Sessions are mechanisms that allow users to keep their shopping carts safe even if they don't browse our website for a few days. By default, the sessions are saved in the filesystem, and the performance could be improved by storing it in your database.
Open the file named local.xml
under app/etc/
and modify line 55 so that it looks like the following line:
<session_save><![CDATA[db]]></session_save>
By default, the sessions are stored in the filesystem under small files, one per session. This option will simply store the contents of these files into the database. This can improve your performance because the database-retrieving process is faster than the filesystem-retrieving process. Another scenario to choose this way of storage is when you have more than one frontend server.