As part of our work to transition from jessie to stretch for our MySQL server, we'll be migrating user data today around 5pm.
To do this, we'll put the existing jessie server into read-only mode, then make a final import to the new stretch host. We believe this will take about an hour and don't anticipate any issues (we've already tested imports from our regular backups without problems).
Read-only mode is necessary during the import to ensure we get a consistent backup, and so that writes made during the transition are not lost.
Some sites may experience downtime while the server is in read-only mode (if they require writing to the database to show pages). Most sites will experience some level of degradation (e.g. can't log in to admin or edit posts).
Update 5:01pm: We have entered read-only mode.
Update 5:58pm: The backup has completed, we are now importing it into the new database server. Probably another 45 minutes or so.
Update 6:07pm: ETA 25 minutes.
Update 6:15pm: ETA 16 minutes
Update 6:25pm: Unfortunately we hit a snag during the import and have to start it over.
Update 6:29pm: ETA 37 minutes.
Update 6:40pm: ETA 21 minutes.
Update 6:46pm: ETA 10 minutes.
Update 6:56pm: Import has finished, we're now switching back into rw mode. This will involve a few seconds of downtime.
Update 6:59pm: MySQL is now available as usual. Total time in read-only mode was about two hours. Total downtime was less than a minute.