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    <p>There are two things you can do to lessen the impact by this</p> <p><strong>OPTION #1 : Increase the variables <a href="http://dev.mysql.com/doc/refman/5.5/en/server-system-variables.html#sysvar_tmp_table_size" rel="noreferrer">tmp_table_size</a> and/or <a href="http://dev.mysql.com/doc/refman/5.5/en/server-system-variables.html#sysvar_max_heap_table_size" rel="noreferrer">max_heap_table_size</a></strong></p> <p>These options will govern how large an in-memory temp table can be before it is deemed too large and then pages to disk as a temporary MyISAM table. The larger these values are, the less likely you will get 'copying to tmp table on disk'. Please, make sure your server has enough RAM and <a href="http://dev.mysql.com/doc/refman/5.5/en/server-system-variables.html#sysvar_max_connections" rel="noreferrer"><strong>max_connections</strong></a> is moderately configured should a single DB connection need a lot of RAM for its own temp tables.</p> <p><strong>OPTION #2 : Use a RAM disk for tmp tables</strong></p> <p>You should be able to configure a RAM disk in Linux and then set the <a href="http://dev.mysql.com/doc/refman/5.5/en/server-options.html#option_mysqld_tmpdir" rel="noreferrer"><strong>tmpdir</strong></a> in mysql to be the folder that has the RAM disk mounted.</p> <p>For starters, configure a RAM disk in the OS</p> <p>Create a folder in the Linux called /var/tmpfs</p> <pre><code>mkdir /var/tmpfs </code></pre> <p>Next, add this line to /etc/fstab (for example, if you want a 16GB RAM disk)</p> <pre><code>none /var/tmpfs tmpfs defaults,size=16g 1 2 </code></pre> <p>and reboot the server.</p> <p>Note : It is possible to make a RAM disk without rebooting. Just remember to still add the aforementioned line to /etc/fstab to have the RAM disk after a server reboot.</p> <p>Now for MySQL:</p> <p>Add this line in /etc/my.cnf</p> <pre><code>[mysqld] tmpdir=/var/tmpfs </code></pre> <p>and restart mysql.</p> <p><strong>OPTION #3 : Get tmp table into the RAM Disk ASAP (assuming you apply OPTION #2 first)</strong></p> <p>You may want to force tmp tables into the RAM disk as quickly as possible so that MySQL does not spin its wheels migrating large in-memory tmp tables into a RAM disk. Just add this to /etc/my.cnf:</p> <pre><code>[mysqld] tmpdir=/var/tmpfs tmp_table_size=2K </code></pre> <p>and restart mysql. This will cause even the tiniest temp table to be brought into existence right in the RAM disk. You could periodically run <code>ls -l /var/tmpfs</code> to watch temp tables come and go.</p> <p>Give it a Try !!!</p> <p><strong>CAVEAT</strong></p> <p>If you see nothing but temp tables in /var/tmpfs 24/7, this could impact OS functionality/performance. To make sure /var/tmpfs does not get overpopulated, look into tuning your queries. Once you do, you should see less tmp tables materializing in /var/tmpfs.</p>
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    1. COA few other points if you are on linux (specifically ubuntu) You might need to add your tmpdir path to app armour in (/etc/apparmor.d/usr.sbin.mysqld) Also dont forget to chmod 777 the dir as well!
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