The storage engine is the component of the database that is responsible for managing how data is stored, both in memory and on disk. MongoDB supports multiple storage engines, as different engines perform better for specific workloads. Choosing the appropriate storage engine for your use case can significantly impact the performance of your applications.
WiredTiger is the default storage engine starting in MongoDB 3.2. It is well-suited for most workloads and is recommended for new deployments. WiredTiger provides a document-level concurrency model, checkpointing, and compression, among other features. In MongoDB Enterprise, WiredTiger also supports Encryption at Rest.
MMAPv1 is the original MongoDB storage engine and is the default storage engine for MongoDB versions before 3.2. It performs well on workloads with high volumes of reads and writes, as well as in-place updates.
The In-Memory Storage Engine is available in MongoDB Enterprise. Rather than storing documents on-disk, it retains them in-memory for more predictable data latencies.