I just updated our Phabricator install today to rP3d3fff4991ab921853fd0d1fc96f8b1e65b962d7. Previously we were probably somewhere around rP45718268a96ef8290ec1e5bed0ab5b978ca9f1a2. Today we have had two occasions in which users have received "Can't connect to MySQL server" error messages. The first time that it happened I figured that it was some AWS issue, but the second time makes me suspect that there are performance issues with the latest update.
The following screenshot shows the CloudWatch metrics for our RDS database over the past two weeks:
None of the metrics shown in the above screenshot seem worrying to me, but I did notice that memory usage was quite high around the time of the second downtime. I'm not sure if I am onto the right track, but I noticed that some queries on the home page are doing full table scans... I will post some screenshots from DarkConsole in a few minutes.