2.5. Creating Disks with Backing Images
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2.5. Creating Disks with Backing Images

A few years ago I found out about backing images (or “base-images”)[17]. Back then I was doing lots of development on host provisioning tools and needed to be able to quickly revert machines I was working on to a desired initial state. In this use case backing images were especially handy when working on features that frequently destroyed the machine if it didn't work right.

TODO: Link to snapshot section

Snapshots were an option, but how they worked wasn't documented very well at the time. I went with backing images instead, as they worked perfectly for what I needed them to do. I could work iteratively and commit changes in the COW image [16] (copy-on-write) back to the base-image [17] when I was satisfied. I also could use the same base-image for multiple COWs at once. This meant that other people on my team working on the same project could all use the same base-image.

TODO: Maybe create an analogy relating base image workflow with version control

Example 2.20. Creating a Disk with a Backing Image

$ mkdir base-images
$ mkdir webserver01
$ cd base-images

$ qemu-img create -f qcow2 image-webserver-base.qcow2 10G
Formatting 'image-webserver-base.qcow2', fmt=qcow2 size=10737418240 encryption=off cluster_size=0
$ cd ../webserver01

$ qemu-img create -b /srv/images/base-images/image-webserver-base.qcow2 -f qcow2 image-webserver-devel.qcow2
Formatting 'image-webserver-devel.qcow2', fmt=qcow2 size=10737418240 backing_file='/srv/images/base-images/image-webserver-base.qcow2' encryption=off cluster_size=0

$ qemu-img info image-webserver-devel.qcow2
image: image-webserver-devel.qcow2
file format: qcow2
virtual size: 10G (10737418240 bytes)
disk size: 136K
cluster_size: 65536
backing file: /srv/images/base-images/image-webserver-base.qcow2 (actual path: /srv/images/base-images/image-webserver-base.qcow2)

Procedure 2.1. Steps in Detail

  1. I consider it bad practice to a bunch of bunch of disk images in a directory so we made two directories here. /srv/images/base-images/ to hold all the base-images on this system and /srv/images/webserver01 to later hold the disk assigned to the virtual machine.

  2. Next we go into the base images directory and create a small 10G image, type: QCOW2.

  3. Normally what we used to do at this point is create a virtual machine that uses this disk for it's primary drive. It would get a base OS provisioned on it and any other tweaks we needed there each time it was wiped.

  4. Once the machine was what we wanted in a “Golden Master” it was shutdown and the backing image would be made read-only.

  5. The next step was creating the copy-on-write (COW) image. See how in the example we give the -b option with the full path to the base-image[18]? Also note that no size is given after the file name. Size is implicitly the size of the disks backing image.

  6. With the image preparation complete we would modify the virtual machines configuration and set its primary disk drive to the COW in the webserver01 directory.


[16] In this section when we refer to a COW image it is not apropos the COW (copy-on-write) disk format. Saying COW only serves to help make a distinction between the read-only base-image and the image that changes are copied to on writing.

[17] The terms “base-image” and “backing image” are used interchangeably

[18] Some versions of qemu-img can not handle relative paths)

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2.4. Converting Between RAW and QCOW2 Home 2.6. Comitting changes to a backing image