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Extent pools can be expanded by adding more ranks to the pool. All ranks that belong to extent pools with the same internal server affinity are called a rank group . Ranks are organized in two rank groups. Rank group 0 is controlled by server 0, and rank group 1 is controlled by server 1. 

## 4.4.1 Dynamic extent pool merge 

Dynamic extent pool merge  is a capability that is provided by the Easy Tier manual mode facility, illustrated in Figure 4-4. It allows one extent pool to be merged into another extent pool if they meet these criteria: 

GLYPH<SM590000> Have extents of the same size 

GLYPH<SM590000> Have the same storage type (FB or CKD) 

GLYPH<SM590000> Have the same DS8900F internal server affinity 

The logical volumes in both extent pools remain accessible to the host systems. Dynamic extent pool merge can be used for the following reasons: 

GLYPH<SM590000> Consolidation of two smaller extent pools with the equivalent storage type (FB or CKD) and extent size into a larger extent pool. Creating a larger extent pool allows logical volumes to be distributed over a greater number of ranks, which improves overall performance in the presence of skewed workloads. Newly created volumes in the merged extent pool allocate capacity as specified by the selected extent allocation algorithm. Logical volumes that existed in either the source or the target extent pool can be redistributed over the set of ranks in the merged extent pool by using the Migrate Volume function. 

GLYPH<SM590000> Consolidating extent pools with different storage tiers to create a merged extent pool with a mix of storage drive technologies. This type of an extent pool is called a multi-tiered pool and is a prerequisite for using the Easy Tier automatic mode feature. 

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