5.5.2.5. Accounting of Space Utilized

One constant in a system administrator's life is that there is never enough free space, and even if there is, it will not remain free for long. Therefore, a system administrator should at least be able to easily determine the level of free space available for each file system. In addition, file systems with well-defined user identification capabilities often include the capability to display the amount of space a particular user has consumed.
This feature is vital in large multi-user environments, as it is an unfortunate fact of life that the 80/20 rule often applies to disk space -- 20 percent of your users will be responsible for consuming 80 percent of your available disk space. By making it easy to determine which users are in that 20 percent, you can more effectively manage your storage-related assets.
Taking this a step further, some file systems include the ability to set per-user limits (often known as disk quotas) on the amount of disk space that can be consumed. The specifics vary from file system to file system, but in general each user can be assigned a specific amount of storage that a user can use. Beyond that, various file systems differ. Some file systems permit the user to exceed their limit for one time only, while others implement a "grace period" during which a second, higher limit is applied.