To enable async I/O in Oracle, the disk_asynch_io parameter needs to be set to true:

disk_asynch_io=true

Note this parameter is set to true by default in Oracle 9i and 10g:

SQL> show parameter disk_asynch_io;

NAME TYPE VALUE
———————————— ———– ——————————
disk_asynch_io boolean TRUE
SQL>

If you use filesystems instead of raw devices, block devices (available in 10gR2) or ASM for datafiles, then you need to ensure that the datafiles reside on filesystems that support asynchronous I/O (e.g., OCFS/OCFS2, ext2, ext3). To do async I/O on filesystems the filesystemio_options parameter needs to be set to “asynch” in addition to disk_asynch_io=true:

filesystemio_options=asynch

This parameter is platform-specific. By default, this parameter is set to none for Linux and thus needs to be changed:

SQL> show parameter filesystemio_options;

NAME TYPE VALUE
———————————— ———– ——————————
filesystemio_options string none
SQL>

The filesystemio_options can have the following values with Oracle9iR2:
asynch: This value enables asynchronous I/O on file system files.
directio: This value enables direct I/O on file system files.
setall: This value enables both asynchronous and direct I/O on file system files.
none: This value disables both asynchronous and direct I/O on file system files.

If you also want to enable Direct I/O Support which is available in RHEL 3/4, set filesystemio_options to “setall”.

Ensure that the datafiles reside on filesystems that support asynchronous I/O (e.g., OCFS, ext2, ext3).

Tuning Asynchronous I/O for Oracle 9i and 10g

For RHEL 3 it is recommended to set aio-max-size to 1048576 since Oracle uses I/Os of up to 1MB. It controls the maximum I/O size for asynchronous I/Os. Note this tuning parameter is not applicable to 2.6 kernel, i.e RHEL 4.

To determine the maximum I/O size in bytes, execute:

$ cat /proc/sys/fs/aio-max-size
131072

To change the maximum number of bytes without reboot:

# echo 1048576 > /proc/sys/fs/aio-max-size

Alternatively, you can use sysctl(8) to change it:

# sysctl -w fs.aio-max-size=1048576

To make the change permanent, add the following line to the /etc/sysctl.conf file. This file is used during the boot process:

$ echo “fs.aio-max-size=1048576” >> /etc/sysctl.conf

Checking Asynchronous I/O Usage

To verify whether $ORACLE_HOME/bin/oracle was linked with async I/O, you can use the Linux commands ldd and nm.

In the following example, $ORACLE_HOME/bin/oracle was relinked with async I/O:

$ ldd $ORACLE_HOME/bin/oracle | grep libaio
libaio.so.1 => /usr/lib/libaio.so.1 (0x0093d000)
$ nm $ORACLE_HOME/bin/oracle | grep io_getevent
w io_getevents@@LIBAIO_0.1
$

In the following example, $ORACLE_HOME/bin/oracle has NOT been relinked with async I/O:

$ ldd $ORACLE_HOME/bin/oracle | grep libaio
$ nm $ORACLE_HOME/bin/oracle | grep io_getevent
w io_getevents
$

Referencia:

http://www.puschitz.com/TuningLinuxForOracle.shtml