Comment 2 for bug 1262571

Reviewed: https://review.openstack.org/65962
Committed: https://git.openstack.org/cgit/openstack/ceilometer/commit/?id=ba6641afacfc52e7391d2095751ee96d62a64c25
Submitter: Jenkins
Branch: master

commit ba6641afacfc52e7391d2095751ee96d62a64c25
Author: Eoghan Glynn <email address hidden>
Date: Thu Jan 9 16:30:10 2014 +0000

    Replace mongo aggregation with plain ol' map-reduce

    Fixes bug 1262571

    Previously, the mongodb storage driver an aggregation pipeline
    over the meter collection in order to construct a list of resources
    adorned with first & last sample timestamps etc.

    However mongodb aggregation framework performs sorting in-memory,
    in this case operating over a potentially very large collection.
    It is also hardcoded to abort any sorts in an aggregation pipeline
    that will consume more than 10% of physical memory, which is
    observed in this case.

    Now, we avoid the aggregation framework altogether and instead
    use an equivalent map-reduce.

    Change-Id: Ibef4a95acada411af385ff75ccb36c5724068b59