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:
GFS file attribute
GFS directory attribute
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.
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.
Setting the directio Flag
gfs_tool setflag directio File
Clearing the directio Flag
gfs_tool clearflag directio File
Specifies the file where the directio flag is assigned.
In this example, the command sets the directio flag on the file named datafile in directory /gfs1.
gfs_tool setflag directio /gfs1/datafile
The gfs_tool command can be used to assign (set) a direct I/O attribute flag, inherit_directio, to a GFS directory. Enabling the inherit_directio flag on a directory causes all newly created regular files in that directory to automatically inherit the directio flag. Also, the inherit_directio flag is inherited by any new subdirectories created in the directory. The inherit_directio flag can also be cleared.
Setting the inherit_directio flag
gfs_tool setflag inherit_directio Directory
Clearing the inherit_directio flag
gfs_tool clearflag inherit_directio Directory
Specifies the directory where the inherit_directio flag is set.
In this example, the command sets the inherit_directio flag on the directory named /gfs1/data/.
gfs_tool setflag inherit_directio /gfs1/data/