[libvirt] Globally Reserve Resources for Host

Eric Blake eblake at redhat.com
Thu Nov 1 15:09:56 UTC 2012

On 11/01/2012 07:16 AM, Dusty Mabe wrote:
> Hi,
> I am interested in the capability to globally reserve resources(cpu and
> memory) for a KVM host. I know you can configure memory limits for each
> guest (http://libvirt.org/formatdomain.html#elementsMemoryTuning), but
> would like the ability to reserve host cpu and memory without having to
> actively do it by modifying each guests xml.
> For clarity, what I mean by "reserve resources" is that there are certain
> cpus and a certain amount of memory that guests will never have access to.
> This can be achieved using cgroups.
> Does anyone think this functionality would be useful? This is primarily to
> prevent the host from being starved when the allocation of guests have the
> host overcommitted/oversubscribed.

Yes, this functionality would be useful.  In fact, I even proposed a
possible design, and Dan chimed in with some improvements (but no one
has coded anything towards that design) ages ago:

Basically, you would create a virGroupPtr that describes the entire
resources you are willing to allow to VMs, then ensure that all VMs are
members of that virGroupPtr.

The idea of a virGroupPtr also makes it possible to isolate
host-specific management out of per-guest XML, making it easier to
migrate guests.  For example, instead of hard-coding that a guest is
pinned to host cpus 4-7, you would state that a guest's cpu usage is
determined by a named virGroup policy, and as long as that named policy
provides 4 cpus on both source and destination, the actual decision of
WHICH 4 host cpus are in the group can differ between the hosts, so that
you don't have to have the same cpus available on the migration
destination, and don't have to write a migration hook to rewrite the XML
to redo the pinning.

Eric Blake   eblake at redhat.com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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