proc_pid_io - Man Page
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 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
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.