A DB instance outage can occur when a DB instance is rebooted, when the DB instance is put into astate that prevents access to it, and when the database is restarted. A reboot can occur when youmanually reboot your DB instance or when you change a DB instance setting that requires a rebootbefore it can take effect.
A DB instance reboot occurs immediately when one of the following occurs:You change the backup retention period for a DB instance from 0 to a nonzero value or from anonzero value to 0 and set Apply Immediately to true.
You change the DB instance class, and Apply Immediately is set to true. You change storage typefrom standard to PIOPS, and Apply Immediately is set to true. A DB instance reboot occurs during themaintenance window when one of the following occurs:
You change the backup retention period for a DB instance from 0 to a nonzero value or from anonzero value to 0, and Apply Immediately is set to false.
You change the DB instance class, and Apply Immediately is set to false.
Reference:
http://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/CHAP_Troubleshooting.html#CHAP_Troubleshooting.Security