In Windows Server 2012, dedup was more focused on consolidation for files at rest and that have gone unchanged for x days, where x=5 by default. Windows Server 2012 R2 provides deduplication that consolidates files which are more in use, possibly with outstanding open handles. Dedup does this by skipping hot ranges of a file during the optimization job. Hot ranges are defined as a range of a file that has been modified with using the threshold time period. Deduplication determines file heat by consuming the USN journal. A hot range bitmap is created based on the consumed USN journal records.
NOTE: Since the USN journal is disabled by default, it will now be enabled when the Deduplication role is enabled for that volume.
The access pattern of a typical VHD/VHDx file is such that a large portion is not changing, at least not frequently, and we classify this region as being cold in the context of this feature. A relatively small portion of the file is under constant flux due to the fact that it contains the VHD metadata, or the data corresponds to the workload that the VHD/VHDx file is being used for - being classified as hot.