Windows® Internals, Sixth Edition, Part 2

Change Logging

Many types of applications need to monitor volumes for file and directory changes. For example, an automatic backup program might perform an initial full backup and then incremental backups based on file changes. An obvious way for an application to monitor a volume for changes is for it to scan the volume, recording the state of files and directories, and on a subsequent scan detect differences. This process can adversely affect system performance, however, especially on computers with thousands or tens of thousands of files.

An alternate approach is for an application to register a directory notification by using the FindFirstChangeNotification or ReadDirectoryChangesW Windows function. As an input parameter, the application specifies the name of a directory it wants to monitor, and the function returns whenever the contents of the directory change. Although this approach is more efficient than volume scanning, it requires the application to be running at all times. Using these functions can also require an application to scan directories because FindFirstChangeNotification doesn’t indicate what changed—just that something in the directory has changed. An application can pass a buffer to ReadDirectoryChangesW that the FSD fills in with change records. If the buffer overflows, however, the application must be prepared to fall back on scanning the directory.

NTFS provides a third approach that overcomes the drawbacks of the first two: an application can configure the NTFS change journal facility by using the DeviceIoControl function’s FSCTL_CREATE_USN_JOURNAL file system control code (USN is update sequence number) to have NTFS record information about file and directory changes to an internal file called the change journal. A change journal is usually large enough to virtually guarantee that applications get a chance to process changes without missing any. Applications use the FSCTL_QUERY_USN_JOURNAL file system control code to read records from a change journal, and they can specify that the DeviceIoControl function not complete until new records are available.

Per-User Volume Quotas

Systems administrators often need to track or limit user disk space usage on shared storage volumes, so NTFS includes quota-management support. NTFS quota-management support allows for per-user specification of quota enforcement, which is useful for usage tracking and tracking when a user reaches warning and limit thresholds. NTFS can be configured to log an event indicating the occurrence to the System event log if a user surpasses his warning limit. Similarly, if a user attempts to use more volume storage then her quota limit permits, NTFS can log an event to the System event log and fail the application file I/O that would have caused the quota violation with a “disk full” error code.

NTFS tracks a user’s volume usage by relying on the fact that it tags files and directories with the security ID (SID) of the user who created them. (See Chapter 6 in Part 1 for a definition of SIDs.) The logical sizes of files and directories a user owns count against the user’s administrator-defined quota limit. Thus, a user can’t circumvent his or her quota limit by creating an empty sparse file that is larger than the quota would allow and then fill the file with nonzero data. Similarly, whereas a 50-KB file might compress to 10 KB, the full 50 KB is used for quota accounting.

By default, volumes don’t have quota tracking enabled. You need to use the Quota tab of a volume’s Properties dialog box, shown in Figure 12-20, to enable quotas, to specify default warning and limit thresholds, and to configure the NTFS behavior that occurs when a user hits the warning or limit threshold. The Quota Entries tool, which you can launch from this dialog box, enables an administrator to specify different limits and behavior for each user. Applications that want to interact with NTFS quota management use COM quota interfaces, including IDiskQuotaControl, IDiskQuotaUser, and IDiskQuotaEvents.

Figure 12-20. Volume Properties dialog box

Link Tracking

Shell shortcuts allow users to place files in their shell namespace (on their desktop, for example) that link to files located in the file system namespace. The Windows Start menu uses shell shortcuts extensively. Similarly, object linking and embedding (OLE) links allow documents from one application to be transparently embedded in the documents of other applications. The products of the Microsoft Office suite, including PowerPoint, Excel, and Word, use OLE linking.