O_Direct; Gfs File Attribute - Red Hat GLOBAL FILE SYSTEM 5.2 Manual

Chapter 3. Managing GFS
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
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
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
opened file.
To cause the
O_DIRECT
beginning of a source file before any includes, or define it on the cc line when compiling.
8.2. GFS File Attribute
The command can be used to assign (set) a direct I/O attribute flag,
gfs_tool
GFS file. The
directio
You can use the
gfs_tool stat filename
The output for this command includes a
the flags that are set for the indicated file.
Usage
Setting the
directio
gfs_tool setflag directio File
Clearing the
directio
gfs_tool clearflag directio File
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, or when a GFS direct I/O attribute is attached to a file, all
O_DIRECT
flag on an
O_DIRECT
flag to be defined with recent glibc libraries, define
flag can also be cleared.
to check what flags have been set for a GFS file.
Flags:
Flag
Flag
O_DIRECT
system call, direct I/O is used for the
open()
at the end of the display followed by a listing of
flag. Alternatively, GFS
at the
_GNU_SOURCE
, to a
directio