Sometimes you want to restart your Apache server after changing some configuration in your virutual hosts,sites etc, but you have few hundred clients currently downloading files from your server and you dont want to disconnect them.You need to use the following command
sudo /etc/init.d/apache2 graceful
This will gracefully restart your Apache with new configuration without affecting your client connections.
ref : http://www.ubuntugeek.com/restart-apache-server-without-affecting-existing-connections.html