text: Storage virtualization can, and is, being implemented on many layers. Figure 1-2 on page 3 
  shows an example where a file system block gets mirrored by the host’s operating system (left 
  side of the figure) by using features of the logical volume manager (LVM), or by IBM Spectrum 
  Virtualize system at the storage pool level (as shown on the right side of Figure 1-2 on 
  page 3). Although the result is similar (the data block is written to two different arrays), the 
  effort that is required for per-host configuration is disproportionately larger than for a 
  centralized solution with organization-wide storage virtualization that is done on a dedicated 
  system and managed from a single GUI. 