HOW TO CHECK IF ASYNCHRONOUS I/O IS WORKING ON LIN

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PURPOSE
-------
In this document we are going to explain how to check that asynchronous I/O
is working.

SCOPE & APPLICATION
-------------------
Many times there is a requirement to check if Asynchronous I/O is working
on Linux Platform, so we can try to use it for our datafiles access
inside database.
SOLUTION
--------
slabinfo maintains statistics about objects in memory. Some of the structs used
by Asynchronous I/O are threated as objects in the virtual memory, so we can
look for those structs on slabinfo. The ones related to AIO are named kio*.
$ cat /proc/slabinfo | grep kio
for example:
output with async io enabled.
$ cat /proc/slabinfo | grep kio
kioctx               270    270    128    9    9    1 :  252  126
kiocb              66080  66080     96 1652 1652    1 :  252  126
kiobuf               236    236     64    4    4    1 :  252  126
$
output with async io disabled.
$ cat /proc/slabinfo | grep kio
kioctx                 0      0    128    0    0    1 :  252  126
kiocb                  0      0     96    0    0    1 :  252  126
kiobuf                 0      0     64    0    0    1 :  252  126
$
There are 3 caches involved.
The kioctx and kiocb are Async I/O data structures that are defined in aio.h.
If it shows a non zero value that means async io is enabled.
If you have the source code loaded, you can review it at file aio.h.
This file is located under:
/usr/src/linux-/include/linux/aio.h
These data structures are using to track the I/O requests, and are allocated as
part of the  __init aio_setup() call in aio.c.


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