fcache_fb_policy man page on HP-UX

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fcache_fb_policy(5)					   fcache_fb_policy(5)

       fcache_fb_policy - policy for flushing dirty file data to disk

   Allowed values

       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 deter‐
       mine 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  set‐
		     ting.   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 cer‐
		     tain  threshold.  These flushes are controlled by a vari‐
		     ety 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 set‐
		     ting 1, but the I/O requests for flushing data are sched‐
		     uled immediately (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

       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 application can

   When Should the Value of This Tunable Be Changed to 2?
       This tunable should be changed to 2 if applications running on the sys‐
       tem  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?

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

       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  val‐
       ues.  For information about the effects of installation on tunable val‐
       ues, 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

       was developed by HP.

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

			   Tunable Kernel Parameters	   fcache_fb_policy(5)

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