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, 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 which this task has caused to be read from storage. This is simply the sum of bytes which this process passed to read(2) and similar system calls. It includes things such as terminal I/O and is unaffected by whether or not actual physical disk I/O was required (the read might have been satisfied from pagecache).

    wchar: characters written
        The number of bytes which this task has caused, or shall cause to be written to disk. Similar caveats apply here as with rchar.

    syscr: read syscalls
        Attempt to count the number of read I/O operations—that is, system calls such as read(2) and pread(2).

    syscw: write syscalls
        Attempt to count the number of write I/O operations—that is, system calls such as write(2) and pwrite(2).

    read_bytes: bytes read
        Attempt to count the number of bytes which this process really did cause to be fetched from the storage layer. This is accurate for block-backed filesystems.

    write_bytes: bytes written
        Attempt to count the number of bytes which this process caused to be sent to the storage layer.

    cancelled_write_bytes:
        The big inaccuracy here is truncate. If a process writes 1 MB to a file and then deletes the file, it will in fact perform no writeout. But it will have been accounted as having caused 1 MB of write. In other words: this field represents the number of bytes which this process caused to not happen, by truncating pagecache. A task can cause "negative" I/O too. If this task truncates some dirty pagecache, some I/O which another task has been accounted for (in its write_bytes) will not be happening.

    Note: In the current implementation, things are a bit racy on 32-bit systems: if process A reads process B's /proc/pid/io while process B is updating one of these 64-bit counters, process A could see an intermediate result.

    Permission to access this file is governed by a ptrace access mode PTRACE_MODE_READ_FSCREDS check; see ptrace(2).

SEE ALSO
proc(5)