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 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 in-
voked 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 in-
voked 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 an-
other process are truncated. cancelled_write_bytes ap-
plies 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 opera-
tions 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 6.9.1 2024-05-02 proc_pid_io(5)
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