Chapter 10. Advanced features for storage efficiency 417
Volume parameters
When creating a thin provisioned volume in Custom mode, some of its parameters might be 
modified, as shown in Figure 10-10.
Figure 10-10   Thin VDisk parameters
In a DRP pool, thin provisioned volume fine-tuning is not required. Real capacity (rsize) value 
is ignored, and Grain Size is fixed to 8 KB.
When a thin-provisioned volume is created in a standard pool, Real capacity (rsize) defines 
both initial volume real capacity and an amount of contingency capacity, which is used by 
autoexpand.
Write I/Os to the grains of the thin volume in a standard pool (that were not previously written 
to) cause grains of the real capacity to be used to store metadata and user data. Write I/Os to 
the grains (that were previously written to) update the grain where data was written. The grain 
is defined when the volume is created, and can be 32 KiB, 64 KiB, 128 KiB, or 256 KiB.
Smaller granularities can save more space, but they have larger metadata directories. When 
you use thin-provisioning with FlashCopy, specify the same grain size for the thin-provisioned 
volume and FlashCopy.
Host considerations
Do not use defragmentation applications on thin-provisioned volumes. The defragmentation 
process can write data to different areas of a volume, which can cause a thin-provisioned 
volume to grow up to its virtual size.
10.3  Unmap
Spectrum Virtualize systems running V8.1.0 and later support the SCSI Unmap command. 
This enables hosts to notify the storage controller of capacity that is no longer required, which 
can improve capacity savings. 
10.3.1  SCSI unmap command
Unmap is a set of SCSI primitives that allow hosts to indicate to a SCSI target that space 
allocated to a range of blocks on a target storage volume is no longer required. This 
command allows the storage controller to take measures and optimize the system so that the 
space can be reused for other purposes. 
The most common use case, for example, is a host application, such as VMware freeing 
storage within a file system. The storage controller can then optimize the space, such as 
reorganizing the data on the volume so that space is better used.