proc_pid_io(5) — Linux manual page

NAME | DESCRIPTION | CAVEATS | SEE ALSO | COLOPHON

proc_pid_io(5)             File Formats Manual            proc_pid_io(5)

NAME         top

       /proc/pid/io - I/O statistics

DESCRIPTION         top

       /proc/pid/io (since Linux 2.6.20)
              This file contains I/O statistics for the process and its
              waited-for children, for example:

                  # 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, sendfile(2),
                     copy_file_range(2), and ioctl(2)
                     BTRFS_IOC_ENCODED_READ[_32] (including 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, sendfile(2),
                     copy_file_range(2), and ioctl(2)
                     BTRFS_IOC_ENCODED_WRITE[_32] (including 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 negative I/O if caches dirtied
                     by another process are truncated.
                     cancelled_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         top

       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         top

       getrusage(2), proc(5)

COLOPHON         top

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Linux man-pages 6.9.1          2024-05-02                 proc_pid_io(5)

Pages that refer to this page: getrusage(2)