A quick and easy way to turn on maintenance mode inside of Magento is to create a file called maintenance.flag and put it in your application root folder. This way in case you need to upgrade your system, perform maintenace, or otherwise take your site offline, you simply drop this file and you are set to go! When you want to bring things back up again, simply delete the file. The file doesn’t have to have anything in it, but I would put a date in there and the reason for taking it offline, just in case.
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