Mattermost version 6.0 introduces many exciting changes to the product, which come with important notes to consider when performing the upgrade.
There is a very fundamental change to the clustering code, which means that now in a cluster, nodes from different versions cannot run. With v6.0 you are unable to also run v5.x simultaneously. This means you should expect to schedule downtime especially in a clustered environment with this upgrade.
Furthermore, 6.0 introduces database schema changes and longer migration times should be expected, especially on MySQL installations. Based on your database size and setup the migration can take a significant amount of time and may even lock the tables for posts, so there is no way to post or to receive messages. There are a few schema changes upcoming, but one in particular is related to this change in the mattermost server.
We will add more details on additional changes and queries, expected migration durations and possible mitigation strategies in the upcoming weeks.