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)