How to increase open file limit for mysql

he solution is to modify the mysql.conf file that defines the upstart service, it is located at /etc/init/mysql.conf and add the following lines before the pre-start block:

# NB: Upstart scripts do not respect
# /etc/security/limits.conf, so the open-file limits
# settings need to be applied here.
limit nofile 32000 32000

check :

mysql> show global variables like 'open%';