Common Misconfigured HP-UX Resources (April 2006)

Some databases use the async driver (/dev/async), which performs asynchronous I/O,
but bypasses the buffer cache and reads directly into shared memory segments.
Be careful not to mix buffered I/O and direct I/O. This results in increased overhead to keep the
direct and buffered data in sync.
Buffer Cache Guidelines
Providing general guidelines for tuning the buffer cache is very difficult. So much depends on the
application mix that is running on the system, but some generalizations can be made.
If you are using a database, the database buffering will likely be more efficient than the system
buffering. The database is more likely to understand the I/O patterns and keep any relevant buffers in
memory. Given a choice, memory should be assigned to the database global area rather than the
system buffer cache.
HP-UX 11 v1 is more efficient at handling large buffer caches than HP-UX 11.0. The term “large” is
relative, but for this discussion consider a buffer cache larger than 1 GB or greater than 50 percent of
memory to be large. In general, the buffer cache on 11.0 should be configured to 1 GB or less due to
scaling issues with large caches, but you can increase this size on HP-UX 11i v1 and HP-UX 11i v2.
However, if you are using a large buffer cache on HP-UX 11i v1 you should have PHKL_27808 or
any superseding patch installed to increase the buffer cache virtual map size.
The larger the buffer cache, the longer sync operations will take. This particularly affects file system
mount and unmount times. If file systems need to be mounted or unmounted quickly (for example
during an Serviceguard package switch), then a smaller buffer cache is better.
If the buffer cache is configured too small, the system could be constantly searching for available
buffers. The buffer cache should probably be configured to a minimum of 200 MB on most systems.
Applications that benefit most from large caches are often file servers, such as NFS or Web servers,
where large amounts of data are frequently accessed. Some database applications that do not
manage their own file access may also fall into this category. Please check with your application
vendor for any vendor-specific recommendations.
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