Write Operation Performance

Indexes

Each index on a collection adds some amount of overhead to the performance of write operations.

For each insert or delete write operation on a collection, MongoDB either inserts or removes the corresponding document keys from each index in the target collection. An update operation may result in updates to a subset of indexes on the collection, depending on the keys affected by the update.

In general, the performance gains that indexes provide for read operations are worth the insertion penalty. However, in order to optimize write performance when possible, be careful when creating new indexes and evaluate the existing indexes to ensure that your queries actually use these indexes.

For indexes and queries, see Query Optimization. For more information on indexes, see Indexes and Indexing Strategies.

Journaling

To provide durability in the event of a crash, MongoDB uses write ahead logging to an on-disk journal. MongoDB writes the in-memory changes first to the on-disk journal files. If MongoDB should terminate or encounter an error before committing the changes to the data files, MongoDB can use the journal files to apply the write operation to the data files.

While the durability assurance provided by the journal typically outweigh the performance costs of the additional write operations, consider the following interactions between the journal and performance:

• If the journal and the data file reside on the same block device, the data files and the journal may have to contend for a finite number of available I/O resources. Moving the journal to a separate device may increase the capacity for write operations.

• If applications specify write concerns that include the j option, mongod will decrease the duration between journal writes, which can increase the overall write load.

• The duration between journal writes is configurable using the commitIntervalMs run-time option. Decreasing the period between journal commits will increase the number of write operations, which can limit MongoDB's capacity for write operations. Increasing the amount of time between journal commits may decrease the total number of write operation, but also increases the chance that the journal will not record a write operation in the event of a failure.

For additional information on journaling, see Journaling.