Comment 13 for bug 1852502

Without --oplog you would have to take the Juju controllers offline to create a stable backup. Oplog allows you to start dumping the collections and include the write-ahead-log which means that if you start dumping collection A, by the time you get to collection Z, you have a consistent view across all of the collections.

It is interesting that with capped collections, we fundamentally have data that is fairly 'ephemeral' and would be ok if it wasn't consistent in the output. Certainly the option being discussed is to discard it entirely with '--no-logs'.