The innodb_write_io_threads value determines the number of I/O threads for writing to InnoDB. The default is 4, the minimum possible is 1, the maximum is 64.
I will give an example of viewing the current value:
show variables like 'innodb_write_io_threads';
You cannot change the value without restarting the MySQL server, I will give an example of an error:
SET global innodb_write_io_threads=32;
ERROR 1238 (HY000): Variable 'innodb_write_io_threads' is a read only variable
Therefore, we will indicate it in the MySQL server configuration file:
[mysqld]
innodb_write_io_threads=32
And restart the MySQL server:
service mysql restart
See also my article:
How to change innodb_thread_concurrency in MySQL