I’ve been playing around with the LXC support in libvirt recently, and I’m trying to use a model where each LXC instance is backed by a dedicated LVM volume. This means that the process of starting an instance is:

  • mount the instance root filesystem if necessary
  • start the instance

It’s annoying to have to do this by hand. I could simply add all the LXC filesystems to /etc/fstab, but this would mean and extra step when creating and deleting each instance.

I thought about creating a wrapper script to handle the mounting (and unmounting, perhaps) for me, but this leads to another problem: I need a way to figure out which backing device corresponding to the virtual instance I’m trying to boot.

I could just adopt a strict naming scheme, so that for a given instance “foo” the backing store would be /dev/vg_something/domain-foo, but that’s too easy! In order to make life interesting:

  • I’ve added some namespace-prefixed metadata to the instance XML description, and
  • Written a shell script to extract this data (and more) using xmllint.

Metadata#

The Libvirt domain XML format allows for a general metadata section:

The metadata node can be used by applications to store custom metadata in the form of XML nodes/trees. Applications must use custom namespaces on their XML nodes/trees, with only one top-level element per namespace (if the application needs structure, they should have sub-elements to their namespace element).

This means that if you want to add metadata to a domain description, you need to define a namespace and use a namespace prefix. For example, if I want to add a device element pointing at the backend storage device for my domain, it might look like this:

<metadata xmlns:ob="http://oddbit.com/ns/libvirt/1">
    <ob:device>/dev/vg0/instance</ob:device>
</metadata>

Extracting metadata with xmllint#

The xmllint tool, part of libxml, can extract nodes from an XML document using xpath expressions. Unfortunately, while xmllint does have namespace support, it’s not particularly convenient. Using the --shell mode there’s a helpful setns command:

$ xmllint --shell domain.xml
/ > setns x=http://oddbit.com/ns/libvirt/1
/ > xpath //x:device
Object is a Node Set :
Set contains 1 nodes:
1  ELEMENT ob:device

But that’s not available from the command line. We can use the namespace-uri() and local-name() xpath functions to get to the same place, albeit more verbosely. An equivalent to the above shell session would be:

$ xmllint --xpath '//*[namespace-uri()="http://oddbit.com/ns/libvirt/1" and local-name()="device"]'
<ob:device>/dev/vg0/instance</ob:device>

Putting it all together#

The following shell script looks through all inactive LXC domains and figures out:

  • If it should try to start them, using the <oddbit:autostart> element,

  • What the backend storage is, using the <oddbit:device> element, and

  • Where to mount it, by looking for the libvirt <filesystem> element with a target of /.

    #!/bin/sh

tmpfile=$(mktemp)
trap 'rm -f $tmpfile' EXIT

virsh -c lxc:/// list --name --inactive | while read domain; do
  [ "$domain" ] || continue

  virsh dumpxml $domain > $tmpfile

  autostart=$(xmllint --xpath '//domain/metadata/*[namespace-uri()="http://oddbit.com/ns/libvirt/1" and local-name()="autostart"]/text()' $tmpfile)

  [ "$autostart" = True ] || continue

  device=$(xmllint --xpath '//domain/metadata/*[namespace-uri()="http://oddbit.com/ns/libvirt/1" and local-name()="device"]/text()' $tmpfile)
  mount=$(xmllint --xpath 'string(//filesystem/target[@dir = "/"]/../source/@dir)' $tmpfile)

  echo "$domain $autostart $device $mount"
done

I’m not actually planning on using this in practice. I’ll probably go the naming scheme route. But this was fun to figure out.