Installation guide

Enabling Asynchronous I/O and Direct I/O Support

Asynchronous I/O permits Oracle to continue processing after issuing I/Os requests, which leads to higher I/O

performance. Red Hat Enterprise Linux also allows Oracle to issue multiple simultaneous I/O requests with a

single system call. This reduces context switch overhead and allows the kernel to optimize disk activity.

Oracle 10g Release 2 is shipped with asynchronous I/O support enabled by default. It does not need to be re-

linked as previous versions of Oracle database servers but you may have to apply a patch, see below.

Enabling Asynchronous I/O in Oracle 10g

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

disk_asynch_io=true

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

SQL> show parameter disk_asynch_io;

NAME TYPE VALUE

------------------------------------ ----------- ----------------------

disk_asynch_io boolean TRUE

SQL>

If you use file systems instead of raw devices or ASM for data files, then you need to ensure that the data files

reside on file systems that support asynchronous I/O (e.g., ext3, gfs, etc.). To do async I/O on file systems, 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 variable can have the following values: asynch: Enables asynchronous I/O

on file system files

directio: Enables direct I/O on file system files

setall: Enables both asynchronous and direct I/O on file system files

none: Disables both asynchronous and direct I/O on file system files

If you also wish to enable Direct I/O Support, set filesystemio_options to "setall".

For Red Hat Enterprise Linux 5, it is strongly recommended to use “setall” for ext2, ext3, GFS, NFS and