¿Qué significan los contadores en /proc/[pid]/io?

Mientras que la página de manual de proc está lamentablemente atrasada (y también lo están la mayoría de las páginas de manual/documentación sobre cualquier cosa que no esté relacionada con el desarrollo del espacio de usuario de corte de cookies), este material está afortunadamente documentado completamente en la fuente del kernel de Linux bajo Documentation/filesystems/proc.txt. Aquí están los bits relevantes:

rchar
-----

I/O counter: chars 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() and pread().
It includes things like tty IO and it is unaffected by whether or not actual
physical disk IO was required (the read might have been satisfied from
pagecache)


wchar
-----

I/O counter: chars 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.


read_bytes
----------

I/O counter: bytes read
Attempt to count the number of bytes which this process really did cause to
be fetched from the storage layer. Done at the submit_bio() level, so it is
accurate for block-backed filesystems. <please add status regarding NFS and
CIFS at a later time>


write_bytes
-----------

I/O counter: bytes written
Attempt to count the number of bytes which this process caused to be sent to
the storage layer. This is done at page-dirtying time.