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<h2 class="title" style="clear: both"><a id="configpage"></a>Configuring the Database Page Size</h2>
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<a href="configpage.html#selectpage_size">Selecting the Page Size</a>
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<p>
When using the BDB SQL interface, you configure your database page size in
exactly the same way as you do when using
SQLite. That is, use <code class="literal">PRAGMA page_size</code>
to report and set the page size. This PRAGMA must be called
before you create your first SQLite table. See the
<a class="ulink" href="http://www.sqlite.org/pragma.html#pragma_page_size" target="_top">PRAGMA page_size</a>
documentation for more information.
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<p>
When you use
<code class="literal">PRAGMA cache_size</code> to size your in-memory
cache, you provide the cache size in terms of a number of
pages. Therefore, your database page size influences how large your
cache is, and so determines how much of your
database will fit into memory. If you adjust the database
page size, you may also want to adjust the in-memory cache
size, as described in
<a class="xref" href="configcache.html" title="Configuring the In-Memory Cache">Configuring the In-Memory Cache</a>.
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The size of your pages can also affect how efficient your
application is at performing disk I/O. It will also
determine just how fine-grained the fine-grained locking
actually is. This is because Berkeley DB locks database pages
when it acquires a lock.
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<h3 class="title"><a id="selectpage_size"></a>Selecting the Page Size</h3>
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<p>
Note that the default value for your page size is
probably correct for the physical hardware that you are using. In
almost all situations, the default page size value will
give your application the best possible I/O performance. For
this reason, tuning the page size should rarely, if
ever, be attempted.
</p>
<p>
That said, when using the BDB SQL interface, the page size affects
how much of your tables are locked when read and/or
write locks are acquired. (See
<a class="xref" href="lockingnotes.html#dbusage" title="Internal Database Usage">Internal Database Usage</a>
for more information.)
Increasing your page size will typically improve the
bandwidth you get accessing the disk, but it also may
increase contention if too many key data pairs are on
the same page. Decreasing your page size frequently
improves concurrency, but may increase the number of
locks you need to acquire and may decrease your disk
bandwidth.
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<p>
When changing your page size, make sure the value you
select is a power of 2 that is greater than 512 and
less than or equal to 64KB. (Note that the standard
SQLite <code class="literal">MAX_PAGE_SIZE</code> limit is not
examined for this upper bound.)
</p>
<p>
Beyond that, there are some additional things
that you need to consider when selecting your page
size. For a thorough treatment of selecting your page size,
see the section on <a href="../programmer_reference/general_am_conf.html#am_conf_pagesize" class="olink">Selecting a page size</a> in the
<em class="citetitle">Berkeley DB Programmer's Reference Guide</em>.
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