fcache_fb_policy(5)						File Formats Manual					       fcache_fb_policy(5)

NAME

       fcache_fb_policy - policy for flushing dirty file data to disk

VALUES

   Failsafe
   Default
   Allowed values
       Minimum:
       Maximum:

DESCRIPTION

       HP-UX  asynchronously flushes dirty data to disk to manage the risk of data loss and avoid burst mode I/O.  However, excessive flushing can
       cause extra I/O and reduce performance.	The tunable is used to determine the trade-off between write performance  and  data  availability.
       The tunable has three settings:

	      This is the default setting. It is a high-performance setting.
		     It  minimizes  the performance impact of optional flushing.  For example, in case of mapped access to a file, the dirty pages
		     are only flushed out asynchronously when they are unmapped by a call (see munmap(2)).  The asynchronous flushing  allows  the
		     call  to  complete  quicker than if the flushing was syncrhonous.	For file system access, the dirty data is flushed every 30
		     seconds or so by the syncer daemon.  This setting is a high-performance setting.  It disables the VxFS Flush Behind  feature.
		     Refer to the next paragraph, for more details about VxFS Flush Behind feature.

	      Please note that the semantics of default behavior are liable to change as HP-UX identifies more optimal settings.

	      This setting is a lower performance setting, but it provides more frequent
		     flushing  to  disk.  It  increases the amount of flushing for file system access by enabling the Flush Behind feature of VxFS
		     File System.  Dirty pages are flushed when the number of dirty pages in memory for a file reaches a certain threshold.  These
		     flushes  are  controlled by a variety of VxFS tunables, such as and See the vxtunefs(1M) manpage for more details on the VxFS
		     tunables.

	      This setting corresponds to the HP-UX 11i v2 default behavior.
		     It does the same amount of flushing as the tunable setting 1, but the I/O requests for flushing data  are	scheduled  immedi-
		     ately (synchronously).

   Who Is Expected to Change This Tunable?
       System administrators who run applications that do large file writes may change this tunable.

   Restrictions on Changing
       Changes to this tunable will take effect immediately.

   When Should the Value of This Tunable Be Changed to 1?
       This  tunable  should be changed from 0 to 1 if applications running on the system require more data integrity than the HP-UX syncer daemon
       provides.

       In addition, should be changed to 1 if the VxFS tunable, is set.  The tunable lets an administrator lower the number of dirty  buffers  per
       file  that  the file system will generate before writing them to disk.  The default value of is zero.  The default value places no limit on
       the number of dirty buffers per file.  With the default setting, there is no problem bypassing the Flush Behind feature.   However,  if	is
       enabled, the application performing writes may encounter longer delays as it waits for to flush some of the dirty buffers before the appli-
       cation can continue.

   When Should the Value of This Tunable Be Changed to 2?
       This tunable should be changed to 2 if applications running on the system have been highly tailored for HP-UX 11i v2 behavior.  It provides
       the most backwards compatible behavior.

   What Are the Side Effects of Changing the Value?
       Changing  the  value  from 0 to 1 will cause increased I/O activity.  Changing the value from 0 to 2 will cause lower response time for and
       system calls.  It may result in loss of overall write performance.

   What Other Tunables Should Be Changed at the Same Time?
       None.

WARNINGS

       All HP-UX kernel tunable parameters are release specific.  This parameter may be removed or have its meaning changed in future releases	of
       HP-UX.

       Installation  of  optional  kernel  software, from HP or other vendors, may cause changes to tunable parameter values.  After installation,
       some tunable parameters may no longer be at the default or recommended values.  For information about the effects of installation  on  tun-
       able  values,  consult  the documentation for the kernel software being installed.  For information about optional kernel software that was
       factory installed on your system, see the at

AUTHOR

       was developed by HP.

SEE ALSO

       syncer(1M), vxtunefs(1M), fsync(2), munmap(2), write(2).

							     Tunable Kernel Parameters					       fcache_fb_policy(5)