MariaDB/MySQL IO limit in test environment
InnoDB only processes in "RAM" -- specifically its "buffer_pool".  InnoDB will persist things to "disk".  So InnoDB will be doing a lot of ram-to-ram copies if you use a RAM-disk.  However, you could save some RAM by decreasing innodb_buffer_pool_size and various other settings.  Caution:  don't lower them too much, or there can be failures.