when you update drupal using drush, while/before it downloads the new version, it empties the folder it is in and stores that in a backup folder, then untars the latest version into the empty folder.
what happens if you are using it in the root folder of your site, is that although you have put your site into maintenance mode, the actual /htdocs folder is empty for a few seconds, meaning your entire site and any other apps contained within the /htdocs folder are offline.
normally i can wait until offpeak times to do drupal updates, but of late there have been several critical security updates which require immediate action. what do other people do to avoid taking the site completely down whilst updating drupal? (even if only for a few seconds)
i have considered having a staging vhost which just contains the drupal folder and files on a bare bones installation. if that works, then have a small script which deletes only the drupal folders/files on the production vhost and mv's the updated ones over. i could use an atomic operation such as mv which is almost instantaneous?
(p.s. it goes without saying that i have a mirror copy of the production server on local where i test all updates before doing on live ;)