Description

On https://docs.mongodb.com/manual/faq/concurrency/#how-does-concurrency-affect-a-replica-set-primary, this FAQ response appears to be MMAP-specific:

How does concurrency affect a replica set primary?

With replica sets, when MongoDB writes to a collection on the primary, MongoDB also writes to the primary's oplog, which is a special collection in the local database. Therefore, MongoDB must lock both the collection's database and the local database. The mongod must lock both databases at the same time to keep the database consistent and ensure that write operations, even with replication, are "all-or-nothing" operations.

Scope of changes

• Discuss w/ replication + storage team what the WiredTiger behavior is (for 4.2)
• Discuss w/ replication + storage team what the MMAPv1 behavior is (for 4.0, 3.6)
• Update documentation as required.