The 9.01 Series 700 dynamic buffer cache functionality is
provided on 10.x Series 800 computers. The programming interfaces
have not been changed. The configuration parameters nbuf
and bufpages specify buffer cache size.
When these parameters are not specified on 9.0 Series 800, a default
percentage of 10 percent of available memory is used. Not specifying
these parameters in the 10.x kernel will configure a dynamic buffer
cache as in 9.0 Series 700. If these parameters are specified in
the 10.x kernel, they will indicate a fixed size buffer cache as
in the 9.0 Series 800 release.
Using dynamic buffer cache causes the buffer cache to grow
or shrink while the system is running (in response to competing
demands for memory.) The actual portion of physical memory used
in the buffer cache, expressed as a percentage, is bounded by the
kernel configuration parameters dbc_min_pct
and dbc_max_pct.
In many cases, file system I/O speeds up when there are no
competing demands for memory. If the minimum and maximum limits
are set to the same value, the size of the cache is fixed and behavior
and performance should be the same as with 9.0 Series 800 systems.
On small- and medium-sized machines, file system access should
be faster when paging traffic is low. There may be no speedup or
slight degradation on very large systems. In such cases, create
a fixed-size buffer cache by setting the dbc_min_pct
and dbc_max_pct kernel configuration
parameters to the same value to fix the size of the buffer cache.
(For example, set them both to 12 for a buffer cache that uses exactly
12 percent of memory.)
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