If you're running some command that takes a while, perhaps compressing a large file, you often want to know how far along it is. This is when the
/proc/_pid_/io file is useful.
I might have a
bzip2 process running with PID 3722, so
/proc/3722/io might look like this:
rchar: 1791093411 wchar: 224891624 syscr: 1308428 syscw: 2992919 read_bytes: 250478592 write_bytes: 183074816 cancelled_write_bytes: 0
The most useful are "rchar" and "wchar" - the number of bytes read and written by this process.
The kernel documentation tells you more about this file.