Always create the JournalFlusher, regardless of ephemeral or non-durable storage engine settings, because using it to wait for write concern should be a performance gain for all settings and also simplifies the logic.

Today we skip creating the JournalFlusher thread for both ephemeral or non-durable (nojournal=true) storage engine settings.
https://github.com/mongodb/mongo/blob/db3a17bbfe2e265722ed88df961e79f3e1a68067/src/mongo/db/storage/wiredtiger/wiredtiger_kv_engine.cpp#L989-L993

Originally, the JournalFlusher was purely a periodically running task. However, recently, that changed. The JournalFlusher is now a periodic task that can also be pinged for an immediate run.
(Old in v4.2)
https://github.com/mongodb/mongo/blob/e329dd322df4a226b143031c99b5f943d3a9be4a/src/mongo/db/storage/wiredtiger/wiredtiger_kv_engine.cpp#L251
(New in v4.4)
https://github.com/mongodb/mongo/blob/db3a17bbfe2e265722ed88df961e79f3e1a68067/src/mongo/db/storage/wiredtiger/wiredtiger_kv_engine.cpp#L305-L307

Moving the write_concern.cpp waitUntilDurable calls onto the JournalFlusher thread via waitForJournalFlush has been a big performance win. Currently, we use logic in waitForJournalFlush to skip the JournalFlusher thread when it doesn’t exist and instead call waitUntilDurable directly.
https://github.com/mongodb/mongo/blob/db3a17bbfe2e265722ed88df961e79f3e1a68067/src/mongo/db/storage/wiredtiger/wiredtiger_kv_engine.cpp#L2377-L2383

We would want to override the periodical functionality of the JournalFlusher thread when ephemeral or !durable, I think, and make it purely run when requested. Taking checkpoints so frequently for non-durable (nojournal) might slow down the system; and ephemeral only updates the JournalListener (durable timestamp).