src/docbkx/upgrading.xml

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    <title>Upgrading</title>
    <para>
        Review <xref linkend="configuration" />, in particular the section on Hadoop version.
    </para>
    <section xml:id="upgrade0.90">
    <title>Upgrading to HBase 0.90.x from 0.20.x or 0.89.x</title>
          <para>This version of 0.90.x HBase can be started on data written by
              HBase 0.20.x or HBase 0.89.x.  There is no need of a migration step.
              HBase 0.89.x and 0.90.x does write out the name of region directories
              differently -- it names them with a md5 hash of the region name rather
              than a jenkins hash -- so this means that once started, there is no
              going back to HBase 0.20.x.
          </para>
          <para>
             Be sure to remove the <filename>hbase-default.xml</filename> from
             your <filename>conf</filename>
             directory on upgrade.  A 0.20.x version of this file will have
             sub-optimal configurations for 0.90.x HBase.  The
             <filename>hbase-default.xml</filename> file is now bundled into the
             HBase jar and read from there.  If you would like to review
             the content of this file, see it in the src tree at
             <filename>src/main/resources/hbase-default.xml</filename> or
             see <xref linkend="hbase_default_configurations" />.
          </para>
          <para>
            Finally, if upgrading from 0.20.x, check your 
            <varname>.META.</varname> schema in the shell.  In the past we would
            recommend that users run with a 16kb
            <varname>MEMSTORE_FLUSHSIZE</varname>.
            Run <code>hbase> scan '-ROOT-'</code> in the shell. This will output
            the current <varname>.META.</varname> schema.  Check
            <varname>MEMSTORE_FLUSHSIZE</varname> size.  Is it 16kb (16384)?  If so, you will
            need to change this (The 'normal'/default value is 64MB (67108864)).
            Run the script <filename>bin/set_meta_memstore_size.rb</filename>.
            This will make the necessary edit to your <varname>.META.</varname> schema.
            Failure to run this change will make for a slow cluster <footnote>
            <para>
            See <link xlink:href="https://issues.apache.org/jira/browse/HBASE-3499">HBASE-3499 Users upgrading to 0.90.0 need to have their .META. table updated with the right MEMSTORE_SIZE</link>
            </para>
            </footnote>
            .

          </para>
          </section>
    </chapter>