[CentOS] Disk read io very high, but no process perform io read

Fri Mar 5 07:54:14 UTC 2021
yf chu <cyflhn at 163.com>

We have experienced a very weird problem. The load of the server machine is very high. We use "pidstat" and find that the disk read io is very high.  
But the processes running on this server do not perform any disk read operation. 
We also noticed that when we execute "top" command, for most of processes, the values in "SHR" column are zero. 
Compared with other normal servers, we found that by executing "free -m", the result shows that the buff/cache value in this server is lower than the value of other normal servers.
We also found that there were lots of major page faults by executing "ps -o majflt,minflt".
Swap is not enabled on this server. What could be the reason for this issue?

The centos version:CentOS Linux release 7.3.1611
kernel version:3.10.0-693.21.1.std7a.el7.0.x86_64