text: Figure 7-3 Simple view of block virtualization 
  
  Several of the MDisk extents are unused; that is, no volume extent maps to them. These 
  unused extents are available for use in creating volumes, migration, expansion, and so on. 
  
  The default and most common type of volumes in IBM Spectrum Virtualize are managed 
  mode volumes. Managed mode volumes are allocated from a set of MDisk (by default, all 
  MDisks belonging to a storage pool) and can be subject of the full set of virtualization 
  functions. 
  
  In particular, they offer full flexibility in mapping between logical volume representation (logical 
  blocks) and physical storage used to store these blocks. This requires that physical storage 
  (MDisks) are fully managed by IBM Spectrum Virtualize, which means that the LUs that are 
  presented to the IBM Spectrum Virtualize by the back-end storage systems does not contain 
  any data when it is added to the storage pool. 