I'm running some map reduce jobs, specifying the out parameter as:

The output step takes many minutes to complete and seems to lock the entire mongod process while it's running. I've read that the nonAtomic option could perhaps prevent this locking behavior. I'm running multiple map reduce jobs in parallel which all specify the same output collection and db in the reduce step. Does specifying

ensure that the individual documents in the output collection are updated atomically? Could there exist race conditions between the parallel MR reduce operations since they are reducing to the same collection?