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

Fri Mar 12 07:27:34 UTC 2021
J Martin Rushton <martinrushton56 at btinternet.com>

I haven't been following this thread closely, so may be off target.

When pages are moved out of the working set they are either "clean" or 
"dirty".  Clean pages have not been modified since they were originally 
moved into memory whereas dirty pages have been changed.  A dirty page 
can become clean if it is written back to disk.  Typically (though not 
always) this is a write to swap.  Read only pages, such as code or data 
will always be clean, so can be dropped when required.

When a hard fault occurs then pages have to be read from disk, 
somewhere.  That somewhere could be swap, but can also be program images 
or files.  It may be that what you observe is this latter process.


On 12/03/2021 01:29, yf chu wrote:
> yes. I suspect it has something to do with swapping. but swap is turned off on this server.
> here is the result of free -m.
>   total        used        free      shared  buff/cache   available
> Mem:         128174       97449       24400        4158        6325       25232
> Swap:             0           0           0
> We have other servers. The processes running on these servers are same. but on other servers, the size of buff/cache is larger than the size on the server which experienced the problem and the size of "free" is smaller than the size on the server which experienced the problem.

J Martin Rushton MBCS