Each index on a collection adds some amount of overhead to the performance of write operations.
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.
The capability of the storage system creates some important physical limits for the performance of MongoDB's write operations. Many unique factors related to the storage system of the drive affect write performance, including random access patterns, disk caches, disk readahead and RAID configurations.
Solid state drives (SSDs) can outperform spinning hard disks (HDDs) by 100 times or more for random workloads.
Production Notes for recommendations regarding additional hardware and configuration options.
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
mongodwill decrease the duration between journal writes, which can increase the overall write load.
- The duration between journal writes is configurable using the
commitIntervalMsrun-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.