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If a pool contains one type of MDisk, Easy Tier enters balancing mode. In this mode, Easy Tier moves extents from busy MDisks to less busy MDisks of the same tier. 

## Tiers of storage 

The MDisks (external LUs or array type) that are presented to the IBM SAN Volume Controller are likely to have different performance attributes because of the type of disk or RAID array on which they are located. 

Depending on performance, the system divides available storage into the following tiers: 

GLYPH<SM590000> Tier 0 flash 

Tier 0 flash drives are high-performance flash drives that use enterprise flash technology. 

GLYPH<SM590000> Tier 1 flash 

Tier 1 flash drives represent the Read-Intensive (RI) flash drive technology. Tier 1 flash drives are lower-cost flash drives that typically offer capacities larger than enterprise class flash, but lower performance and write endurance characteristics. 

GLYPH<SM590000> Enterprise tier 

Enterprise tier exists when the pool contains MDisks on enterprise-class hard disk drives, which are disk drives that are optimized for performance. 

GLYPH<SM590000> Nearline tier 

Nearline tier exists when the pool has MDisks on nearline-class disks drives that are optimized for capacity. 

For array type MDisks, the system automatically sets its tier because it knows the capabilities of array members, physical drives, or modules. External MDisks needs manual tier assignment when they are added to a storage pool. 

Note:  The tier of MDisks that are mapped from certain types of IBM System Storage Enterprise Flash is fixed to tier0_flash, and cannot be changed. 

Although IBM SAN Volume Controller can distinguish four tiers, Easy Tier manages only a three tier storage architecture. MDisk tiers are mapped to Easy Tier tiers, depending on the pool configuration. 

Figure 10-2 shows the possible combinations for the pool configuration with four MDisk tiers. 

<img src='content_image/1008153.jpg'>

