WiredTiger uses Multiversion Concurrency Control (MVCC) to maintain multiple views of data for the lifetime of sessions that can read it. WiredTiger’s in-memory (cache) format is different from the on-disk format.

In MongoDB 3.6, the cache implementation improves efficiency for small updates to large documents by adding the capability to store deltas rather than copying the full document for each in-memory version required. When flushed to disk, a full version of the document must still be constructed.