title: When should I run cleanup in Cassandra?
body: <p>Cassandra <code>nodetool</code> has a command called <code>cleanup</code>:</p>
  
  <blockquote>
    <p>cleanup [keyspace][cf_name]</p>
    
    <p>Triggers the immediate cleanup of keys no longer belonging to this
    node. This has roughly the same effect on a node that a major
    compaction does in terms of a temporary increase in disk space usage
    and an increase in disk I/O. Optionally takes a list of column family
    names.</p>
  </blockquote>
  
  <p>My questions are:</p>
  
  <ol>
  <li>When will a node having keys not belonging to it?</li>
  <li>When should I issue a cleanup?</li>
  <li>Should I do cleanup regularly (e.g. once per week)?</li>
  </ol>