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<title>Berkeley DB Reference Guide: Selecting a Queue extent size</title>
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<td><h3><dl><dt>Berkeley DB Reference Guide:<dd>Access Methods</dl></h3></td>
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<h1 align=center>Selecting a Queue extent size</h1>
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<p>In Queue databases, records are allocated sequentially and directly
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mapped to an offset within the file storage for the database. As
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records are deleted from the Queue, pages will become empty and will
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not be reused in normal queue operations. To facilitate the reclamation
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of disk space a Queue may be partitioned into extents. Each extent is
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kept in a separate physical file.
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<p>Extent files are automatically created as needed and marked for deletion
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when the head of the queue moves off the extent. The extent will not
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be deleted until all processes close the extent. In addition, Berkeley DB
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caches a small number of extents that have been recently used; this may
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delay when an extent will be deleted. The number of extents left open
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depends on queue activity.
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<p>The extent size specifies the number of pages that make up each extent.
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By default, if no extent size is specified, the Queue resides in a
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single file and disk space is not reclaimed. In choosing an extent size
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there is a tradeoff between the amount of disk space used and the
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overhead of creating and deleting files. If the extent size is too
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small, the system will pay a performance penalty, creating and deleting
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files frequently. In addition, if the active part of the queue spans
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many files, all those files will need to be open at the same time,
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consuming system and process file resources.
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