text: 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: 
  
  (cid:2) Tier 0 flash 
  
  Tier 0 flash drives are high-performance flash drives that use enterprise flash technology. 
  
  (cid:2) 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. 
  
  (cid:2) 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. 
  
  (cid:2) Nearline tier 
  
  Nearline tier exists when the pool has MDisks on nearline-class disks drives that are 
  optimized for capacity. 