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<p>
Cache size is important to your application because if it is set to too
small of a value, your application's performance will suffer from too
much disk I/O. On the other hand, if your cache is too large, then your
application will use more memory than it actually needs.
Moreover, if your application uses too much memory, then on most
operating systems this can result in your application being swapped out
of memory, resulting in extremely poor performance.
</p>
<p>
You select your cache size using either
<span>
<tt class="methodname">Db::set_cachesize()</tt>, or
<tt class="methodname">DbEnv::set_cachesize()</tt>,
</span>
depending on whether you are using a database environment or not. You
cache size must be a power of 2, but it is otherwise limited only by
available memory and performance considerations.
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<p>
Selecting a cache size is something of an art, but fortunately you
can change it any time, so it can be easily tuned to your
application's changing data requirements. The best way to
determine how large your cache needs to be is to put your
application into a production environment and watch to see how much
disk I/O is occurring. If your application is going to disk quite a
lot to retrieve database records, then you should increase the size
of your cache (provided that you have enough memory to do so).
</p>
<p>
You can use the <tt class="literal">db_stat</tt> command line utility with the
<tt class="literal">-m</tt> option to gauge the effectiveness of your cache.
In particular, the number of pages found in the cache is shown, along
with a percentage value. The closer to 100% that you can get, the
better. If this value drops too low, and you are experiencing
performance problems, then you should consider increasing the size of
your cache, assuming you have memory to support it.
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