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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