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On Mon, Oct 6, 2014 at 6:03 AM, Gary Kotton <gkotton at vmware.com> wrote:

>  Hi,
> At the moment the resource tracker in Nova ignores that statistics that
> are returned by the hypervisor and it calculates the values on its own. Not
> only is this highly error prone but it is also very costly – all of the
> resources on the host are read from the database. Not only the fact that we
> are doing something very costly is troubling, the fact that we are over
> calculating resources used by the hypervisor is also an issue. In my
> opinion this leads us to not fully utilize hosts at our disposal. I have a
> number of concerns with this approach and would like to know why we are not
> using the actual resource reported by the hypervisor.
> The reason for asking this is that I have added a patch which uses the
> actual hypervisor resources returned and it lead to a discussion on the
> particular review (https://review.openstack.org/126237).
>

So it sounds like you have mentioned two concerns here:

1. The current method to calculate hypervisor usage is expensive in terms
of database access.
2. Nova ignores that statistics that are returned by the hypervisor and
uses its own calculations.


To #1, maybe we can doing something better, optimize the query, cache the
result etc. As for #2 nova intentionally doesn't use the hypervisor
statistics for a few reasons:

* Make scheduling more deterministic, make it easier to reproduce issues
etc.
* Things like memory ballooning and thin provisioning in general, mean that
the hypervisor is not reporting how much of the resources can be allocated
but rather how much are currently in use (This behavior can vary from
hypervisor to hypervisor today AFAIK -- which makes things confusing). So
if I don't want to over subscribe RAM, and the hypervisor is using memory
ballooning, the hypervisor statistics are mostly useless. I am sure there
are more complex schemes that we can come up with that allow us to factor
in the properties of thin provisioning, but is the extra complexity worth
it?

That being said I am fine with discussing in a spec the idea of adding an
option to use the hypervisor reported statistics, as long as it is off by
default.




> Thanks
> Gary
>
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