Using virt-customize to make custom guests with a single backing file

Yesterday I hinted that virt-customize could be used to make custom guests sharing a single backing file. Here is how you do that.

Firstly download a cloud image, or use virt-builder to create one:

$ virt-builder fedora-20 -o backing.img
[   0.0] Downloading: http://libguestfs.org/download/builder/fedora-20.xz
[   1.0] Planning how to build this image
[   1.0] Uncompressing
[  11.0] Opening the new disk
[  15.0] Setting a random seed
[  15.0] Setting passwords
Setting random password of root to 8obMIvmrWe6CCkAv
[  16.0] Finishing off

Now use qemu-img to create overlays for each guest:

$ qemu-img create -b backing.img -f qcow2 guest1.img
$ qemu-img create -b backing.img -f qcow2 guest2.img

You must leave the backing file untouched. In particular don’t try to customize it, else you’ll corrupt all the guests using that backing file.

Now you can customize each guest overlay:

$ virt-customize -a guest1.img \
    --hostname guest1 --timezone Europe/London \
    --install gcc
[   0.0] Examining the guest ...
[   3.0] Setting a random seed
[   3.0] Setting the hostname: guest1
[   3.0] Setting the timezone: Europe/London
[   3.0] Installing packages: gcc

$ virt-customize -a guest2.img \
  --hostname guest2 --install /usr/bin/soffice
[   0.0] Examining the guest ...
[   5.0] Setting a random seed
[   5.0] Setting the hostname: guest2
[   5.0] Installing packages: /usr/bin/soffice

As expected, each guest overlay uses a different amount of space depending on what has been installed:

$ ls -lh guest?.img
-rw-r--r--. 1 rjones rjones 613M Mar 26 15:49 guest1.img
-rw-r--r--. 1 rjones rjones 924M Mar 26 16:04 guest2.img

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