Chapter 10. Advanced features for storage efficiency 431
10.6.2  Evaluating compression and deduplication
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 both 
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 
can access 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 
deleted from the file system. For example, you can fill a 100 GB file system and make it 100% 
used, and 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, the DRET accesses the 
data that belongs to the files that are deleted.
For more information and the latest version of this utility, see this IBM Support web page.
10.7  Data deduplication and compression on external storage
Starting from IBM Spectrum Virtualize V8.1.x, it supports over-provisioning on selected 
back-end controllers. This means that 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 
Storwize V7000. 
Thin-provisioned MDisks from controllers that are supported by this feature can be used as 
managed mode MDisks in IBM Storwize V7000 and added to storage pools (including DRPs). 
Implementation steps for thin-provisioned MDisks are same as for fully allocated storage 
controllers. Extra caution is used when planning capacity for such configurations.
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 data is 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%.
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 deleted, but 
might still have traces on the device.