Chapter 10. Advanced features for storage efficiency 405
If a pool contains one type of MDisk, Easy Tier enters balancing mode. With it, it 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 Storwize V7000 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:
/SM590000Tier 0 flash
Tier 0 flash drives are high-performance flash drives that use enterprise flash technology.
/SM590000Tier 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.
/SM590000Enterprise tier
Enterprise tier exists when the pool contains MDisks on enterprise-class hard disk drives, 
which are disk drives that are optimized for performance.
/SM590000Nearline 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.
Although IBM Storwize V7000 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.
Figure 10-2   Tier combinations
Note: The tier of MDisks mapped from certain types of IBM System Storage Enterprise 
Flash is fixed to tier0_flash, and cannot be changed.