9.7. Direct I/O
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 by only a few applications that manage their own caches, such as databases.
Direct I/O is invoked by an application 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
9.7.1. 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 beginning of a source file before any includes, or define it on the cc line when compiling.
 | Note |
|---|---|
Linux kernels from some distributions do not support use of the O_DIRECT flag. |
9.7.2. GFS File Attribute
The gfs_tool command can be used to assign a direct I/O attribute flag, directio, to a regular GFS file. The directio flag can also be cleared.
9.7.3. GFS Directory Attribute
The gfs_tool command can be used to assign 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.