title: What would cause data loss during a REPAIR TABLE in MySQL using MyISAM?
body: <p>The MySQL documentation for <a href="http://dev.mysql.com/doc/refman/5.1/en/repair-table.html" rel="nofollow noreferrer">REPAIR TABLE</a> states that </p>
  
  <blockquote>
    <p>It is best to make a backup of a table
    before performing a table repair
    operation; under some circumstances
    the operation might cause data loss.
    Possible causes include but are not
    limited to file system errors.</p>
  </blockquote>
  
  <p>I'd like to know if there are any other causes of data loss besides file system errors. Has anyone seen this happen in the wild? How likely is it that the repair will lose data if there are no file system errors?</p>
  
  <p>My specific situation is as follows. I have Sun T5120 server running Solaris 10 (SPARC) and am using MySQL 5.1.30. I have a table which uses the MyISAM engine which occasionally becomes corrupted. Some of the times that the table has been corrupted have been due to unexpected power outages on our development system which didn't have an UPS. I'm not sure that all corruption was due to power outages, so there may be some additional reasons that this is happening. Such as the causes listed <a href="http://dev.mysql.com/doc/refman/5.1/en/corrupted-myisam-tables.html" rel="nofollow noreferrer">here</a>.</p>
  
  <p>I'd like to setup an automatic repair solution in case these suspected "additional reasons" happen in the production system, or the production UPS fails. I could set a cron job to run <code>mysqlcheck --auto-recover</code> as is suggested in <a href="https://stackoverflow.com/questions/739958/what-causes-myisam-to-become-corrupt/740160#740160">this answer</a>, or I could modify my process that is inserting into that table to instantly perform the <code>REPAIR TABLE EXTENDED</code> command when it detects corruption. However, both of those approaches use <code>REPAIR TABLE</code> and are, thus, susceptible to data loss.</p>
  
  <p>I could backup the table before attempting the repair, as the documentation suggests, but the table is rather large and I'm not sure I will have space available for the backup. I've done some searching, but haven't found any explanation of why <code>REPAIR TABLE</code> would cause data loss besides what is mentioned in the documentation. So is it likely that the repair will lose data when you have a sound file system, or is the documentation just being cautious?</p>