should survive a bit of “oopsie”. BUT: I still don’t want to leave it to a chance and loose several month of my work inside those VMs. So I needed impromptu backup solution that is portable. That’s where QEMU/QCOW2 external snapshots come handy. First we need to lay out images, and so we do:
# qemu-img create -f qcow2 -b vm1-base.qcow2 vm1-staging.qcow2
# qemu-img create -f qcow2 -b vm1-staging.qcow2 vm1-running.qcow2
Great, now we have a chain of images like so: vm1-base <- vm1-staging <- vm1-running. Our libvirt VM definitionwill use that last image: vm1-running .
# virsh edit vm1
... replace reference to vm1-base.qcow2 with vm1-running.qcow2 ...
Now we’re running off snapshot and whatever happens both vm1-base and vm1-staging will not be modified and those shouldn’t be affected if we have a power failure etc.
On with the libvirt! Under /etc/libvirt we need a directory called “hooks“:
# mkdir -p /etc/libvirt/hooks
now we create qemu hook:
$ cat > /etc/libvirt/hooks/qemu
#!/bin/sh
guest_name=$1
state=$2
phase=$3
other=$4
RESNAP_SCRIPT=/usr/local/sbin/resnap.sh
date >> /tmp/libvirt.log
echo "$0 $@" >> /tmp/libvirt.log
if [ -e "$LOCK_FILE" ]
then
# we've been asked to skip resnapping. politely exit now
echo "Lock found: $LOCK_FILE . Skipping resnap..." >> /tmp/libvirt.log
exit 0
fi
if [ $state == "release" -a $phase == "end" ]
then
# 1. grab qemu image name
# 2. create a clone/snapshot
## http://libvirt.org/hooks.html#names
## "...A hook script must not call back into libvirt,
## as the libvirt daemon is already waiting for the script to exit."
# very crude attempt to sidestep "virsh dump_xml"
vm_definition_xml=$(grep -Fl "<name>$guest_name</name>" /etc/libvirt/qemu/*.xml)
source_image=$(xmllint --xpath 'string(/domain/devices/disk/source/@file)' $vm_definition_xml)
image_type=$(file -b $source_image | awk '{print $1, $2;}')
echo "$source_image $image_type" >> /tmp/libvirt.log
if [ "$image_type" == 'QEMU QCOW' ]
then
( date; echo "$RESNAP_SCRIPT $source_image" ) >> /tmp/resnap.log
$RESNAP_SCRIPT $source_image
fi
fi
^D
And if we really need to skip re-snap we just do:
$ touch /tmp/qemu_vm1.lock
Now a small matter of getting RESNAP_SCRIPT done 😉
# cat > /usr/local/sbin/resnap.sh
#!/bin/sh
image=$1
base_img=$(qemu-img info --output json $image | python -c 'import simplejson,sys; i=simplejson.load(sys.stdin); print i.get("full-backing-filename",i.get("backing-filename","ITSABASE"))')
if [ "$base_img" == "ITSABASE" ]
then
exit 1
fi
qemu-img commit $image
mv $image $image.commited
qemu-img create -f qcow2 -b $base_img $image
Most likely you’ll need to restart libvirtd (I had to, anyway):
systemctl restart libvirtd.service
and we’re all set.
End result will be: whenever I shutdown a VM – hook script will kick in and commit all the changes to “vm1-staging.qcow2”, then create a backup copy of my commited changes after which it will create brand new snapshot vm1-runnint.qcow2 for me to use next time I start up vm1.
Couple of notes:
- depending on requirements resnap.sh script could be easily modified to create chain of commits that could be commited instead of committing the “vm1-staging” to the base, like so ” commit vm1-running.qcow2.commited.1; commit vm1-running.qcow2.commited 2 …” this should work if you want to have backup to your backup and if you’re going to watch your disk usage carefully as those do add up.
- resnap.sh will kick in for any QCOW2 image, it is smart enough to bail on base images but it will commit any snapshots back to whatever is next up the “backing-chain”, so be warned. To avoid this – extend the script and have either a whitelist or a blacklist of VMs you’ll be starting up.
- we create files in /tmp and not /var as my intent was to not have those files around upon reboot.
