I am facing a problem upgrading multiple servers from Ubuntu 16.04 to 18.04. The servers are running the supported versions of Mongo, 2.6.10 on 16.04 and 3.6.3 after the upgrade. The upgrade breaks the existing MongoDB databases, and as far as I understand the way to fix it is to downgrade mongo and to the rolling updates.
This does not feel like the intended way to upgrade the systems, and I have so far been unable to find a propper way to do it. Is it really the intended result to have to downgrade Mongo manually and upgrade step by step?
The problem seems to be the compatibility when upgrading from 2.6 to 3.6 directly, which is what happens if you upgrade from 16.04 to 18.04.
** IMPORTANT: UPGRADE PROBLEM: The data files need to be fully upgraded to version 3.4 before attempting an upgrade to 3.6; see http://dochub.mongodb.org/core/3.6-upgrade-fcv for more details.
I appreciate any suggestions, thank you in advance.