fcache_fb_policy.5 (2010 09)

f
fcache_fb_policy(5) fcache_fb_policy(5)
(Tunable Kernel Parameters)
NAME
fcache_fb_policy - policy for flushing dirty file data to disk
VALUES
Failsafe
0
Default
0
Allowed values
Minimum: 0
Maximum: 2
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
fcache_fb_policy
tunable is used to determine the trade-off between write performance and data
availability. The tunable has three settings:
fcache_fb_policy = 0
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
munmap() call (see mun-
map(2)). The asynchronous flushing allows the
munmap() 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, fcache_fb_policy = 1
, 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.
fcache_fb_policy = 1
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 write_pref_io,
max_diskq, write_throttle, and write_nstream. See
the vxtunefs (1M) manpage for more details on the VxFS tunables.
fcache_fb_policy = 2
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,
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 the default
write_throttle setting, there is no problem bypassing the Flush Behind feature. However, 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.
HP-UX 11i Version 3: September 2010 − 1 − Hewlett-Packard Company 1