416 Implementing the IBM Storwize V7000 with IBM Spectrum Virtualize V8.2.1
10.2.1  Concepts
Volumes can be configured as thin-provisioned or fully allocated. Both of them can be 
configured in standard pools and data reduction pools.
In IBM Storwize V7000, each volume has virtual capacity and real capacity parameters. 
Virtual capacity is the volume storage capacity that is available to a host Operating System 
(OS) and is used by it to create a file system. Real capacity is the storage capacity that is 
allocated to a volume from a pool, it shows the actual amount of space that is used on 
physical storage.
Fully allocated volumes are created with the same amount of real capacity and virtual 
capacity. This type uses no storage efficiency features. 
A thin-provisioned volume presents a different capacity to mapped hosts than the capacity 
that the volume uses in the storage pool, so real and virtual capacities might not be equal. 
The virtual capacity of a thin-provisioned volume is typically significantly larger than its real 
capacity. As more information is written by the host to the volume, more of the real capacity is 
used. The system identifies read operations to unwritten parts of the virtual capacity and 
returns zeros to the server without using any real capacity.
The autoexpand feature prevents a thin-provisioned volume from using up its capacity and 
going offline. As a thin-provisioned volume uses capacity, the autoexpand feature maintains a 
fixed amount of unused real capacity, called the contingency capacity. For thin-provisioned 
volumes in standard pools, the autoexpand feature can be turned on and off. For 
thin-provisioned volumes in data reduction pools, the autoexpand feature is always enabled.
A thin-provisioned volume can be converted non-disruptively to a fully allocated volume, or 
vice versa, by using the volume mirroring function. For example, you can add 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. Usually, if the 
Storwize V7000 is to detect zeros on the volume, you must use software on the host side to 
write zeros to all unused space on the disk or file system.
10.2.2  Implementation
For more information about creating thin-provisioned volumes, see Chapter 7, “Volumes” on 
page 241.
Metadata
In a standard pool, the system uses the real capacity to store data that is written to the 
volume, and metadata that describes the thin-provisioned configuration of the volume. 
Metadata uses volume real capacity and usually needs less than 0.5% of virtual capacity to 
store its data. 
This means that if your host used 100% of virtual capacity, some extra space is required on 
your storage pool to store thin provisioning metadata. In a worst case, the real size of a thin 
provisioned volume can be 100.5% of its virtual capacity. 
In a data reduction pool, metadata for a thin provisioned volume is stored separately from 
user data, and does not count in the volumes real capacity.