MySQL 5.7 Release Notes
variable controls server verbosity for writing error, warning,
and note messages to the error log. Permitted values are 1
(errors only), 2 (errors and warnings), 3 (errors, warnings, and
notes), with a default of 3. If the value is greater than 2, the
server logs aborted connections and access-denied errors for new
connection attempts. See Section B.3.2.9, “Communication Errors and Aborted Connections”.