In implementing account-based storage quotas we faced two alternatives, the first was to use native quota managed packages in Linux itself (see http://www.yolinux.com/TUTORIALS/LinuxTutorialQuotas.html for example) or to implement alerting and management at a higher level. To ease the burden on support and server administrators we elected to implement quotas at our application level rather than utilize the file system level. This has resulted in a simple and robust implementation which you can customize!
WholesaleBackup’s implementation is straightforward and open source. If you enable a quota for a backup account, it is checked each time a backup transaction occurs, and if the user is over the quota you set, your backup server administrative alert email account (which you specified at installation time) is sent an email indicating that quota has been exceeded for that user. From the WholesaleBackup web portal you can easily edit the quota value, frequency of alerts, and see when an alert was last sent. In addition, all alerts are logged for future reference.
You can easily extend our implementation, since we’ve provided the schema for the underlying database as well as the simple script which checks for quota conditions and sends email alerts, to perform more sophisticated processing such as suspending accounts and emailing clients directly.