Nova metadata recorded in libvirt guest instance XML

Posted: February 20th, 2015 | Filed under: Coding Tips, Fedora, libvirt, OpenStack, Virt Tools | Tags: , , , | No Comments »

One of the issues encountered when debugging libvirt guest problems with Nova, is that it isn’t always entirely obvious why the guest XML is configured the way it is. For a while now, libvirt has had the ability to record arbitrary application specific metadata in the guest XML. Each application simply declares the XML namespace it wishes to use and can then record whatever it wants. Libvirt will treat this metadata as a black box, never attempting to interpret or modify it. In the Juno release I worked on a blueprint to make use of this feature to record some interesting information about Nova.

The initial set of information recorded is as follows:

  • Version – the Nova version number, and any vendor specific package suffiix (eg RPM release number). This is useful as the user reporting a bug is often not entirely clear what particular RPM version was installed when the guest was first booted.
  • Name – the Nova instance display name. While you can correlated Nova instances to libvirt guests using the UUID, users reporting bugs often only tell you the display name. So recording this in the XML is handy to correlate which XML config corresponds to which Nova guest they’re talking about.
  • Creation time – the time at which Nova booted the guest. Sometimes useful when trying to understand the sequence in which things happened.
  • Flavour – the Nova flavour name, memory, disk, swap, ephemeral and vcpus settings. Flavours can be changed by the admin after a guest is booted, so having the original values recorded against the guest XML is again handy.
  • Owner – the tenant user ID and name, as well as their project
  • Root image – the glance image ID, if the guest was booted from an image

The Nova version number information in particular has already proved very useful in a couple of support tickets, showing that the VM instance was not booted under the software version that was initially claimed. There is still scope for augmenting this information further though. When working on another support issues it would have been handy to know the image properties and flavour extra specs that were set, as the user’s bug report also gave misleading / incorrect information in this area. Information about cinder block devices would also be useful to have access to, for cases where the guest isn’t booting from an image.

While all this info is technically available from the Nova database, it is far easier (and less dangerous) to ask the user to provide the libvirt XML configuration than to have them run random SQL queries. Standard OS trouble shooting tools such as  sosreport from RHEL/Fedora already collect the libvirt guest XML when run. As a result, the bug report is more likely to contain this useful data in the initial filing, avoiding the need to ask the user to collect further data after the fact.

To give an example of what the data looks like, a Nova guest booted with

$ nova boot --image cirros-0.3.0-x86_64-uec --flavor m1.tiny vm1

Gets the following data recorded

$ virsh -c qemu:///system dumpxml instance-00000001
<domain type='kvm' id='2'>
    <nova:instance xmlns:nova="">
      <nova:package version="2015.1"/>
      <nova:creationTime>2015-02-19 18:23:44</nova:creationTime>
      <nova:flavor name="m1.tiny">
        <nova:user uuid="ef53a6031fc643f2af7add439ece7e9d">admin</nova:user>
        <nova:project uuid="60a60883d7de429aa45f8f9d689c1fd6">demo</nova:project>
      <nova:root type="image" uuid="2344a0fc-a34b-4e2d-888e-01db795fc89a"/>

The intention is that as long as the XML namespace URI ( isn’t changed, the data reported here will not change in a backwards incompatible manner. IOW, we will add further elements or attributes to the Nova metadata, but not change or remove existing elements or attributes. So if OpenStack related troubleshooting / debugging tools want to extract this data from the libvirt XML they can be reasonably well assured of compatibility across future Nova releases.

In the Kilo development cycle there have been patches submitted to record similar kind of data for VMWare guests, though obviously it uses a config data format relevant to VMWare rather than XML. Sadly this useful debugging improvement for VMWare had its feature freeze exception request rejected, pushing it out to Liberty, which is rather a shame :-(

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