2011-09-13 17:44:24 +00:00
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
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<title>Sizing a database environment</title>
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<link rel="stylesheet" href="gettingStarted.css" type="text/css" />
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<meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
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<link rel="start" href="index.html" title="Berkeley DB Programmer's Reference Guide" />
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<link rel="up" href="env.html" title="Chapter 9. The Berkeley DB Environment" />
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<link rel="prev" href="env_create.html" title="Creating a database environment" />
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<link rel="next" href="env_open.html" title="Opening databases within the environment" />
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</head>
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<body>
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<div xmlns="" class="navheader">
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<div class="libver">
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2012-11-14 21:35:20 +00:00
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<p>Library Version 11.2.5.3</p>
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2011-09-13 17:44:24 +00:00
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</div>
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<table width="100%" summary="Navigation header">
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<tr>
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<th colspan="3" align="center">Sizing a database environment</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="env_create.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 9.
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The Berkeley DB Environment
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</th>
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<td width="20%" align="right"> <a accesskey="n" href="env_open.html">Next</a></td>
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</tr>
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</table>
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<hr />
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</div>
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<div class="sect1" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h2 class="title" style="clear: both"><a id="env_size"></a>Sizing a database environment</h2>
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</div>
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</div>
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</div>
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<p>
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The Berkeley DB environment allocates memory to hold shared
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structures, either in shared regions or in process data space (if the
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<a href="../api_reference/C/envopen.html#envopen_DB_PRIVATE" class="olink">DB_PRIVATE</a> flag is specified). There are three distinct memory
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regions:
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</p>
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<div class="itemizedlist">
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<ul type="disc">
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<li>
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<p>
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The memory pool (also known as the database page cache),
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</p>
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</li>
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<li>
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<p>
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the area containing mutexes, and
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</p>
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</li>
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<li>
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<p>
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the main region which holds all other shared structures.
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</p>
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</li>
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</ul>
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</div>
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<p>
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The shared structures in the main region are used by the lock,
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transaction, logging, thread and replicatoin subsystems.
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</p>
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<p>
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Determining the amount of space allocated for each of these shared
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structures is dependent upon the structure in question. The sizing
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of the memory pool is discussed in
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<a class="xref" href="mp_config.html" title="Configuring the memory pool">Configuring the memory pool</a>. The amount
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of memory needed for mutexes is calculated from the number of
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mutexes needed by various subsystems and can be adjusted using the
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<a href="../api_reference/C/mutexset_increment.html" class="olink">DB_ENV->mutex_set_increment()</a> method.
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</p>
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<p>
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For applications using shared memory (that is, they do not specify
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<a href="../api_reference/C/envopen.html#envopen_DB_PRIVATE" class="olink">DB_PRIVATE</a>), a maximum memory size for the main region must be
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specified or left to default. The maximum memory size is specified
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using the <a href="../api_reference/C/envset_memory_max.html" class="olink">DB_ENV->set_memory_max()</a> method.
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</p>
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<p>
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The amount of memory needed by an application is dependent
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on the resources that the application uses. For a very rough
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estimate, add all of the following together:
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</p>
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<div class="orderedlist">
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<ol type="1">
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<li>
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<p>
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The environment has an overhead of about 80 kilobytes
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without statistics enabled or 250 kilobytes with statistics
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enabled.
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</p>
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</li>
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<li>
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<p>
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Identify the amount of space you require for your locks:
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</p>
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<div class="orderedlist">
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<ol type="a">
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<li>
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<p>
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Estimate the number of threads of control that will
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simultaneously access the environment.
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</p>
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</li>
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<li>
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<p>
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Estimate the number of concurrency locks that, on average,
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will be required by each thread. For information on
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sizing concurrency locks, see
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<a class="xref" href="lock_max.html" title="Configuring locking: sizing the system">Configuring locking: sizing the system</a>.
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</p>
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</li>
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<li>
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<p>
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Multiply these two numbers, then multiply by 1/2 to
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arrive at the number of kilobytes required to
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service your locks.
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</p>
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</li>
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</ol>
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</div>
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</li>
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<li>
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<p>
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Estimate the number of open database handles you will use
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at any given time. For each database handle, there is an
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overhead of about 1/2 kilobyte.
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</p>
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</li>
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<li>
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<p>
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Add 1 kilobyte for each active transaction.
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</p>
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</li>
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</ol>
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</div>
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<p>
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Note that these are very rough guidelines. It is best to
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overestimate the needs of your applications, because if the memory
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allocation is exhausted the application must be shutdown to
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increase the allocation.
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</p>
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<p>
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The estimate for maximum memory need not be exact. In most
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situations there is little penalty for over estimating. For systems
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using memory mapped files for the shared environment, this only
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allocates the address space in the process to hold the maximum
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memory. The backing file will only be extended as needed. For
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systems running with <a href="../api_reference/C/envopen.html#envopen_DB_PRIVATE" class="olink">DB_PRIVATE</a> specified, the maximum memory
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serves only as a limit and memory is allocated from the process
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data space as needed. No maximum need be set for private
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environments.
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</p>
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<p>
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For locking and thread information, groups of objects are allocated
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when needed so that there is less contention in the allocator
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during performance critical operations. Once allocated to a
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particular use, this memory will only be used for that structure.
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To avoid runtime contention, or to ensure a minimum number of a
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particular type of object, the <a href="../api_reference/C/envset_memory_init.html" class="olink">DB_ENV->set_memory_init()</a> method can be
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used. This method can set the initial numbers of particular types
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of structures to allocate at environment creation time.
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</p>
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</div>
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<td width="40%" align="left"><a accesskey="p" href="env_create.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="env.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="env_open.html">Next</a></td>
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</tr>
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<tr>
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<td width="40%" align="left" valign="top">Creating a database environment </td>
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<td width="20%" align="center">
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<a accesskey="h" href="index.html">Home</a>
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</td>
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<td width="40%" align="right" valign="top"> Opening databases within the environment</td>
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</tr>
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</table>
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</div>
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</body>
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</html>
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