man > proc_pid_io(5)

proc_pid_io(5)                           File Formats Manual                          proc_pid_io(5)

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) and similar system calls.

              wchar: characters written
                     The number of bytes returned by successful write(2) and similar system calls.

              syscr: read syscalls
                     The  number  of  "file  read" system calls—those from the read(2) family, send‐
                     file(2), copy_file_range(2), and ioctl(2) 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) family, send‐
                     file(2), copy_file_range(2), and ioctl(2) 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)   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), proc(5)

Linux man-pages 6.7                          2024-03-18                               proc_pid_io(5)