mongodump/mongorestore and oplog.bson

[Eric Snow]

When running mongodump with the --oplog, you end up getting an
oplog.bson file dumped, along with a directory for each dumped
database.  mongorestore likewise has an --oplog option, which
presumably causes it to use that oplog.bson file.

Who could tell me about that oplog.bson file?  In my testing for
backups, it always comes up empty.  If I can always expect it to be
empty then I'll be a happy camper.  Otherwise, what are the cases
where it would not be empty and is there a safe way I could parse/edit
oplog.bson in those cases?  My guess is that it will be non-empty when
mongdump is run while mongo has actions queued up or when requests
come in while mongodump is running.  Regardless, any insight here
would be helpful.  Thanks!

-eric