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)