Red Hat GFS 5.2.1 -, Руководство администратора

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96 Chapter 9. Managing GFS
Device
Specifies the device node of the file system.
Table 9-4 describes the GFS-specific options that can be used when adding journals to a GFS file
system.
Flag Parameter Description
-h
Help. Displays short usage message, then exits.
-J
MegaBytes
Specifies the size of the new journals in MBytes.
Default journal size is 128 MBytes. The minimum size
is 32 MBytes. To add journals of different sizes to the
file system, the
gfs_jadd
command must be run for
each size journal. The size specified is rounded down so
that it is a multiple of the journal-segment size that was
specified when the file system was created.
-j
Number
Specifies the number of new journals to be added by the
gfs_jadd
command. The default value is 1.
-T
Test. Do all calculations, but do not write any data to
the disk and do not add journals to the file system.
Enabling this flag helps discover what the
gfs_jadd
command would have done if it were run without this
flag. Using the
-v
flag with the
-T
flag turns up the
verbosity level to display more information.
-q
Quiet. Turns down the verbosity level.
-V
Display command version information, then exit.
-v
Turn up the verbosity of messages.
Table 9-4. GFS-specific Options Available When Adding Journals
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