title: Migrating Solr from 3.6 to 4.0, method to do it, and is it safe?
body: <p>We are using Solr 3.6 in master-slave configuration, 100s of cores, 100s of millions documents and need round-the-clock uptime. We wish to upgrade to Solr 4.0.</p>
  
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  <p>I believe the correct method is to do a <strong>filesystem copy</strong> of the old indexes to the new location and configuring SolrConfig with:</p>
  
  <pre><code>&lt;luceneMatchVersion&gt;LUCENE_33&lt;/luceneMatchVersion&gt;
  </code></pre>
  
  <p><strong>Question 1:</strong> Will this make Solr use the obsolete format? Will this <strong>convert</strong> the index to the 4.0 format in an online manner?</p>
  
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  <p><strong>Question 2:</strong> Is there a better method that needs <strong>lesser downtime</strong> to convert the indexes? Can I <strong>replicate</strong> from the old server to the new?</p>
  
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  <p><strong>Question 3:</strong> And can anyone confirm if Solr 4.0 is safe for as heavy use as ours?</p>