Comment 4 for bug 1743901

I believe we apt update but as part of "enable-os-update" that can be set to False.

As for what is specifically failing, it is this line:
[initandlisten] exception in initAndListen: 28662 Cannot start server. Detected data files in /var/lib/juju/db created by the 'mmapv1' storage engine, but the specified storage engine was 'wiredTiger'., terminating

AFAICT, we have always started Mongo 3.2 with the --storageEngine wiredTiger (it is part of the systemd script).

So I don't know how the directory could have been created with an mmap storage pool.

Has this been running a while? Is there some reason why a different mongo would have been installed on the machine?
(Its possible that some other package decided it wanted to have /usr/bin/mongo installed, and we had failed to install /usr/lib/juju/mongo3.2/bin/mongo, and thus we fell back to the system mongo during startup, which seemed to be working, until you installed the real mongo, and it decided to try to use it?)