When you replace a comprehensive file tree via FTP, you will realize that it takes some time to do all the uploading. If your upload is going to replace old content, then one would usually first delete the old content, then upload the new one. (It is not a good idea to let the FTP client overwrite old contents, unless one knows exactly how the client behaves. For example, some clients merge existing trees, not deleting old files that are expunged rather than replaced, thus potentially raising security problems.)

However, the procedure of first deleting and then copying can leave your site naked or uncomplete for a couple of endless minutes. In the worst case, a user retrieves a page from your incomplete site and causes a database corruption or messes up the internal settings (this is not too unlikely with CMS applications). Therefore, the better way is to first upload the new content under a different name and then rename folders.

For example, if your web documents are in /var/www/docroot/, you would usually do the following: You delete everything in that directory and then upload the new content — which has the above mentioned side effects. Instead, you should better do the following: Upload all new content to /var/www/docroot_new/, rename /var/www/docroot/ to /var/www/docroot_old/, finally rename /var/www/docroot_new/ to /var/www/docroot/. Afterwards, you can delete /var/www/docroot_old/. Renaming is very fast, as only the base directory’s name has to be changed in the filesystem.

With this method, your website will only be naked for a few seconds, and there won’t be an incomplete installation at any time.