fcache_fb_policy(5)fcache_fb_policy(5)NAMEfcache_fb_policy - policy for flushing dirty file data to disk
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
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
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.
SEE ALSOsyncer(1M), vxtunefs(1M), fsync(2), munmap(2), write(2).
Tunable Kernel Parameters fcache_fb_policy(5)