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DFS: The missing link?

May be the missing link.

NTFS Change Journal

As files, folders, and other NTFS objects are added, deleted, and modified, NTFS enters change journal records in streams, one for each volume on the computer.

The total size of all the records currently in the journal varies, but there is a configurable maximum size. The change journal can exceed the maximum size until the size reaches an outer threshold, at which point a portion of the oldest records are deleted until the change journal is restored to its maximum size. The maximum size of the change journal is configurable but cannot be reduced, only increased.

The change journal conveys significant scalability benefits to applications that might otherwise need to scan an entire volume for changes. File system indexing, replication managers, virus scanners, and incremental backup applications can benefit from using the change journal.

The change journal is much more efficient than time stamps or file notifications for determining changes in a particular namespace. Applications that must rescan an entire volume to determine changes can now scan once and subsequently refer to the change journal. The I/O cost depends on how many files have changed, not on how many files exist on the volume.

The APIs are fully documented and can be leveraged by independent software vendors (ISVs). Microsoft uses the change journal in Windows Server 2003 components such as the Indexing Service and File Replication Service. ISVs can use this feature to enhance the scalability and robustness of a range of products including backup, antivirus, and auditing tools.

http://technet.microsoft.com/en-us/library/bb742450.aspx

The Change Journal is initially an empty file on the disk volume. As changes occur to the volume, records are appended to the end of this file. Each record is assigned a 64-bit identifier called an Update Sequence Number (USN). When Microsoft was first developing the Change Journal, it was internally called the USN Journal. That’s why the structures and defines in the winioctl.h header file refer to the Change Journal as the USN Journal. When a record is added to the journal, it is assigned a USN. USNs are generated in increasing order, so that you can compare USNs to find out the order of events (lower USNs are older events). USNs are not contiguous, so it’s possible that the first USN record might be 0 and the second USN record might be 128.

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