MongoDB Documentation, Release 3.0.4 1.8.1 1.8.0 1.7.6 1.7.5 1.7.4 1.7.3 • You can merge MapReduce output into an existing collection. Output from the Reduce phase will replace existing keys in the output collection if it already exists. Other keys will remain in the collection. • You can now re-reduce your output with the contents of an existing collection. Each key output by the reduce phase will be reduced with the existing document in the output collection. • You can replace the existing output collection with the new results of the MapReduce job (equivalent to setting a permanent output collection in previous releases) • You can compute MapReduce inline and return results to the caller without persisting the results of the job. This is similar to the temporary collections generated in previous releases, except results are limited to 8MB. For more information, see the out field options in the mapReduce document. • Sharding migrate fix when moving larger chunks. • Durability fix with background indexing. • Fixed mongos concurrency issue with many incoming connections. • All changes from 1.7.x series. • Bug fixes. • Journaling (page 308). • Extent allocation improvements. • Improved replica set connectivity for mongos . • getLastError improvements for sharding . • mongos routes slaveOk queries to secondaries in replica sets . • New mapReduce output options. • Sparse Indexes (page 505). • Initial covered index (page 66) support. • Distinct can use data from indexes when possible. • mapReduce can merge or reduce results into an existing collection. • mongod tracks and mongostat displays network usage. See mongostat . Chapter 12. Release Notes 918 Additional Changes and Enhancements