The size of the cache used for the underlying database can be specified as part of the db_open call to open the database, specifically by setting the db_cachesize element of the DB_INFO structure.
Choosing a cache size is, unfortunately, an art. Your cache must be at least large enough for your working set plus some overlap for unexpected situations.
When using the Btree access method, you must have a cache big enough for the minimum working set for a single access. This will include a root page, one or more internal pages (depending on the depth of your tree), and a leaf page. If your cache is any smaller than that, each new page will force out the least-recently-used page, and you'll end up requesting the root page of the tree anew on each database request.
If your keys are of moderate size (a few tens of bytes) and your pages are on the order of 4K to 8K, most Btrees will be only three levels (e.g., if we assume 20 byte keys with 20 bytes of data associated with each key, a 8KB page can hold roughly 400 keys and 200 key/data pairs, so a fully populated three level Btree will hold 32 million key/data pairs, and a tree with only a 50% page-fill factore will still hold 16 million key/data pairs). We rarely expect trees to exceed five levels, although Berkeley DB will support trees up to 255 levels.
Even a small initial increase of the size of the cache over the minimum working set gives you a good return, as this allows you to cache more internal pages, which are more likely to be accessed in a Btree requests than any single leaf page.
Regardless, the rule-of-thumb is that cache is good, and more cache is better. Generally, applications benefit from increasing the cache size up to a point, where the performance will stop increasing when the cache size increases, or, that the performance will only increase a very little bit in response to increasing the cache size. When this point is reached, one of two things have happened: First, either the cache is large enough that the application is almost never having to retrieve information from disk. Second, your application is doing truly random accesses, and therefore increasing size of the cache doesn't significantly increase the odds of finding the next requested information in the cache. The latter is fairly rare -- almost all applications show some form of locality of reference!
For example, even if our accesses are truly random within a B+-tree, your access pattern will favor internal pages to leaf pages, so your cache should be large enough to hold all internal pages. This will result in your requiring at most one I/O per operation to retrieve the appropriate leaf page.
Finally, you can use the db_stat utility to monitor the effectiveness of your cache. The following output is excerpted from the output of that utility's -m option:
prompt> db_stat -m 131072 Cache size (128K). 4273 Requested pages found in the cache (97%). 134 Requested pages not found in the cache. 18 Pages created in the cache. 116 Pages read into the cache. 93 Pages written from the cache to the backing file. 5 Clean pages forced from the cache. 13 Dirty pages forced from the cache. 0 Dirty buffers written by trickle-sync thread. 130 Current clean buffer count. 4 Current dirty buffer count.
The statistics for this cache are that there have been 4,273 requests of the cache, and only 116 of those requests required an I/O from disk. This means that the cache is working well, yielding a 97% cache hit rate. The db_stat utility will present these statistics both for the cache as a whole and for each file within the cache separately.