Oracle Internals Notes

Direct I/O

Direct I/O is I/O to file system based files which bypasses the operating system's file system buffer cache. Direct I/O saves memory and boosts the performance of database applications like Oracle that cache their own data independently (more). It also avoids performance problems associated with using database block sizes that do not match the file system buffer size exactly. (more). Whether direct I/O is available, and the way in which it can be enabled, are dependent on the operating system and file system type in use. Direct I/O can sometimes be enabled for an entire file system, using a file system mount option. Application programs may also be able to request direct I/O for a particular file by setting a flag when opening the file.

Most operating systems support numerous types of file systems. Some aspects of file system operation are independent of file system type and are implemented in a common part of the operating system kernel. However, many other aspects of file system operation, including direct I/O availability, are dependent on the type of file system in use. For example, some file systems cannot support direct I/O because their file system structure is such that file system blocks do not align with device block boundaries and so I/O requests cannot be mapped directly from file system blocks to device blocks. File systems that can support direct I/O may only be able to do so via a mount option if the file system independent layer of the operating system does not support a direct I/O flag in its open() system call.

The following table details the availability of program selectable direct I/O for some important operating systems. Oracle may use this operating system feature automatically when opening database files, or it may be used based on the setting of the _filesystemio_options parameter. Of course, the file system must support direct I/O for that setting to be effective. See the second table below for details about which file systems support program selectable direct I/O.

Operating system Program selectable direct I/O

Solaris available, used based on _filesystemio_options

Windows NT/2000 available, used automatically

Tru64 Unix available, used automatically from 8.1.6.3 (and in 8.0.6.3) on 5.1 with PK4
or 5.1A with PK1 or later; can be disabled with _tru64_directio_disabled

AIX available, usage unknown

Irix available, usage unknown

Linux available (2.4 kernels), not yet used by Oracle

HP-UX not available

Reliant Unix not available

... ...

Operating system	Program selectable direct I/O
Solaris	available, used based on _filesystemio_options
Windows NT/2000	available, used automatically
Tru64 Unix	available, used automatically from 8.1.6.3 (and in 8.0.6.3) on 5.1 with PK4 or 5.1A with PK1 or later; can be disabled with _tru64_directio_disabled
AIX	available, usage unknown
Irix	available, usage unknown
Linux	available (2.4 kernels), not yet used by Oracle
HP-UX	not available
Reliant Unix	not available
...	...

The following table details direct I/O availability for some important file systems.

File system Mount option Program selectable

Solaris UFS forcedirectio yes

Solaris VxFS convosync=direct yes

HP-UX HFS none no

HP-UX BaseJFS none no

HP-UX OnlineJFS convosync=direct no

AIX JFS none yes

AIX VxFS convosync=direct yes

Irix EFS none yes

Irix XFS none yes

Reliant Unix VxFS convosync=direct no

Reliant Unix UFS none no

Tru64 Unix AdvFS none yes

Tru64 Unix UFS none no

... ... ...

File system	Mount option	Program selectable
Solaris UFS	`forcedirectio`	yes
Solaris VxFS	`convosync=direct`	yes
HP-UX HFS	none	no
HP-UX BaseJFS	none	no
HP-UX OnlineJFS	`convosync=direct`	no
AIX JFS	none	yes
AIX VxFS	`convosync=direct`	yes
Irix EFS	none	yes
Irix XFS	none	yes
Reliant Unix VxFS	`convosync=direct`	no
Reliant Unix UFS	none	no
Tru64 Unix AdvFS	none	yes
Tru64 Unix UFS	none	no
...	...	...

If your operating system or file system is not listed in the tables above, and if you are aware of the details, please click on the feedback icon below and send us an email so that we can add it to the list.