Disk Quota Management

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How does quota management work? According to Precise/W. Quinn Associates (Reston, Virginia; www.wquinn.com), a disk quota software vendor, several different approaches to quota management are available.

Using APIs

One method is to monitor file creation and allocation events after the file operation completes while utilizing operating system security features to restrict access. This approach harnesses WIN32 APIs that act on file system events and file operations after they have completed. When the quota is exceeded, users are prevented from creating additional files. The operating system detects changes in file size at the point when a file is written to disk. Because it is necessary to wait until after the I/O is complete, delays may occur; for example, if the system uses a lot of caching, the cache has to be flushed enough for detection of the exceeded quota to occur.

This approach, though, carries with it several disadvantages. As mentioned above, using APIs means that no action can normally be taken until a file is closed. Suppose that User X has a quota of 500 MB. He has already used 485 MB and then decides to create or copy a 1-GB file. That action would result in User X being almost 1 GB over the quota before any restrictions could be enforced. On the plus side, though, this feature does not expose users to data loss if they exceed their quotas by a small amount. In addition, using APIs can lead to other problems such as restricted access to specific directories and can result in changing security parameters, which should be avoided if possible.

Operating System Integration

Another approach is to integrate disk quota management with the operating system using a driver. This method, used by the built-in quota utility for Windows 2000, supports user quotas on volumes; that is, the administrator can control how much data each user is permitted to store on an NTFS volume. It does not, however, support directory quotas. But, on the plus side, administrators can generate reports and use the event monitor to track quotas.

Under this approach, when users exceed their quotas, they must delete some files, swap ownership of some files to another user, or obtain an increased quota allocation from the administrator. Each file's owner is directly tracked, and each user has an assigned amount of space; the administrator can set quotas for specific users on any volume or can set default quotas for new users. Windows 2000 also adds the ability to query the level of quota tracking, default quota limits, and the quota information for each user, a feature unavailable previously.

Windows 2000 administrators can set hard quotas, see who is using what, set warning thresholds, and more — quite an improvement over NT and its various third-party utilities. Quotas can also be disabled if desired, and the administrator can establish soft quotas in order to track quota usage without enforcement. In this arrangement, administrators at least know what is happening, although they are limited in what they can do about it. Alternatively, hard quotas can be set, tracked, and enforced.

What about quota thresholds? Various thresholds can be set to keep track of user quotas, not just to prevent them from being exceeded but to alert users and administrators when space limits are being approached.

While this approach integrates well with the operating system and NTFS file system, it has several disadvantages. For one thing, directory quotas are not supported. As these are important for controlling disk allocation in shared directories and for reporting on space usage by directory, most organizations require this functionality. Note that Windows NT quota tools provided this function to a greater or lesser degree, but typically did not support individual user quotas. Further, only one set of warnings is generated. If a quota is exceeded, an alert goes to the user and an entry is made in the event log. It is far better to initiate a scheme whereby a series of warnings are generated, starting at, say, 70% usage exceeded, then at 80%, 90%, and finally when the quota is exceeded. That way, users have a chance to delete files and exert some control over their allocations.

I/O Filters

Another method of setting quotas uses I/O filters. Such filters intercept I/O operations before they complete and so enable the monitoring of file creations and allocation changes on a real-time basis. By comparing this data to existing quota information, decisions can be made regarding space availability and space restrictions. This method has several advantages, particularly in that it allows both user and directory quotas, which can be set on both NTFS and FAT volumes. Further, system managers can utilize many configuration options to specify exactly how quotas and thresholds are to be enforced. In addition to the features of other approaches discussed previously, this method makes it possible for quota policy to be automatically adjusted in order to extend user quotas if needed. When plenty of space is available, the quota policy can be set to add space automatically when users exceed their limits.

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