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MMAPv1 Storage Engine

MMAPv1 is MongoDB’s original storage engine based on memory mapped files. It excels at workloads with high volume inserts, reads, and in-place updates. MMAPv1 is the default storage engine in MongoDB 3.0 and all previous versions.

Journal

In order to ensure that all modifications to a MongoDB data set are durably written to disk, MongoDB, by default, records all modifications to an on-disk journal. MongoDB writes more frequently to the journal than it writes the data files. The journal allows MongoDB to successfully recover data from data files after a mongod instance exits without flushing all changes.

See Journaling for more information about the journal in MongoDB.

Record Storage Characteristics

All records are contiguously located on disk, and when a document becomes larger than the allocated record, MongoDB must allocate a new record. New allocations require MongoDB to move a document and update all indexes that refer to the document, which takes more time than in-place updates and leads to storage fragmentation.

Changed in version 3.0.0.

By default, MongoDB uses Power of 2 Sized Allocations so that every document in MongoDB is stored in a record which contains the document itself and extra space, or padding. Padding allows the document to grow as the result of updates while minimizing the likelihood of reallocations.

Record Allocation Strategies

MongoDB supports multiple record allocation strategies that determine how mongod adds padding to a document when creating a record. Because documents in MongoDB may grow after insertion and all records are contiguous on disk, the padding can reduce the need to relocate documents on disk following updates. Relocations are less efficient than in-place updates and can lead to storage fragmentation. As a result, all padding strategies trade additional space for increased efficiency and decreased fragmentation.

Different allocation strategies support different kinds of workloads: the power of 2 allocations are more efficient for insert/update/delete workloads; while exact fit allocations is ideal for collections without update and delete workloads.

Power of 2 Sized Allocations

Changed in version 3.0.0.

MongoDB 3.0 uses the power of 2 sizes allocation as the default record allocation strategy for MMAPv1. With the power of 2 sizes allocation strategy, each record has a size in bytes that is a power of 2 (e.g. 32, 64, 128, 256, 512 … 2MB). For documents larger than 2MB, the allocation is rounded up to the nearest multiple of 2MB.

The power of 2 sizes allocation strategy has the following key properties:

  • Can efficiently reuse freed records to reduce fragmentation. Quantizing record allocation sizes into a fixed set of sizes increases the probability that an insert will fit into the free space created by an earlier document deletion or relocation.
  • Can reduce moves. The added padding space gives a document room to grow without requiring a move. In addition to saving the cost of moving, this results in less updates to indexes. Although the power of 2 sizes strategy can minimize moves, it does not eliminate them entirely.

No Padding Allocation Strategy

Changed in version 3.0.0.

For collections whose workloads do not change the document sizes, such as workloads that consist of insert-only operations or update operations that do not increase document size (such as incrementing a counter), you can disable the power of 2 allocation using the collMod command with the noPadding flag or the db.createCollection() method with the noPadding option.

Prior to version 3.0.0, MongoDB used an allocation strategy that included a dynamically calculated padding as a factor of the document size.