Red Hat GFS 6.1 -, Administrator'S Manual

Page: 61 / 86

Chapter 5. Managing GFS 47
Flag Parameter Description
-v
Turns up the verbosity of messages.
Table 5-4. GFS-specific Options Available When Adding Journals
5.7. 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 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
5.7.1.
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.
5.7.2. 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.