MMAPv1

MMAPv1 is the original storage engine used by MongoDB. It stores data in memory-mapped files on disk, relying on the operating system’s paging and caching for I/O. It served as the default storage engine in early MongoDB releases.

Architecture and concurrency: Data and indexes reside in data files that are memory-mapped into the mongod process address space. Updates modify these files in memory as the OS flushes changes to disk. The locking model was coarse-grained, which limited write throughput under heavy workloads; reads could run in parallel with writes, but there was no document-level locking.

Durability and maintenance: Journaling was supported to improve crash recovery, and data files could become fragmented over time, requiring maintenance in long-running deployments. The design favored simplicity and fast reads for bounded working sets but traded off high write concurrency and online data compaction capabilities.

Legacy status: MMAPv1 was deprecated during the 3.x MongoDB releases and removed in MongoDB 4.x, as the project shifted toward the WiredTiger storage engine. For new deployments, WiredTiger is recommended due to its document-level concurrency, compression options, and improved overall performance and scalability. Migrating from MMAPv1 involves enabling WiredTiger and following the migration path provided by MongoDB tools, which typically includes data migration steps and compatibility considerations.