Buffered warning: Changed limits: max_connections: 214 (requested 800)
I would not recommend to edit the original systemd file as it will be overwritten during updates.
To modify the limits, do the following:
mkdir /etc/systemd/system/mysqld.service.d
Inside that directory, create new file limits.conf and add the following to that file:
[Service]
LimitNOFILE = 65535
finally reload systemd with:
systemctl daemon-reload
and restart mysqld to enable the change:
systemctl restart mysqld
Now validate that the change was successful by using the following query:
mysql> show variables like '%file%';
You should find a line like this:
| open_files_limit | 65535
That's it, this way your changes survive MySQL updates.
Having just spent an hour or two facing the exact same problem on CentOS 7 with MySQL 5.6.26, here's my solution. In addition to raising the max open files (for the mysql user), which it looks like you've already done, you need to add "LimitNOFILE=65535" (or similar depending on how high a limit you want to set) to your mysql.service definition file.
So the complete solution for me was as follows
append these two lines to /etc/security/limits.conf
mysql hard nofile 65535
mysql soft nofile 65535
append this line to /usr/lib/systemd/system/mysqld.service (in the [service] section)
LimitNOFILE=65535
then finally reboot and check that those error messages have disappeared from your mysql error log.
EDIT: Thanks to @SieGeL below for reminding me to add, if you update the ".service" file directly your edits will be lost on upgrade. To keep edits after an upgrade use a systemd override by creating an additional conf file in /etc/systemd/system/mysqld.service.d