对于DBA来说，必须注意一个地方，异步IO需要在两个地方启用。

1、OS级别支持异步IO

2、oracle级别启用异步IO

另外：关注异步IO，asmlib和裸设备是有区别的。

Goal
In this document we are going to explain how to check that asynchronous I/O (AIO) is working. AIO can be enabled in Oracle 9i 9.2 and higher.

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
The 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

If Async I/O is 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

and if Async I/O is 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
In the SLAB allocator there are three different caches involved. The kioctx and kiocb are Async I/O data structures that are defined in aio.h header file. 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-<version>/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.

Example strace of dbw0 process with AIO enabled (init.ora parameter filesystemio_options = asynch) shows:

...io_submit(3071864832, 1, {{0xb7302e34, 0, 1, 0, 21}}) = 1gettimeofday({1176916625, 58882}, NULL) = 0io_getevents(-1223102464, 1, 1024, {{0xb7302e34, 0xb7302e34, 8192, 0}}, {600, 0}) = 1...Example strace of dbw0 process with AIO disabled (filesystemio_options = none):

...pwrite64(21, "\6\242\0\0004\21\300\0\220B\243\0\0\0\1\6\207\357\0\0\1"..., 8192, 36077568) = 8192times(NULL) = 1775653082times(NULL) = 1775653082pwrite64(21, "\6\242\0\0<\21\300\0\220B\243\0\0\0\1\6\254\0\0\0\2\0*"..., 8192, 36143104) = 8192...Caveat for ASMLib
If Oracle ASMLib (see http://oss.oracle.com/projects/oracleasm ) is deployed, the kiocb structs are not used. ASMLib does not use the POSIX aio_*() functions. You will never see any kioctx or kiocb structures from ASMLib. It is far lower level than that.

In fact, ASMLib does AIO or SyncIO depending on how the I/O is passed to it, It makes no decisions at all. This is entirely up to kfk and the layers above it, kfk is entirely controlled by the disk_asynch_io parameter. So, we can check whether ASMLib is doing AIO by PL/SQL command "show param disk_asynch_io". (You can disable AIO by setting disk_asynch_io=false)

With ASMLib, AIO is done via ioctl() calls (2.4 kernel), or read() calls (2.6 kernel) on the ASM device. Whether ASMLib uses aio depends on whether oracle is configured to do aio, In oracle 10g, if ASMLib is in use, the i/o is asynchronous, because oracle 10g enables aio by default.

The strace when using ASMlib will show read calls that look like this:

...read(16, "MSA\0\2\0\10\0P\0\0\0\222\377\377\377@\313\373\5\0\0\0"..., 80) = 80...The first 3 characters, byte-swapped, are ASM, indicating an ASMLib I/O command structure.