Why tokudb’s read rate is very large when no reading query?

[郭枫]

I run MariaDB 10 with all tables using tokudb engine, and I find an interesting phenomenon: the slave just sync data from master, and there is no reading query,but disk's reading rate is about 100M+(mysqld is  reading from iotop)~So is there any method that I could dig why reading rate is that high? 

Just a non-reading slave：

MariaDB [(none)]> show processlist;
+--------+-------------+-----------+--------------+---------+--------+----------------------------------+------------------+----------+
| Id     | User        | Host      | db           | Command | Time   | State                            | Info             | Progress |
+--------+-------------+-----------+--------------+---------+--------+----------------------------------+------------------+----------+
| 137210 | system user |           | NULL         | Connect | 171356 | Waiting for master to send event | NULL             |    0.000 |
| 137211 | system user |           | NULL         | Connect | 142654 | executing                        | NULL             |    0.000 |
| 290475 | root        | localhost | c2c_wx_db_78 | Sleep   |    251 |                                  | NULL             |    0.000 |
| 290846 | root        | localhost | NULL         | Query   |      0 | init                             | show processlist |    0.000 |
+--------+-------------+-----------+--------------+---------+--------+----------------------------------+------------------+----------+
4 rows in set (0.00 sec)

But mysqld's reading rate is very high using iotop:

Total DISK READ :     145.94 M/s | Total DISK WRITE :      10.71 M/s
Actual DISK READ:     146.28 M/s | Actual DISK WRITE:      11.07 M/s
  TID  PRIO  USER     DISK READ  DISK WRITE  SWAPIN     IO>    COMMAND                                                                                                                                                                      
 3461 be/4 mysql     145.60 M/s  585.15 K/s  0.00 % 23.81 % mysqld --basedir=/usr/local/mysql --datadir=/data1/zftMysqlData --plugin-dir=/usr/local/~iles-limit=8192 --pid-file=mysql.pid --socket=/data1/zftMysqlData/mysql.sock --port=3306