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<h1 class="topictitle1">Create Node Group (CRTNODGRP) command</h1>
<div><p>This section uses two CL command examples to show how to create
a node group by using the Create Node Group (CRTNODGRP) command.</p>
<div class="p">In the following example, a node group with default partitioning (equal
partitioning across the systems) is created: <pre>CRTNODGRP NODGRP(LIB1/GROUP1) RDB(SYSTEMA SYSTEMB SYSTEMC SYSTEMD)
TEXT('Node group for test files')</pre>
</div>
<p>In this example, the command creates a node group that contains four nodes.
Note that each of the nodes must be defined RDB entries (previously added
to the relational database directory using the ADDRDBDIRE command) and that
one node must be defined as local (*LOCAL).</p>
<p>The partitioning attributes default to assigning one-fourth of the partitions
to each node number. This node group can be used on the NODGRP parameter of
the Create Physical File (CRTPF) command to create a distributed file. For
more information about distributed files, see Creating distributed files with DB2<sup>®</sup> Multisystem.</p>
<div class="p">In the following example, a node group with specified partitioning is created
by using the partitioning file (PTNFILE) parameter: <pre>CRTNODGRP NODGRP(LIB1/GROUP2) RDB(SYSTEMA SYSTEMB SYSTEMC)
PTNFILE(LIB1/PTN1)
TEXT('Partition most of the data to SYSTEMA')</pre>
</div>
<p>In this example, the command creates a node group that contains three nodes
(SYSTEMA, SYSTEMB, and SYSTEMC). The partitioning attributes are taken from
the file called PTN1. This file can be set up to force a higher percentage
of the records to be located on a particular system.</p>
<p>The file PTN1 in this example is a partitioning file. This file is not
a distributed file, but a regular local physical file that can be used to
set up a custom partitioning scheme. The partitioning file must have one 2-byte
binary field. The partitioning file must contain 1024 records in which each
record contains a valid node number.</p>
<div class="fignone" id="crtngc__ftn1"><a name="crtngc__ftn1"><!-- --></a><span class="figcap">Figure 1. Example of the contents of partitioning file PTNFILE</span><br /><img src="rbal3508.gif" alt="In this graphic of a partitioning file with a 2–byte binary field, Row 1 corresponds to partition number 0, and Row 1024 corresponds to partition number 1023." /><br /></div>
<p>If the node group contains three nodes, all of the records in the partitioning
file must have numbers 1, 2, or 3. The node numbers are assigned in the order
that the RDB names were specified on the Create Node Group (CRTNODGRP) command.
A higher percentage of data can be forced to a particular node by having more
records containing that node number in the partitioning file. This is a method
for customizing the partitioning with respect to the amount of data that physically
resides on each system. To customize the partitioning with respect to specific
values residing on specific nodes, use the Change Node Group Attributes (CHGNODGRPA)
command. See Changing node groups using the CHGNODGRPA command with DB2 Multisystem
for more information.</p>
<p>You should note that, because the node group information is stored in the
distributed file, the file is not immediately sensitive to changes in the
node group or to changes in the RDB directory entries that are included in
the node group. You can make modifications to node groups and RDB directory
entries, but until you use the CHGPF command and specify the changed node
group, your files do not change their behavior.</p>
<p>Another concept is that of a <em>visibility node</em>. A
visibility node within a node group contains the file object (part of the
mechanism that allows the file to be distributed across several nodes), but
no data. A visibility node retains a current level of the file object at all
times; the visibility node has no data stored on it. In contrast, a node (sometimes
called a <em>data node</em>) contains data. As an example of how you can use
a visibility node in your node group, assume that the <span class="keyword">iSeries™</span> server
that your sales executives use is part of your node group. These executives
probably do not want to run queries on a regular basis, but on occasion they
might want to run a specific query. From their server, they can run their
queries, access real-time data, and receive the results of their query. So
even though none of the data is stored on their server, because their system
is a visibility node, the executives can run the query whenever necessary.</p>
<p>To specify a node as being a visibility node, you must use the PTNFILE
parameter on the Create Node Group (CRTNODGRP) command. If the partitioning
file contains no records for a particular node number, that node is a visibility
node.</p>
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<div class="familylinks">
<div class="parentlink"><strong>Parent topic:</strong> <a href="rzaf3c2-gen4.htm" title="To enable database files to be visible across a set of iSeries servers, you must first define the group of systems (node group) that you want the files on.">Introduction of node groups with DB2 Multisystem</a></div>
</div>
<div class="relconcepts"><strong>Related concepts</strong><br />
<div><a href="nodework.htm" title="A node group is a system object (*NODGRP), which is stored on the system on which it was created.">How node groups work with DB2 Multisystem</a></div>
<div><a href="crtdf.htm" title="A distributed file is a database file that is spread across multiple iSeries servers.">Distributed files with DB2 Multisystem</a></div>
<div><a href="chgnga.htm" title="The Change Node Group Attributes (CHGNODGRPA) command changes the data partitioning attributes for a node group.">Change Node Group Attributes (CHGNODGRPA) command</a></div>
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