The variable in the config file works just fine for me:
elenst@linux-0bhr:/releases/mariadb-10.1.9-linux-x86_64> cat mdev9474.cnf
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[mysqld]
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group_concat_max_len = 65536
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elenst@linux-0bhr:/releases/mariadb-10.1.9-linux-x86_64> bin/mysqld_safe --defaults-file=/releases/mariadb-10.1.9-linux-x86_64/mdev9474.cnf &
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[1] 15794
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elenst@linux-0bhr:/releases/mariadb-10.1.9-linux-x86_64> 160127 14:45:19 mysqld_safe Logging to '/releases/mariadb-10.1.9-linux-x86_64/data/linux-0bhr.suse.err'.
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160127 14:45:19 mysqld_safe Starting mysqld daemon with databases from /releases/mariadb-10.1.9-linux-x86_64/data
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elenst@linux-0bhr:/releases/mariadb-10.1.9-linux-x86_64> bin/mysql -uroot --protocol=tcp --port=3306
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Welcome to the MariaDB monitor. Commands end with ; or \g.
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Your MariaDB connection id is 2
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Server version: 10.1.9-MariaDB MariaDB Server
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Copyright (c) 2000, 2015, Oracle, MariaDB Corporation Ab and others.
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Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
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MariaDB [(none)]> select @@group_concat_max_len;
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+------------------------+
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| @@group_concat_max_len |
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+------------------------+
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| 65536 |
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+------------------------+
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1 row in set (0.00 sec)
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MariaDB [(none)]> select @@version;
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+----------------+
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| @@version |
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+----------------+
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| 10.1.9-MariaDB |
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+----------------+
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1 row in set (0.00 sec)
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MariaDB [(none)]>
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The reason is usually one of two: either your config file is not read at all, or the option is later overridden in another section or another config file. Given that you are getting the default value, the first reason is more likely.
To check it, first start the server with the explicit --defaults-file=/etc/my.cnf option (it must be the first option on the command line!). If the option now gets set all right, it means your installation is such that it does not read from /etc/my.cnf by default. If the server does not start, it will say why, for example, current user does not have permissions to read the file. If the server starts but the option is still not set, check later sections and include directives in the file, something must be overridding the option.
Regarding your question about documentation, this is the page:
https://mariadb.com/kb/en/mariadb/server-system-variables/
Most variables work both from the config file and from the command-line, so if it is not mentioned specifically, it should be the case. Same goes for group_concat_max_len.
Bug
Major
The variable in the config file works just fine for me:
elenst@linux-0bhr:/releases/mariadb-10.1.9-linux-x86_64> cat mdev9474.cnf[mysqld]group_concat_max_len = 65536elenst@linux-0bhr:/releases/mariadb-10.1.9-linux-x86_64> bin/mysqld_safe --defaults-file=/releases/mariadb-10.1.9-linux-x86_64/mdev9474.cnf &[1] 15794elenst@linux-0bhr:/releases/mariadb-10.1.9-linux-x86_64> 160127 14:45:19 mysqld_safe Logging to '/releases/mariadb-10.1.9-linux-x86_64/data/linux-0bhr.suse.err'.160127 14:45:19 mysqld_safe Starting mysqld daemon with databases from /releases/mariadb-10.1.9-linux-x86_64/dataelenst@linux-0bhr:/releases/mariadb-10.1.9-linux-x86_64> bin/mysql -uroot --protocol=tcp --port=3306Welcome to the MariaDB monitor. Commands end with ; or \g.Your MariaDB connection id is 2Server version: 10.1.9-MariaDB MariaDB ServerCopyright (c) 2000, 2015, Oracle, MariaDB Corporation Ab and others.Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.MariaDB [(none)]> select @@group_concat_max_len;+------------------------+| @@group_concat_max_len |+------------------------+| 65536 |+------------------------+1 row in set (0.00 sec)MariaDB [(none)]> select @@version;+----------------+| @@version |+----------------+| 10.1.9-MariaDB |+----------------+1 row in set (0.00 sec)MariaDB [(none)]>The reason is usually one of two: either your config file is not read at all, or the option is later overridden in another section or another config file. Given that you are getting the default value, the first reason is more likely.
To check it, first start the server with the explicit --defaults-file=/etc/my.cnf option (it must be the first option on the command line!). If the option now gets set all right, it means your installation is such that it does not read from /etc/my.cnf by default. If the server does not start, it will say why, for example, current user does not have permissions to read the file. If the server starts but the option is still not set, check later sections and include directives in the file, something must be overridding the option.
Regarding your question about documentation, this is the page:
https://mariadb.com/kb/en/mariadb/server-system-variables/
Most variables work both from the config file and from the command-line, so if it is not mentioned specifically, it should be the case. Same goes for group_concat_max_len.