MariaDB

This is an issue which occurs due to the maximum number of open files allowed for a process (mysql in this case) being very less that what is required - something that happens because you have lots of large tables / a large DB. To fix this, you just have to increase the limits in a few places.

Here is how you do it:

Step 1: Identify the issue
as limit on the max number of open files (1024) for a single process (mysql) by doing these as root user:
ulimit -a | grep open