qemu-img Backing Files: A Poor Man's Snapshot/Rollback

I often like to formulate detailed steps when trying to reproduce a bug or a working setup. VMs are great for this because they can be manipulated easily. To manipulate their disk images I use qemu-img to create new disk images that use other disk images as a backing store. This is what I like to call a "poor man's" way to do snapshots because the snapshotting process is a bit manual, but that is also why I like it; I don't touch the original disk image at all so I have full confidence I haven't compromised it.

NOTE: I use QEMU/KVM/Libvirt so those are the tools used in this example:

Taking A Snapshot

In order to take a snapshot you should first shutdown the VM and then simply create a new disk image that uses the original disk image as a backing store:

$sudo virsh shutdown F21server Domain F21server is being shutdown$ sudo qemu-img create -f qcow2 -b /guests/F21server.img /guests/F21server.qcow2.snap
Formatting '/guests/F21server.qcow2.snap', fmt=qcow2 size=21474836480 backing_file='/guests/F21server.img' encryption=off cluster_size=65536 lazy_refcounts=off


This new disk image is a COW snapshot of the original image, which means any writes will go into the new image but any reads of non-modified blocks will be read from the original image. A benefit of this is that the size of the new file will start off at 0 and increase only as modifications are made.

To get the virtual machine to pick up and start using the new COW disk image we will need to modify the libvirt XML to point it at the new file:

$sudo virt-xml F21server --edit target=vda --disk driver_type=qcow2,path=/guests/F21server.qcow2.snap --print-diff --- Original XML +++ Altered XML @@ -27,8 +27,8 @@ <devices> <emulator>/usr/bin/qemu-kvm</emulator> <disk type="file" device="disk"> - <driver name="qemu" type="raw"/> - <source file="/guests/F21server.img"/> + <driver name="qemu" type="qcow2"/> + <source file="/guests/F21server.qcow2.snap"/> <target dev="vda" bus="virtio"/> <address type="pci" domain="0x0000" bus="0x00" slot="0x07" function="0x0"/> </disk>$
$sudo virt-xml F21server --edit target=vda --disk driver_type=qcow2,path=/guests/F21server.qcow2.snap Domain 'F21server' defined successfully.  You can now start your VM and make changes as you wish. Be destructive if you like; the original disk image hasn't been touched. After making a few changes I had around 15M of differences between the original image and the snapshot: $ du -sh /guests/F21server.img
21G     /guests/F21server.img
$du -sh /guests/F21server.qcow2.snap 15M /guests/F21server.qcow2.snap  Going Back To go back to the point you started you must first delete the file that you created (/guests/F21server.qcow2.snap) and then you have two options: • Again create a disk image using the origin as a backing file. • Go back to using the original image. If you want to continue testing and going back to your starting point then you will want to delete and recreate the COW snapshot disk image: $ sudo rm /guests/F21server.qcow2.snap
$sudo qemu-img create -f qcow2 -b /guests/F21server.img /guests/F21server.qcow2.snap Formatting '/guests/F21server.qcow2.snap', fmt=qcow2 size=21474836480 backing_file='/guests/F21server.img' encryption=off cluster_size=65536 lazy_refcounts=off  If you want to go back to your original setup then we'll also need to change back the xml to what it was before: $ sudo rm /guests/F21server.qcow2.snap
$sudo virt-xml F21server --edit target=vda --disk driver_type=raw,path=/guests/F21server.img Domain 'F21server' defined successfully.  Committing Changes If you happen to decide that the changes you have made are some that you want to carry forward then you can commit the changes in the COW disk image into the backing disk image. In the case below I have 15M worth of changes that get committed back into the original image. I then edit the xml accordingly and can start the guest with all the changes baked back into the original disk image: $ sudo qemu-img info /guests/F21server.qcow2.snap
image: /guests/F21server.qcow2.snap
file format: qcow2
virtual size: 20G (21474836480 bytes)
disk size: 15M
cluster_size: 65536
backing file: /guests/F21server.img
$sudo qemu-img commit /guests/F21server.qcow2.snap Image committed.$ sudo rm /guests/F21server.qcow2.snap
\$ sudo virt-xml F21server --edit target=vda --disk driver_type=raw,path=/guests/F21server.img
Domain 'F21server' defined successfully.


Fin

This backing file approach is useful because it's much more convenient than making multiple copies of huge disk image files, but it can be used for much more than just snapshotting/reverting changes. It can also be used to start 100 virtual machines from a common backing image, thus saving space...etc.. Go ahead and try it!

Happy Snapshotting!
Dusty