109 lines
7.7 KiB
HTML
109 lines
7.7 KiB
HTML
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<meta name="DC.Title" content="DB2 Multisystem: Basic terms and concepts" />
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<meta name="abstract" content="A distributed file is a database file that is spread across multiple iSeries servers. This section describes some of the main concepts that are used in discussing the creation and use of distributed files by DB2 Multisystem." />
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<title>DB2 Multisystem: Basic terms
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and concepts</title>
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<body id="basicterms"><a name="basicterms"><!-- --></a>
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<!-- Java sync-link --><script language="Javascript" src="../rzahg/synch.js" type="text/javascript"></script>
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<h1 class="topictitle1"><span class="keyword">DB2 Multisystem</span>: Basic terms
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and concepts</h1>
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<div><p>A <em>distributed file</em> is a database file that is spread across
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multiple <span class="keyword">iSeries™</span> servers. This
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section describes some of the main concepts that are used in discussing the
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creation and use of distributed files by DB2<sup>®</sup> Multisystem.</p>
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<p>Each server that has a piece of a distributed file is called a <em>node</em>.
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Each server is identified by the name that is defined for it in the relational
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database directory.</p>
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<p>A group of systems that contains one or more distributed files is called
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a <em>node group</em>. A <em>node group</em> is a system object that contains
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the list of nodes across which the data is distributed. A system can be a
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node in more than one node group.</p>
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<div class="fignone" id="basicterms__fki"><a name="basicterms__fki"><!-- --></a><span class="figcap">Figure 1. Node groups</span><br /><img src="rbal3509.gif" alt="This graphic shows two node groups. Node group one contains systems A, B, and C. Node group two contains systems A, B, and D. Node groups one and two share systems A and B because a system can be a node in more than one node group." /><br /></div>
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<p>A file is distributed across all the systems in a node group through <em>partitioning</em>. <em>Table
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partitioning</em>, further described in Partitioned tables, applies to tables
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partitioned on a single system.</p>
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<p>A <em>partition number</em> is a number from 0 to 1023. Each partition number
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is assigned to a node in the node group. Each node can be assigned many partition
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numbers. The correlation between nodes and partition numbers is stored in
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a <em>partition map</em>. The partition map is also stored as part of the node
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group object. You can provide the partition map when you create a node group;
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otherwise, the system generates a default map.</p>
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<p>You define a partition map by using a partitioning file. A <em>partitioning
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file</em> is a physical file that defines a node number for each
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partition number.</p>
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<p>A <em>partitioning key</em> consists of one or more fields in the file that
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is being distributed. The partitioning key is used to determine which node
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in the node group is to physically contain rows with certain values. This
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is done by using <em>hashing</em>, an operating system function that takes the
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value of the partitioning key for a record and maps it to a partition number.
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The node corresponding to that partition number is used to store the record.</p>
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<p>The following example shows what partition number and nodes might look
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like for a distributed table for two systems. The table has a partitioning
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key of LASTNAME.</p>
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<div class="tablenoborder"><a name="basicterms__fig1"><!-- --></a><table cellpadding="4" cellspacing="0" summary="" id="basicterms__fig1" frame="void" border="0" rules="none"><caption>Table 1. Partition map</caption><thead align="left"><tr><th valign="top" width="NaN%" class="firstcol" id="d0e210">Partition number</th>
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<th valign="top" width="NaN%" id="d0e212">Node</th>
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</tr>
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</thead>
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<tbody><tr><th valign="top" width="NaN%" class="firstcol" id="d0e216" headers="d0e210 ">0</th>
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<td valign="top" width="NaN%" headers="d0e216 d0e212 ">SYSA</td>
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</tr>
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<tr><th valign="top" width="NaN%" class="firstcol" id="d0e221" headers="d0e210 ">1</th>
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<td valign="top" width="NaN%" headers="d0e221 d0e212 ">SYSB</td>
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</tr>
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<tr><th valign="top" width="NaN%" class="firstcol" id="d0e226" headers="d0e210 ">2</th>
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<td valign="top" width="NaN%" headers="d0e226 d0e212 ">SYSA</td>
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</tr>
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<tr><th valign="top" width="NaN%" class="firstcol" id="d0e231" headers="d0e210 ">3</th>
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<td valign="top" width="NaN%" headers="d0e231 d0e212 ">SYSB</td>
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</tr>
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</tbody>
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</table>
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</div>
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<p>In the partition map, partition number 0 contains SYSA,
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partition number 1 contains node SYSB, partition number 2 contains SYSA, and
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partition number 3 contains SYSB. This pattern is repeated.</p>
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<p>The hashing of the partitioning key determines a number
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that corresponds to a partition number. For example, a record that has a value
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of <tt>Andrews</tt> might hash to partition number 1. A record that has a
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value of <tt>Anderson</tt> might hash to partition number 2. If you refer
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to the partition map shown in <a href="#basicterms__fig1">Table 1</a>, records
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for partition number 1 are stored at SYSB, while records for partition number
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2 are stored at SYSA.</p>
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</div>
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<div>
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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="intro.htm" title="DB2 Multisystem is a parallel processing technique that provides greater scalability for databases.">DB2 Multisystem introduction</a></div>
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</div>
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<div class="relconcepts"><strong>Related concepts</strong><br />
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<div><a href="partitionedtables.htm" title="DB2 UDB for iSeries supports partitioned tables using SQL.">Partitioned tables</a></div>
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</div>
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</div>
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</body>
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