I bet you are all tired of this stuff already, but then again, so we are. We ourselves hope it's the last big maintenance we get to do at that site and then everything will stay stable for a while.
We are moving all VMs to new servers with more optimal hardware configuration and optimizing some configs. Among the VMs that will go down, likely for a few hours, are phabricator and jessie build host.
Things should get back to normal once we are done, we'll make another post when it's complete.