There are several preferred practices for using Comprestimator: Note:  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. If the number of empty samples is over 95%, the scan is stopped. Data Reduction Estimator Tool (DRET) is a CLI-operated execution file that runs from the  host to analyze a given block device. It provides a report of what it would expect the  deduplication savings to be from data written to the disk. There are no additional adjustments  or requirements to run this tool on your IBM Spectrum Virtualize solution. DRET displays its accuracy best with sequential workloads, and with volumes that contain the  most active data in a storage environment. DRET is accurate at analyzing a range of  workloads to identify the range at which the capacity of a volume can be used. Furthermore, it  is able to complete these tasks within a reasonable amount of time, as shown in Table 3-3.  When using DRET to analyze a block device 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 making it 0% used. When scanning the block device  used for storing the file system in this example, DRET will access the data that belongs to the  files that are already 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  increases the accuracy level and reduces the risk of analyzing old data that is already  deleted, but might still have traces on the device. Chapter 3. Data reduction and tools  47 Table 3-3 Estimated time per block size 3.8.3 Using Data Reduction Estimation Tool Volume Size Time Taken (minutes) 15 GB 1 35 GB 1 100 GB 41 1 TB 32 GLYPH<SM590000> Run the Comprestimator utility before implementing an IBM Spectrum Virtualize solution,  and before implementing the Data Reduction Pools technology. GLYPH<SM590000> Download the latest version of the Comprestimator utility if you are not using one that is  included in your IBM Spectrum Virtualize solution. GLYPH<SM590000> Use Comprestimator to analyze volumes that contain as much active data as possible  rather than volumes that are nearly empty or newly created. This ensures more accuracy  when sizing your environment for compression and data reduction pools. 