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. 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: Tier 0 flash drives are high-performance flash drives that use enterprise flash technology. 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. Enterprise tier exists when the pool contains MDisks on enterprise-class hard disk drives,  which are disk drives that are optimized for performance. 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. Chapter 10. Advanced features for storage efficiency  429 Figure 10-2 Tier combinations GLYPH<SM590000> Tier 0 flash GLYPH<SM590000> Tier 1 flash GLYPH<SM590000> Enterprise tier GLYPH<SM590000> Nearline tier Tiers of storage