The /tmp partition in cPanel web servers might get filled for a variety of factors. Generally,/tmp partition become full with large MySQL queries that are dumping to the /tmp partition. A easy way to take care of this is to set the /tmp directory for MySQL to the main partition, which has much more space.
You can use the following steps to move the MySQL /tmp directory location:
Edit /etc/my.cnf file to set new tmpdir for MySQL
vi /etc/my.cnf
Place the following line into the file under the [mysqld] section:
tmpdir=/mysqltmp
Save the file with :wq
Create the /mysqltmp directory
mkdir /mysqltmp chmod 1777 /mysqltmp
Restart MySQL and check that the tmpdir is now /mysqltmp
/etc/init.d/mysql restart mysqladmin var | grep tmpdir
You will get the following result:
# mysqladmin var | grep tmpdir | slave_load_tmpdir | /mysqltmp | tmpdir | /mysqltmp