Increasing File Descriptors and Open Files Limit in CentOS 7

Some programs like Apache and MySQL require a higher number of file descriptors.
This is how you can increase that limit for all users in CentOS 7
Note: Commands require root access

1. Find the default limit – check the open files line – it will be 1024

   sudo ulimit -a

2. To increase edit nano /etc/sysctl.conf add the below line, save and exit

   fs.file-max = 100000

3. We also need to increase hard and soft limits
   Edit /etc/security/limits.conf add the below lines before the #End, save and exit 

   * soft nproc 65535
    * hard nproc 65535
    * soft nofile 65535
    * hard nofile 65535

4. Next, run the command

   sudo sysctl -p

5. for MySQL, edit /usr/lib/systemd/system/mysqld.service  and add the below 2 lines at the end, save and exit

   LimitNOFILE=65535
    LimitNPROC=65535

6. then increase the table_open_cache and open_files_limit in my.cnf

   # reload systemctl
    sudo systemctl daemon-reload

7. if you modified MySQL config, restart MySQL and check values for table_open_cache and open_files_limit

   systemctl restart mysqld.service

8. run the below command to check the open files limit – change user based on requirement
   output should say: open files (-n) 65535 

   # for mysql
    su - mysql -c 'ulimit -aHS' -s '/bin/bash'
   
   # for apache
    su - apache -c 'ulimit -aHS' -s '/bin/bash'