ensure that one site collection cannot use so many resources that other site collections can no longer function. Windows SharePoint Services allows you to specify quota for site collections
When you set a quota limit for storage, you can set two values: a warning value and a maximum value. When a site collection passes the warning limit, an e-mail message is sent to the site administrator and owner notifying them that their site collection is near to their storage quota. E-mail messages are sent daily until the storage level drops below the warning level. When a site collection meets the maximum limit, another e-mail message is sent to the owner and administrator, and no new content can be added to the site collection.