mongo capped collection does not create an index on _id field

[s0enke]

Getting this mongodb error occasionally:

Tue Dec 14 16:00:13 [conn1176] assertion 10129 too much data for sort() with no index ns:graylog2.messages query:{ $query: { deleted: { $in: [ false, null ] }, message: { $nin: [ /\#24861/, /Could\ not\ find\ subtype\ object\ for\ subtypeID\ 0\ in\ mm_WebsitePeer::createJimdo/, /Exception\ while\ trying\ to\ ping\ BlogPingR/, /Allowed\ memory\ size\ of\ 67108864\ bytes\ exhausted/, /contains\ an\ suspicious\ activity\ pattern/, /MatrixController\ \-\ create\ failed:\ Already\ created\ a\ new\ module/, /\[PAYMENT\]\[DEBUG\]\ GC\ api\ request\ xml/ ] } }, $orderby: { _id: -1 } }
Tue Dec 14 16:00:13 [conn1176]  ntoskip:0 ntoreturn:100
Tue Dec 14 16:00:13 [conn1176] query graylog2.messages ntoreturn:100 exception  514ms

http://www.mongodb.org/display/DOCS/Capped+Collections says:

"An index is not automatically created on _id for capped collections by default"

creating an index like this fixed the issue for us:

> db.messages.ensureIndex({_id: 1}, {unique: true});

[lennartkoopmann]

The coming release (was schedules for last weekend. re-scheduled for next weekend) will no longer sort by _id but use natural ordering so this problem should no longer exist then.

Thanks!