Comment 9 for bug 1942421

As Ian pointed out in https://bugs.launchpad.net/juju/+bug/1942421/comments/7 I think the easiest way forward would be to first locally upgrade to 2.9.* and then attempt the migration again.

However, it may be something that we should be engaging at an Engineering level, and having one of our engineers working directly with you to understand what is going on. (If you are getting txn-watcher sync errors, that means that Mongo is not reliably responding to us, which is not a great time to be trying to do a large migration. There may be something like ensuring the Replica set is happy before we do other work.)