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Re: [libvirt-users] Libvirt API for getting disk capacity from VM XML



On Thu, Jun 13, 2019 at 03:17:16PM +0530, Varsha Verma wrote:
Hello everyone,
I am doing an outreachy internship at Openstack Ironic. In the sushy-tools
project, we are using libvirt VMs to simulate bare metal machines for
testing purposes.

In the XML description of a domain, there are a bunch of disk elements
giving information about the various storage devices attached to the
domain. Is there some way to get the size/capacity of those disks using the
libvirt API?


Yes, there is, virStorageVolGetXMLDesc:

 https://libvirt.org/html/libvirt-libvirt-storage.html#virStorageVolGetXMLDesc

Look for <capacity/>

The tricky part might be to get the storage volume object.  I'm not sure how
sushy-tools are using libvirt and what the deployment looks like, so here are
few tips and feel free to ask for more info.

If it is a path on the local filesystem, you might just use
`virStorageVolLookupByKey` and be done with it:

 https://libvirt.org/html/libvirt-libvirt-storage.html#virStorageVolLookupByKey

If it is not, you might need to look it up based on the pool it is in.

If you have the pool already, then it is a matter of `virStorageVolLookupBy*`
for example.  If you don't have that you might look for the pool with
`virStoragePoolLookupBy*`, for example `virStoragePoolLookupByName`:

 https://libvirt.org/html/libvirt-libvirt-storage.html#virStoragePoolLookupByName

And one more general libvirt advice (although maybe not really a good one,
definitely not a silver bullet):

You can always have a look at how `virsh` does things.  It might be a bit
confusing at times, but if you have a command name (in our case we'd use the
`vol-dumpxml` command), you can CamelCase it ("VolDumpXML"), prepend "cmd" (as
in "command") and the resulting name (`cmdVolDumpXML`) is the name of the
function that virsh runs for that command.  You can see it in
tools/virsh-volume.c:

 https://github.com/libvirt/libvirt/blob/master/tools/virsh-volume.c#L1211

The main part of how it looks up a volume is `virshCommandOptVol` which is just
a syntax sugar for `virshCommandOptVolBy` in the same file:

 https://github.com/libvirt/libvirt/blob/master/tools/virsh-volume.c#L68

Which is trying to do the lookup by all various possible things.  Same thing can
be found for `pool-*` commands.

HTH, have a nice day,
Martin

--

*Regards,*

*Varsha Verma*

*Fourth Year Undergraduate*

*Department of Electrical Engineering*

*IIT-BHU, Varanasi*

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