MariaDB with XtraDB uses times more of IO events than with InnoDB plugin

If MariaDB is started with default options, it uses several times more IO events than if it's started with InnoDB plugin, or than Percona server, or MySQL server.

On my machine, if I run the server manually with all default options, the values are:

MariaDB 5.5/10.0 with InnoDB plugin, Percona 5.5/5.6, MySQL 5.5/5.6:
2661

MariaDB 5.5/10.0 with XtraDB:
14337

With MTR the values are somewhat different, possibly due to non-default options, but the problem is still obvious:

MariaDB 5.5/10.0 with InnoDB plugin, Percona 5.5/5.6, MySQL 5.5/5.6:
4298

MariaDB 5.5/10.0 with XtraDB:
15050

Apparently, it has been like that from early versions of MariaDB 5.5.

While hitting the maximum is not a fatal problem for MariaDB, it still means that the native AIO will be switched off much earlier than it could have been.

MTR test:

--source include/have_innodb.inc

--echo AIO count:

--exec cat /proc/sys/fs/aio-nr