Note:  IBM Comprestimator can run for a long period (a few hours) when it is scanning a  relatively empty device. The utility randomly selects and reads 256 KB samples from the  device. If the sample is empty (that is, full of null values), it is skipped. A minimum number  of samples with actual data are required to provide an accurate estimation. When a device  is mostly empty, many random samples are empty. As a result, the utility runs for a longer  time as it tries to gather enough non-empty samples that are required for an accurate  estimate. The scan is stopped if the number of empty samples is over 95%. To help with the profiling and analysis of user workloads that must be migrated to the new  system, IBM provides a highly accurate data reduction estimation tool that supports  deduplication and compression. The tool operates by scanning target workloads on any  legacy array (from IBM or third party) and then merging all scan results to provide an  integrated system level data reduction estimate. The Data Reduction Estimator Tool (DRET) utility uses advanced mathematical and statistical  algorithms to perform an analysis with low memory footprint. The utility runs on a host that  has access to the devices to be analyzed. It performs only read operations so it has no effect  on the data stored on the device.  The following sections provide information about installing DRET on a host and using it to  analyze devices on it. Depending on the environment configuration, in many cases DRET is  used on more than one host to analyze more data types. When DRET is used to analyze a block device that is used by a file system, all underlying  data in the device is analyzed, regardless of whether this data belongs to files that were  already deleted from the file system. For example, you can fill a 100 GB file system and make  it 100% used, then delete all the files in the file system to make it 0% used. When scanning  the block device that is used for storing the file system in this example, DRET accesses the  data that belongs to the files that are deleted. Important:  The preferred method of using DRET is to analyze volumes that contain as  much active data as possible rather than volumes that are mostly empty of data. This use  increases the accuracy level and reduces the risk of analyzing old data that is deleted, but  might still have traces on the device. For more information and the latest version of the DRET utility, see this IBM Support web  page.  Starting with IBM Spectrum Virtualize V8.1.x, over-provisioning is supported on selected  back-end controllers. Therefore, if back-end storage performs data deduplication or data  compression on LUs provisioned from it, they still can be used as external MDisks on IBM  SAN Volume Controller.  Thin-provisioned MDisks from controllers that are supported by this feature can be used as  managed mode MDisks in IBM SAN Volume Controller and added to storage pools (including  DRPs).  Chapter 10. Advanced features for storage efficiency  455 10.7 Data deduplication and compression on external storage 10.6.2 Evaluating compression and deduplication