/proc/pid/io - I/O statistics
- /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).