content_image/973573.jpg
Figure 2-8 shows the relationships of the SVC entities to each other. 

<img src='content_image/973573.jpg'>

Each MDisk in the storage pool is divided into several extents. The size of the extent is selected by the administrator when the storage pool is created, and cannot be changed later. The size of the extent is 16 MiB - 8192 MiB. 

It is a preferred practice to use the same extent size for all storage pools in a system. This approach is a prerequisite for supporting volume migration between two storage pools. If the storage pool extent sizes are not the same, you must use volume mirroring to copy volumes between pools. 

The SVC limits the number of extents in a system to 2$^{22}$=~4 million. Because the number of addressable extents is limited, the total capacity of an SVC system depends on the extent size that is chosen by the SVC administrator. 

## 2.2.11 Volumes 

Volumes  are logical disks that are presented to the host or application servers by the SVC. Hosts and application servers can see only the logical volumes that are created from combining extents from a storage pool. 

There are three types of volumes in terms of extents management: 

GLYPH<SM590000> Striped 

A striped volume is allocated one extent in turn from each MDisk in the storage pool. This process continues until the space required for the volume has been satisfied. 

It is also possible to supply a list of MDisks to use. 

