1<!--$Id: extentsize.so,v 1.3 2001/05/02 15:53:22 bostic Exp $--> 2<!--Copyright (c) 1997,2008 Oracle. All rights reserved.--> 3<!--See the file LICENSE for redistribution information.--> 4<html> 5<head> 6<title>Berkeley DB Reference Guide: Selecting a Queue extent size</title> 7<meta name="description" content="Berkeley DB: An embedded database programmatic toolkit."> 8<meta name="keywords" content="embedded,database,programmatic,toolkit,btree,hash,hashing,transaction,transactions,locking,logging,access method,access methods,Java,C,C++"> 9</head> 10<body bgcolor=white> 11<a name="2"><!--meow--></a> 12<table width="100%"><tr valign=top> 13<td><b><dl><dt>Berkeley DB Reference Guide:<dd>Access Methods</dl></b></td> 14<td align=right><a href="../am_conf/recno.html"><img src="../../images/prev.gif" alt="Prev"></a><a href="../toc.html"><img src="../../images/ref.gif" alt="Ref"></a><a href="../am_conf/re_source.html"><img src="../../images/next.gif" alt="Next"></a> 15</td></tr></table> 16<p align=center><b>Selecting a Queue extent size</b></p> 17<p>In Queue databases, records are allocated sequentially and directly 18mapped to an offset within the file storage for the database. As 19records are deleted from the Queue, pages will become empty and will 20not be reused in normal queue operations. To facilitate the reclamation 21of disk space a Queue may be partitioned into extents. Each extent is 22kept in a separate physical file.</p> 23<p>Extent files are automatically created as needed and marked for deletion 24when the head of the queue moves off the extent. The extent will not 25be deleted until all processes close the extent. In addition, Berkeley DB 26caches a small number of extents that have been recently used; this may 27delay when an extent will be deleted. The number of extents left open 28depends on queue activity.</p> 29<p>The extent size specifies the number of pages that make up each extent. 30By default, if no extent size is specified, the Queue resides in a 31single file and disk space is not reclaimed. In choosing an extent size 32there is a tradeoff between the amount of disk space used and the 33overhead of creating and deleting files. If the extent size is too 34small, the system will pay a performance penalty, creating and deleting 35files frequently. In addition, if the active part of the queue spans 36many files, all those files will need to be open at the same time, 37consuming system and process file resources.</p> 38<table width="100%"><tr><td><br></td><td align=right><a href="../am_conf/recno.html"><img src="../../images/prev.gif" alt="Prev"></a><a href="../toc.html"><img src="../../images/ref.gif" alt="Ref"></a><a href="../am_conf/re_source.html"><img src="../../images/next.gif" alt="Next"></a> 39</td></tr></table> 40<p><font size=1>Copyright (c) 1996,2008 Oracle. All rights reserved.</font> 41</body> 42</html> 43
