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
                     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.

SEE ALSO
       getrusage(2), proc(5)

Linux man-pages 6.7               2024-03-18                    proc_pid_io(5)