The 'fsync' flag in writeConcern is undocumented since mmap was removed. Eric managed to find this old documentation that mentions the fsync flag: https://docs.mongodb.com/v2.2/reference/command/getLastError/#dbcmd.getLastError. WT does still act on it, however.

We still parse it from user writeConcern here and act on that flag in waiting for write concern here. flushAllFiles (that fsync flagged write concern calls) behaves differently in the WT integration layer code than a regular j:true flag: we explicitly take a checkpoint instead of flushing the journal.