How to Fix an (errno: 24) in MySQL

When creating a large number of partitions or tables, MySQL may mysteriously stop working and you find this type of error on /var/lib/mysql/hostname.err:

[ERROR] /usr/sbin/mysqld: Can't open file: './database/table.frm' (errno: 24)

errno: 24 simply means that too many files are open for the given process.
There is a read-only mysql variable called open_files_limit that will show how many open files are allowed by the mysqld:

SHOW VARIABLES LIKE 'open%';

 A lot systems set this to something very low, like 1024. Unfortunately, the following will NOT work: SET open_files_limit=100000

[mysqld]
open_files_limit = 100000

Then, be sure to restart mysql:

 /etc/init.d/mysql restart

Now, SHOW VARIABLES LIKE 'open%' should show 100000. The number you use may be different.