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HBASE-4541 clarifying versions in Schema Design 

git-svn-id: https://svn.apache.org/repos/asf/hbase/trunk@1179314 13f79535-47bb-0310-9956-ffa450edef68
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Doug Meil 2011-10-05 16:55:31 +00:00
parent 1131ebe570
commit f973b6081e
1 changed files with 11 additions and 9 deletions
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src/docbkx/book.xml
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@ -445,24 +445,26 @@ admin.enableTable(table);
<title>
Number of Versions
</title>
<para>The number of row versions to store is configured per column
<section xml:id="schema.versions.max"><title>Maximum Number of Versions</title>
<para>The maximum number of row versions to store is configured per column
family via <link xlink:href="http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/HColumnDescriptor.html">HColumnDescriptor</link>.
The default is 3.
The default for max versions is 3.
This is an important parameter because as described in <xref linkend="datamodel" />
section HBase does <emphasis>not</emphasis> overwrite row values, but rather
stores different values per row by time (and qualifier). Excess versions are removed during major
compactions. The number of versions may need to be increased or decreased depending on application needs.
</para>
<para>It is not recommended setting the number of versions to an exceedingly high level (e.g., hundreds or more) unless those old values are
very dear to you because this will greatly increase StoreFile size.
</para>
compactions. The number of max versions may need to be increased or decreased depending on application needs.
</para>
<para>It is not recommended setting the number of max versions to an exceedingly high level (e.g., hundreds or more) unless those old values are
very dear to you because this will greatly increase StoreFile size.
</para>
</section>
<section xml:id="schema.minversions">
<title>
Minimum Number of Versions
</title>
<para>Like number of row versions, the minimum number of row versions to keep is configured per column
<para>Like number of max row versions, the minimum number of row versions to keep is configured per column
family via <link xlink:href="http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/HColumnDescriptor.html">HColumnDescriptor</link>.
The default is 0, which means the feature is disabled.
The default for min versions is 0, which means the feature is disabled.
The minimum number of row versions parameter is used together with the time-to-live parameter and can be combined with the
number of row versions parameter to allow configurations such as
"keep the last T minutes worth of data, at most N versions, <emphasis>but keep at least M versions around</emphasis>"