/proc/<pid>/io and iotop

• We look at the file "/proc/<pid>/io":

       $ cat /proc/1275/io 
       rchar: 1661777
       wchar: 7431
       syscr: 1240
       syscw: 123
       read_bytes: 7335936
       write_bytes: 12288
       cancelled_write_bytes: 0
       

• The fields are:
  • rchar: number of bytes the process read, using any read-like system call (from files, pipes, tty...).
  • wchar: number of bytes the process wrote using any write-like system call.
  • syscr: number of read-like system call invocations that the process performed.
  • syscr: number of write-like system call invocations that the process performed.
  • read_bytes: number of bytes the process directly read from disk.
  • write_bytes: number of bytes the process originally dirtied in the page-cache (assuming they will go to disk later).
  • cancelled_write_bytes: number of bytes the process "un-dirtied" - e.g. using an "ftruncate" call that truncated pages from the page-cache.
• One can use "iotop" to view the top I/O generators in real-time.

Originally written by guy keren