• Storage engine is responsible for managing how data is stored, both in memory and on disk. It is the component of the database.
• MongoDB supports multiple storage engines, as different engines perform better for specific workloads.
• Choosing the excellent storage engine for your use case can extensively have an impact on the overall performance of your applications.
• Here, there are four types of storage engines used by MongoDB :
  • MMAPv1
  • WiredTiger
  • Encrypted
  • In-memory

MMAPv1 :

• This type of storage engine uses memory-mapped files to store its data.
• A segment of virtual memory that has been assigned a direct byte-for-byte correlation with some portion of a file.
• This storage engine allows a great deal of performance for heavy read applications.
• Indexes and Data are mapped into virtual space.
• Data access is placed into RAM.
• It will swap out memory to disk to make space for the newly requested data if the OS runs out of RAM and an application requests for memory.
• MongoDB’s memory was managed by operating system’s virtual memory subsystem.

WiredTiger :

• From Mongo 3.0 WiredTiger is the default storage engine.
• This type storage engine uses document-level concurrency control for write operations. At the same time multiple clients can modify different documents of a collection.
• It uses only intent locks at the global, database and collection levels when the storage engine detects conflicts between two operations, one will incur a write conflict causing MongoDB to transparently retry that operation.
• MongoDB utilizes both the filesystem cache and WiredTiger internal cache. the wired tiger cache will use 50% of RAM minus 1 GB or 256 MB by default.
• It is efficient use of CPU cores and RAM.
• It allows more tuning storage engines than MMVPv1. 7 to 10X better write performance. 80% less storage with compression.
• In WiredTiger internal cache collection level data is uncompressed and uses a different representation from the on-disk format.

Encrypted :

• It is available in MongoDB enterprise only.
• During encryption master keys and database keys are used.
• Master key encrypts the database keys, data is encrypted with the database keys.
• Application-Level Encryption provides encryption on a per-field or per-document basis within the application layer.

In-memory :

• Available in the enterprise editions starting from version 3.2.6.
• It Handles ultra-high throughput with high availability and low latency.
• Low latency on less in-memory infrastructure and more predictable.
• Supports high level infrastructure based on zonal sharding.
• Indexing support and MongodB rich query capability.