Advantages <ul><li>Shared codebase means all sites are using the same version.
One big upgrade of drupal core can be done as opposed to updating the core code for separate installations each of which could be a different version of Drupal.
Modules/Themes can be shared across sites or they can have their own modules/themes. </li></ul>
Disadvantages <ul><li>Updating Drupal core means all sites need to be put offline for the update.
Updating core and contrib modules, means running update.php for each site in your Multi-Site setup. (though you might be able to use Aegir/Drush to run update.php for all sites in one go) </li></ul>
For example, for a fictitious site installed at http://www.drupal.org/mysite/test/, the 'settings.php' is searched in the following directories: 1. sites/www.drupal.org.mysite.test 2. sites/drupal.org.mysite.test 3. sites/org.mysite.test 4. sites/www.drupal.org.mysite 5. sites/drupal.org.mysite 6. sites/org.mysite 7. sites/www.drupal.org 8. sites/drupal.org 9. sites/org 10. sites/default
How to handle the database(s)? <ul><li>Two options </li><ul><li>One database with different prefixed tables.
Separate databases for each site in the Multi-site. </li></ul><li>Which is better? </li><ul><li>It's up to you and the resources available to you. We chose separate databases. </li></ul></ul>
Install Profile API - http://drupal.org/project/install_profile_api </li></ul><li>Write some scripts for common tasks </li><ul><li>Drush/Aegir might be able to do some of this already </li></ul></ul>
Helpful Tips <ul><li>Set some conventions and follow them. </li><ul><li>e.g. databases are named by the 'sitename' and users created for each database with the name 'sitename_user'. </li></ul></ul>
Resources <ul><li>Run Multiple Sites from a single code base - http://drupal.org/getting-started/6/install/multi-site
Multi-Site Group - http://groups.drupal.org/multisite
Multi-Site Installation and Setup Links - http://drupal.org/node/43816
How To: Multisites vs. Multiple Sites - http://gotdrupal.com/videos/multisites-vs-multiple-sites </li></ul>
Resources <ul><li>Sharing Users and content across multiple sites - http://drupal.org/node/201673