As a general rule, set an extent size of either 256 MB or 512 MB for Easy Tier enabled configurations. With these extent sizes, the maximum configurable capacity for an IBM Spectrum Virtualize/Storwize system is 1 PB and 2 PB. For systems with larger capacity requirements, bigger extent sizes must be used. Special considerations apply to Data Reduction Pools (DRP) as discussed in “Easy Tier and remote copy considerations” on page 324. For DRP the extent size recommendation is 4 GB or larger. 

## External controller tiering considerations 

IBM Easy Tier is an algorithm that has been developed by IBM Almaden Research and made available to many members of the IBM storage family, such as the DS8000, IBM Spectrum Virtualize, and Storwize products. The DS8000 is the most advanced in Easy Tier implementation and currently provides features that are not yet available for IBM Spectrum Virtualize and Storwize technology, such as Easy Tier Application, Easy Tier Heat Map Transfer, and Easy Tier Control. 

Before V7.3, IBM Spectrum Virtualize/Storwize had basically only two tiers and no autorebalance feature was available. For this reason, when using external controllers with more advanced tiering capabilities like the DS8000, the preferred practice was to enable tiering at the backend level and leave IBM Spectrum Virtualize/Storwize Easy Tier disabled. 

With V7.3 and the introduction of the autorebalance function, Easy Tier can be effectively enabled at the IBM Spectrum Virtualize/Storwize systems level. However, consider that IBM Spectrum Virtualize is not aware of the tiering functions of an external controller, and vice versa. So each tiering function makes its decisions independently. What you need to avoid is rebalance over rebalance because this situation can cancel any performance benefits. 

Consider the following two options: 

GLYPH<SM590000> Easy Tier is done at the IBM Spectrum Virtualize/Storwize level: 

a. In this case, complete these steps at the backend level: 

i. Set up homogeneous pools according to the tier technology available. 

ii. Create volumes to present to IBM Spectrum Virtualize/Storwize from the homogeneous pool. 

iii. Disable tiering functions. 

b. At an IBM Spectrum Virtualize/Storwize level, you need to complete the following actions: 

i. Discover the MDisks provided by the backend storage and set the tier properly. 

ii. Create hybrid pools that aggregate the MDisks. 

iii. Enable the Easy Tier function. 

GLYPH<SM590000> Easy Tier is done at the backend level. 

a. In this case, complete these actions at the back-end level: 

i. Set up hybrid pools according to the tier technology available. 

ii. Create volumes to present to IBM Spectrum Virtualize/Storwize from the hybrid pools. 

iii. Enable the tiering functions. 

b. At IBM Spectrum Virtualize/Storwize level, you need to complete the following actions: 

i. Discover the MDisks provided by the backend storage and set the same tier for all. 

ii. Create standard pools that aggregate the MDisks. 

iii. Disable the Easy Tier function. 

