Jump to content United States-English
HP.com Home Products and Services Support and Drivers Solutions How to Buy
» Contact HP
More options
HP.com home
HP-UX Reference > F


Tunable Kernel Parameters
HP-UX 11i Version 3: February 2007

Technical documentation

» Feedback
Content starts here

 » Table of Contents

 » Index


fcache_fb_policy — policy for flush behind requests from VxFS file system






Allowed values

Minimum: 0

Maximum: 1


VxFS File System has a feature known as Flush Behind, which flushes out dirty pages belonging to a file using asynchronous I/O. This is done when the number of dirty pages in memory for the file reaches a certain threshold. These flushes are controlled by a variety of VxFS tunables, such as: write_pref_io, max_diskq, write_throttle, and write_nstream. Please see the vxtunefs(1M) manpage for more details on the VxFS tunables.

The flush behind feature helps to keep the number of dirty pages associated with a file low. Flush behind may also improve data integrity and avoid data corruption in case of a system crash. The disadvantage of the flush behind, however, is the high overhead to initiate the I/O from the thread context. Moreover, re-writes of data blocks may stall as I/Os may already be outstanding to the block due to a previous flush behind.

HP-UX provides a Syncer daemon that satisfies the flush behind feature requirements to a large extent. It runs approximately every 30 seconds (can be tuned) to flush dirty pages to guarantee file system integrity. Please see the syncer(1M) manpage for more details on the syncer daemon. It would be performance optimal on HP-UX 11i V3 to disable VxFS flush behind and rely on the syncer daemon to ensure data availability and integrity in case of a system crash.

The fcache_fb_policy tunable is used to determine the trade-off between write performance and data availability. The fcache_fb_policy tunable has two settings:

fcache_fb_policy = 0

This setting is a high-performance setting, where no flush behind is implemented. Applications shall depend on the syncer daemon to provide file data integrity. Or, the application must do explicit flushing via fsync(2) if it wants to reduce the number of dirty pages on the system.

fcache_fb_policy = 1

This setting corresponds to the HP-UX 11i V2 default behavior. This setting enables file system flush behind. It is designed to minimize the number of dirty pages for a file. A side-effect is that it minimizes data loss, should a system failure occur before the syncer daemon had a chance to flush the dirty pages to disk.

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?

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, fcache_fb_policy should be changed to 1 if the VxFS tunable, write_throttle, is set. The write_throttle 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 write_throttle is zero. The default value places no limit on the number of dirty buffers per file. With default write_throttle setting there is no problem by-passing flush behind. But if write_throttle is enabled, the application performing writes may encounter longer delays as it waits for syncer to flush some of the dirty buffers before the application can continue.

What Are the Side Effects of Changing the Value?

Changing the value from 0 to 1 will cause increased I/O activity. It may result in loss of overall write(2) performance.

What Other Tunables Should Be Changed at the Same Time?



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 tunable values, consult the documentation for the kernel software being installed. For information about optional kernel software that was factory installed on your system, see HP-UX Release Notes at http://docs.hp.com.


fcache_fb_policy was developed by HP.


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

Printable version
Privacy statement Using this site means you accept its terms Feedback to webmaster
© 1983-2007 Hewlett-Packard Development Company, L.P.