Installation guide

7. Direct I/O

Direct I/O is a feature of the file system whereby file reads and writes go directly from the applic-

ations to the storage device, bypassing the operating system read and write caches. Direct I/O

is used only by applications (such as databases) that manage their own caches.

An application invokes direct I/O by opening a file with the O_DIRECT flag. Alternatively, GFS can

attach a direct I/O attribute to a file, in which case direct I/O is used regardless of how the file is

opened.

When a file is opened with O_DIRECT, or when a GFS direct I/O attribute is attached to a file, all I/

O operations must be done in block-size multiples of 512 bytes. The memory being read from or

written to must also be 512-byte aligned.

One of the following methods can be used to enable direct I/O on a file:

• O_DIRECT

• GFS file attribute

• GFS directory attribute

7.1. O_DIRECT

If an application uses the O_DIRECT flag on an open() system call, direct I/O is used for the

opened file.

To cause the O_DIRECT flag to be defined with recent glibc libraries, define _GNU_SOURCE at the be-

ginning of a source file before any includes, or define it on the cc line when compiling.

7.2. GFS File Attribute

The gfs_tool command can be used to assign (set) a direct I/O attribute flag, directio, to a

GFS file. The directio flag can also be cleared.

Usage

Setting the directio Flag

gfs_tool setflag directio File

Clearing the directio Flag

gfs_tool clearflag directio File

File

Specifies the file where the directio flag is assigned.

Example

7.1. O_DIRECT