# proc_pid_io - I/O statistics - man(5) - [phpMan]

[_proc_pid_io_(5)](https://www.chedong.com/phpMan.php/man/procpidio/5/markdown)                           File Formats Manual                          [_proc_pid_io_(5)](https://www.chedong.com/phpMan.php/man/procpidio/5/markdown)

## NAME
       /proc/pid/io - I/O statistics

## DESCRIPTION
       _/proc/_pid_/io_ (since Linux 2.6.20)
              This file contains I/O statistics for the process and its waited-for children, for ex‐
              ample:

                  # **cat /proc/3828/io**
                  rchar: 323934931
                  wchar: 323929600
                  syscr: 632687
                  syscw: 632675
                  read_bytes: 0
                  write_bytes: 323932160
                  cancelled_write_bytes: 0

              The fields are as follows:

              _rchar_: characters read
                     The number of bytes returned by successful [**read**(2)](https://www.chedong.com/phpMan.php/man/read/2/markdown) and similar system calls.

              _wchar_: characters written
                     The number of bytes returned by successful [**write**(2)](https://www.chedong.com/phpMan.php/man/write/2/markdown) and similar system calls.

              _syscr_: read syscalls
                     The  number  of  "file  read" system calls—those from the [**read**(2)](https://www.chedong.com/phpMan.php/man/read/2/markdown) family, **send‐**
                     [**file**(2)](https://www.chedong.com/phpMan.php/man/file/2/markdown), [**copy_file_range**(2)](https://www.chedong.com/phpMan.php/man/copyfilerange/2/markdown), and [**ioctl**(2)](https://www.chedong.com/phpMan.php/man/ioctl/2/markdown) **BTRFS_IOC_ENCODED_READ**[**_32**]  (includ‐
                     ing when invoked by the kernel as part of other syscalls).

              _syscw_: write syscalls
                     The  number  of "file write" system calls—those from the [**write**(2)](https://www.chedong.com/phpMan.php/man/write/2/markdown) family, **send‐**
                     [**file**(2)](https://www.chedong.com/phpMan.php/man/file/2/markdown), [**copy_file_range**(2)](https://www.chedong.com/phpMan.php/man/copyfilerange/2/markdown), and [**ioctl**(2)](https://www.chedong.com/phpMan.php/man/ioctl/2/markdown) **BTRFS_IOC_ENCODED_WRITE**[**_32**] (includ‐
                     ing when invoked by the kernel as part of other syscalls).

              _read_bytes_: bytes read
                     The number of bytes really fetched from the storage layer.   This  is  accurate
                     for block-backed filesystems.

              _write_bytes_: bytes written
                     The number of bytes really sent to the storage layer.

              _cancelled_write_bytes_:
                     The  above  statistics fail to account for truncation: if a process writes 1 MB
                     to a regular file and then removes it, said 1 MB will not be written, but  _will_
                     have  nevertheless  been  accounted as a 1 MB write.  This field represents the
                     number of bytes "saved" from I/O writeback.  This can yield to having done neg‐
                     ative  I/O  if  caches  dirtied  by  another  process  are   truncated.    _can‐_
                     _celled_write_bytes_ applies to I/O already accounted-for in _write_bytes_.

              Permission   to   access   this   file   is   governed   by   [**ptrace**(2)](https://www.chedong.com/phpMan.php/man/ptrace/2/markdown)   access  mode
              **PTRACE_MODE_READ_FSCREDS**.

## CAVEATS
       These counters are not atomic: on systems where 64-bit integer operations may tear, a counter
       could be updated simultaneously with a read, yielding an incorrect intermediate value.

## SEE ALSO
       [**getrusage**(2)](https://www.chedong.com/phpMan.php/man/getrusage/2/markdown), [**proc**(5)](https://www.chedong.com/phpMan.php/man/proc/5/markdown)

Linux man-pages 6.7                          2024-03-18                               [_proc_pid_io_(5)](https://www.chedong.com/phpMan.php/man/procpidio/5/markdown)