/proc/pid/io - I/O statistics

/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.

getrusage(2), proc(5)