Hi,
I have installed 
MariaDB 10.1.35
in a container (LXC) with Debian 9.5.
 
Starting the database service I get this warning:
Nov 02 03:23:50 ct125-netdot mysqld[25687]: 2018-11-02  3:23:50 140396535071680 [Warning] Could not increase number of max_open_files to more than 4096 (request: 4182)
 
MariaDB [(none)]> SHOW GLOBAL VARIABLES LIKE 'open_files_limit';
+------------------+-------+
| Variable_name    | Value |
+------------------+-------+
| open_files_limit | 4096  |
+------------------+-------+
1 row in set (0.00 sec)

 
 
Checking MariaDB documentation "Server System Variables" here (https://mariadb.com/kb/en/library/server-system-variables/#open_files_limit) I get this info:

open_files_limit

mysql soft nofile 65535
mysql hard nofile 65535

rebooting, and then create /etc/systemd/system/mysqld.service.d/filelimit.conf or /etc/systemd/system/mariadb.service.d/filelimit.conf (corresponding to the service name).

[Service]
LimitNOFILE=infinity
 
I have modified file /etc/security/limits.conf accordingly:
mysql           soft    nofile          4182
mysql           hard    nofile          4182

 
And I have create file /etc/systemd/system/mariadb.service.d/filelimit.conf:
root@ct125-netdot:~# more /etc/systemd/system/mariadb.service.d/filelimit.conf
[Service]
LimitNOFILE=4182
 
Why is MariaDB writing this warning?
 
THX