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MongoDB Storage Engines

The storage engine that is used can be seen with the command db.serverStatus(). It is a mongod option: --storageEngine.

MMAPv1​

In March 2015, there were two choices: MMAPv1 (original) and WiredTiger (new).

Comparison made by Percona in Jan 2019: MMAPV1 Vs WiredTiger by Percona

WiredTiger​

Following the acquisition of the company by MongoDB, WireTiger was introduced in in MongoDB 3.0. It is the first pluggable storage engine.

Features:

  • Document level locking
  • Compression:
    • Snappy (default) - fast
    • Zlib - more compression
    • None
  • Lacks some pitfalls of MMAPv1
  • Performance gains

Background:

  • Built separately from MongoDB
  • Used by other's DB
  • Open source

Internals:

  • Stores data in btrees
  • Writes are initially separate, incorporated later
  • Two caches:
    • WT caches - 1/2 of RAM (default)
    • FS cache
  • Checkpoint: every minute or more
  • No need for a journal