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One of the basic storage pool parameters is the extent size. All MDisks in the storage pool have the same extent size, and all volumes that are allocated from the storage pool inherit its extent size. 

There are two implications of a storage pool extent size: 

GLYPH<SM590000> Maximum volume, MDisk, and managed storage capacity depend on extent size (see http://www.ibm.com/support/docview.wss?uid=ibm10744461 ). The bigger the extent defined for the specific pool, the larger is the maximum size of this pool, the maximum MDisk size in the pool, and the maximum size of a volume created in the pool. 

GLYPH<SM590000> Volume sizes must be a multiple of the extent size of the pool in which the volume is defined. Therefore, the smaller the extend size, the better control over volume size. 

The SAN Volume Controller supports extent sizes 16 mebibytes (MiB) - 8192 MiB. The extent size is a property of the storage pool and is set when the storage pool is created. 

The extent size of a storage pool cannot be changed. If you need to change extent size, the storage pool must be deleted and a new storage pool configured. 

Table 3-2 lists all of the available extent sizes in a SAN Volume Controller, and the maximum managed storage capacity for each extent size. 

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When planning storage pool layout, consider the following aspects: 

GLYPH<SM590000> Pool extent size: 

– Generally, use 128 MiB or 256 MiB. The IBM Storage Performance Council (SPC) benchmarks use a 256 MiB extent. 

– Pick the extent size and then use that size for all storage pools. 

– You cannot migrate volumes between storage pools with different extent sizes. However, you can use volume mirroring to create copies between storage pools with different extent sizes. 

