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On Wed, Nov 28, 2012 at 7:04 AM, Julien Danjou <julien at danjou.info> wrote:

> On Wed, Nov 28 2012, Doug Hellmann wrote:
>
> > There's a race condition, though. If you and I are processing events at
> the
> > same time, we get the same "last value" from the storage but then try to
> > update it to our current value.
>
> Right. Now that I think about it, that problems with multiple collector
> would also exists if we try to transform cumulative to delta on
> notifications.
>

Right. That's why it has to be done at the source, where the data is being
collected.

I talked about this a bit with the folks from Yahoo at the summit. They're
using this approach in their own monitoring, and seeing good results. They
use a little sqlite db on the compute node to persist the last few samples
from the meters so they can detect resets and the agent emits delta values
instead of cumulative values.

We have a further advantage in that we're already plugging into nova to
trigger stats collection. If we add a few more notification hooks for cases
where these counters get reset, we can be sure of detecting the reset no
matter what the polling interval is.

I think applying both techniques would give us the results we want.

Doug


>
> --
> Julien Danjou
> /* Free Software hacker & freelance
>    http://julien.danjou.info */
>
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