With the introduction of WiredTiger and its many benefits, such as document-level locking, many MongoDB users are questioning whether it's worth discussing MMAPv1 anymore.
In reality, the only cases where we should consider using MMAPv1 instead of WiredTiger would be the following:
- Legacy systems: If we have a system that fits our needs, we may upgrade to MongoDB 3.0+ and not transition to WiredTiger.
- Version downgrade: Once we upgrade to MongoDB 3.0+ and convert our storage to WiredTiger, we cannot downgrade to a version lower than 2.6.8. This should be kept in mind if we want the flexibility to downgrade at a later time.
As shown previously, WiredTiger is a better choice than MMAPv1, and we should use it whenever we get the chance. This book is oriented around WiredTiger, and assumes we will be able to use the latest stable version of MongoDB (3.4 at the time of writing).
MMAPv1, as of version 3.4, only supports collection-level locking, as opposed to the document-level locking supported by WiredTiger. This can lead to a performance penalty in high contention database loads and is one of the main reasons why we should use WiredTiger whenever possible.