Chapter 7. Volumes 253
The real capacity of a thin-provisioned volume can be changed if the volume is not in image 
mode. Thin-provisioned volumes use the grains of real capacity that is provided in ascending 
order as new data is written to the volume. If the user initially assigns too much real capacity 
to the volume, the real capacity can be reduced to free storage for other uses.
A thin-provisioned volume can be configured to autoexpand. This feature causes the IBM 
Spectrum Virtualize to automatically add a fixed amount of extra real capacity to the 
thin-provisioned volume as required. Autoexpand does not cause the real capacity to grow 
much beyond the virtual capacity. Instead, it attempts to maintain a fixed amount of unused 
real capacity for the volume, which is known as the contingency capacity. 
The contingency capacity is initially set to the real capacity that is assigned when the volume 
is created. If the user modifies the real capacity, the contingency capacity is reset to be the 
difference between the used capacity and real capacity.
A volume that is created without the autoexpand feature, and therefore has a zero 
contingency capacity, goes offline when the real capacity is used and it must expand. 
To facilitate management of the auto expansion of thin-provisioned volumes, a capacity 
warning should be set for the storage pools from which they are allocated. When the used 
capacity of the pool exceeds the warning capacity, a warning event is logged. For example, if 
a warning of 80% is specified, an event is logged when 20% of the pool capacity remains free.
A thin-provisioned volume can be converted nondisruptively to a fully allocated volume (or 
vice versa) by using the volume mirroring function. You can create a thin-provisioned copy to 
a fully allocated primary volume and then remove the fully allocated copy from the volume 
after they are synchronized.
The fully allocated-to-thin-provisioned migration procedure uses a zero-detection algorithm 
so that grains that contain all zeros do not cause any real capacity to be used.
7.1.9  Compressed volumes
This custom type of volume is where data is compressed as it is written to disk, which saves 
more space. To use the compression function, you must obtain the IBM Real-time 
Compression license.
7.1.10  Deduplicated volumes
Deduplication is a specialized data compression technique for eliminating duplicate copies of 
data. However, in contrast to the standard file-compression tools that work on single files or 
sets of files, deduplication is a technique applied on a larger scale, such as a file system or 
volume. In IBM Spectrum Virtualize deduplication can be enabled for thin provisioned and 
compressed volumes created in data reduction pools. 
Deduplication works by identifying repeating chunks in the data written to the storage system. 
Spectrum Virtualize calculates a signature for each data chunk (by using a hash function), 
and checks if that signature is already present in the deduplication database. 
Note: Thin-provisioning is supported in standard pools and data reduction pools. However, 
data reduction pools support reclaiming capacity when it is no longer used by host data 
and then can redistribute it automatically for other uses. For more information, see 7.1.11, 
“Capacity reclamation” on page 254.