PHP-fpm Too Many Open Files 24 Error

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You need to set open file descriptor rlimit for the PHP master process. The default value is system defined but one can increase it as per needs.

php-fpm too many open files error and solution

Edit php-fpm.conf
# vi /etc/php/7.0/fpm/php-fpm.conf
Find rlimit_files
;rlimit_files = 1024
And update it as follows (or as per your need):
rlimit_files = 4096
Save and close the file. Restart php7-0.fpm, run:
# systemctl restart php7.0-fpm

Increase FD limit at the OS level

Find user/group name:
$ grep ^user /etc/php/7.0/fpm/pool.d/www.conf
$ grep ^group /etc/php/7.0/fpm/pool.d/www.conf

www-data
www-data

Edit /etc/security/limits.conf, enter:
# vi /etc/security/limits.conf
Append as follows (replace user/group name with your actual names):

www-data       soft    nofile  4096
www-data       hard    nofile  4096

Restart required process. Save and close the file. Verify it:
$ su - www-data
To see the hard and soft values, issue the command as follows:
$ ulimit -Hn
$ ulimit -Sn

Reference: www.cyberciti.biz