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.\" Copyright (C) 1994, 1995, Daniel Quinlan <quinlan@yggdrasil.com>
.\" Copyright (C) 2002-2008, 2017, Michael Kerrisk <mtk.manpages@gmail.com>
.\" Copyright (C) 2023, Alejandro Colomar <alx@kernel.org>
.\"
.\" SPDX-License-Identifier: GPL-3.0-or-later
.\"
.TH proc_pid_io 5 (date) "Linux man-pages (unreleased)"
.SH NAME
/proc/pid/io \- I/O statistics
.SH DESCRIPTION
.TP
.IR /proc/ pid /io " (since Linux 2.6.20)"
.\" commit 7c3ab7381e79dfc7db14a67c6f4f3285664e1ec2
This file contains I/O statistics for the process, for example:
.IP
.in +4n
.EX
.RB "#" " cat /proc/3828/io"
rchar: 323934931
wchar: 323929600
syscr: 632687
syscw: 632675
read_bytes: 0
write_bytes: 323932160
cancelled_write_bytes: 0
.EE
.in
.IP
The fields are as follows:
.RS
.TP
.IR rchar ": characters read"
The number of bytes which this task has caused to be read from storage.
This is simply the sum of bytes which this process passed to
.BR read (2)
and similar system calls.
It includes things such as terminal I/O and
is unaffected by whether or not actual
physical disk I/O was required (the read might have been satisfied from
pagecache).
.TP
.IR wchar ": characters written"
The number of bytes which this task has caused, or shall cause to be written
to disk.
Similar caveats apply here as with
.IR rchar .
.TP
.IR syscr ": read syscalls"
Attempt to count the number of read I/O operations\[em]that is,
system calls such as
.BR read (2)
and
.BR pread (2).
.TP
.IR syscw ": write syscalls"
Attempt to count the number of write I/O operations\[em]that is,
system calls such as
.BR write (2)
and
.BR pwrite (2).
.TP
.IR read_bytes ": bytes read"
Attempt to count the number of bytes which this process really did cause to
be fetched from the storage layer.
This is accurate for block-backed filesystems.
.TP
.IR write_bytes ": bytes written"
Attempt to count the number of bytes which this process caused to be sent to
the storage layer.
.TP
.IR cancelled_write_bytes :
The big inaccuracy here is truncate.
If a process writes 1 MB to a file and then deletes the file,
it will in fact perform no writeout.
But it will have been accounted as having caused 1 MB of write.
In other words: this field represents the number of bytes which this process
caused to not happen, by truncating pagecache.
A task can cause "negative" I/O too.
If this task truncates some dirty pagecache,
some I/O which another task has been accounted for
(in its
.IR write_bytes )
will not be happening.
.RE
.IP
.IR Note :
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.
.IP
Permission to access this file is governed by
.BR ptrace (2)
access mode
.BR PTRACE_MODE_READ_FSCREDS .
.SH SEE ALSO
.BR getrusage (2),
.BR proc (5)
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