Ceilometers database keeps growing even though TTL in the ceilometer.conf is defined

Issue

We have been investigating this issue for some time and of course made sure the samples are NOT deleted, So the count is rising, even though ttl is set to 600 (10 minutes)

This shows the ttl is defined:

rs:PRIMARY> db.meter.getIndexes()
[
        {
                "v" : 1,
                "key" : {
                        "_id" : 1
                },
                "name" : "_id_",
                "ns" : "ceilometer.meter"
        },
        {
                "v" : 1,
                "key" : {
                        "resource_id" : 1,
                        "user_id" : 1,
                        "counter_name" : 1,
                        "timestamp" : 1,
                        "source" : 1
                },
                "name" : "meter_idx",
                "ns" : "ceilometer.meter",
                "background" : false
        },
        {
                "v" : 1,
                "key" : {
                        "resource_id" : 1,
                        "project_id" : 1,
                        "counter_name" : 1,
                        "timestamp" : 1,
                        "source" : 1
                },
                "name" : "meter_project_idx",
                "ns" : "ceilometer.meter",
                "background" : true
        },
        {
                "v" : 1,
                "key" : {
                        "timestamp" : -1
                },
                "name" : "timestamp_idx",
                "ns" : "ceilometer.meter"
        },
        {
                "v" : 1,
                "key" : {
                        "timestamp" : 1
                },
                "name" : "meter_ttl",
                "expireAfterSeconds" : 600,
                "ns" : "ceilometer.meter"
        }
]
rs:PRIMARY>

This shows the rising sample count:

rs:PRIMARY> db.meter.count()
15585779

few minutes later

rs:PRIMARY> db.meter.count()
15595350

Checking the ttl execution (should run every minute) shows that it does not run anymore.

rs:PRIMARY> db.serverStatus().metrics.ttl
{ "deletedDocuments" : NumberLong(26999680), "passes" : NumberLong(2470) }

few minutes later

rs:PRIMARY> db.serverStatus().metrics.ttl
{ "deletedDocuments" : NumberLong(26999680), "passes" : NumberLong(2470) }

And, of course i've ran a query, such as the following, to see that indeed there are samples that should have been deleted (the specified date is yesterday, beyond the ttl), this gives lots of results:

 db.meter.find({"timestamp" : {$lt : ISODate("2015-08-09T01:20:00Z")}})