[Libvir] Need more information on virDomainBlockStats, virDomainInterfaceStats and virNodeGetCellsFreeMemory

Richard W.M. Jones rjones at redhat.com
Mon Mar 3 21:57:38 UTC 2008

On Mon, Mar 03, 2008 at 11:31:10AM +0530, Ajishrao.r wrote:
> Hi,
>        I am trying to use libvirt, any one please help more about
> virDomainBlockStats, virDomainInterfaceStats and virNodeGetCellsFreeMemory
> any pointer or docs?

Is there anything in particular you want to know?

If you want to see virDomainBlockStats and virDomainInterfaceStats in
action I suggest getting virt-top which makes use of these calls:


(It's also in Fedora).  If you run virt-top with a recent version of
libvirt and export LIBVIRT_DEBUG=1 then it will show you the libvirt
calls being made.

Basically what you do is call virDomainGetXMLDesc for a domain, parse
out the <target dev="foo"> for each block device and network
interface, and then pass that string ("foo") back to
virDomainBlockStats or virDomainInterfaceStats as the path parameter.
If the call is supported by the hypervisor then what you get back is a
structure with the stats you requested.

For reasons of efficiency you probably don't want to call
virDomainGetXMLDesc frequently.  You only need to call it when the
domain's devices change, and for most programs you can ignore that
possibility and just call it once per domain.

All of the stats monotonically increment, so you need to do your own
subtraction and divide by the time between calls in order to get
rates.  Again, see the code in virt-top.  We expose all the stats as
64 bit counters so hopefully they don't wrap for a Very Long Time, but
in some cases the underlying kernel or hypervisor is using 32 bit
counters (particularly Linux's /proc/net/dev with a 32 bit kernel
IIRC) so be careful about that.


Richard Jones, Emerging Technologies, Red Hat  http://et.redhat.com/~rjones
virt-top is 'top' for virtual machines.  Tiny program with many
powerful monitoring features, net stats, disk stats, logging, etc.

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