Basically all the session files stored in the tmp directory and some time due to the excessive connection to mysql, we are facing the tmp directory 100% full problem at that time we can use the following command in cron and remove the session file more then 30 minutes old.
find /tmp -type f -name sess_* -cmin +30 |xargs rm -f
In above command I have used +30 for the 30 minutes, you can use the values as per your requirement.
