CentOS 7, systemd, redis, max open files

在CentOS 7安裝好Redis，預設redis會開啟10000的檔案，所以啟動之後log總會顯示以下錯誤：

Current maximum open files is 1024. maxclients has been reduced to 4064 to compensate for low ulimit. If you need higher maxclients increase 'ulimit -n'.

即使修改/etc/security/limits.conf也沒有用，這個檔案一開頭就說：

This file sets the resource limits for the users logged in via PAM.
It does not affect resource limits of the system services.

因為CentOS 7用systemd換掉ntsysv，網路上有提示解法，就是修改redis的systemd配置檔案/lib/systemd/system/redis.service正確來說是/etc/systemd/system/redis.service.d/limit.conf，把LimitNOFILE=10240後面的數字改成所需的。

修改之後，要先重新載入systemd，否則會出現警告。如下

$ service redis restart
Redirecting to /bin/systemctl restart  redis.service
Warning: Unit file of redis.service changed on disk, 'systemctl daemon-reload' recommended.