upgrade instructions for gitlab lack information how to do the backup
In the recent announcement of 8.9 it is mentioned that the upgrade of mattermost will need manual intervention. It links to http://docs.gitlab.com/omnibus/gitlab-mattermost/#upgrading-gitlab-mattermost-from-versions-prior-to-8.9
The second step is to "Backup your Mattermost database."... I'm searching for half an hour now and for an outsider it's quite hard to figure this out.
given a standard omnibus install it would be great to list a command here.