InnoDB Write buffer efficiency: 0.00%

[gate2vn]

I am using mysqltuner.pl for checking optimization. Somehow, the read buffer is good, but write buffer is always 0%. Here is my.cnf related to innodb

innodb_buffer_pool_instances=13
innodb_buffer_pool_size=13G
innodb_write_io_threads=16
innodb_flush_method=O_DIRECT
innodb_flush_log_at_trx_commit=2
innodb_change_buffer_max_size=50
innodb_thread_concurrency=12
innodb_log_buffer_size=1G

And the result from mysqltuner.

-------- InnoDB Metrics -----------------------------------------------------
[--] InnoDB is enabled.
[OK] InnoDB buffer pool / data size: 13.0G/12.4G
[OK] InnoDB buffer pool instances: 13
[!!] InnoDB Used buffer: 6.60% (56267 used/ 851955 total)
[OK] InnoDB Read buffer efficiency: 100.00% (29461228058 hits/ 29461261330 total)
[!!] InnoDB Write buffer efficiency: 0.00% (0 hits/ 1 total)
[OK] InnoDB log waits: 0.00% (0 waits / 2064979 writes)

Any idea for optimizing write buffer please? The engine is MariaDB 10.0.21 x86_64.

Thank you.