Red Hat GLOBAL FILE SYSTEM 5.2 Installationsanleitung PDF herunterladen (Seite 40)

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

applications 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

8.1. 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

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

8.2. 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.

You can use the gfs_tool stat filename to check what flags have been set for a GFS file.

The output for this command includes a Flags: at the end of the display followed by a listing of

the flags that are set for the indicated file.

Usage

Setting the directio Flag

gfs_tool setflag directio File

Clearing the directio Flag

gfs_tool clearflag directio File

Chapter 3. Managing GFS

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Red Hat GLOBAL FILE SYSTEM 5.2 Installationsanleitung Seite 40