text: IBM Spectrum Virtualize uses Small Computer System Interface (SCSI) protocol to 
  communicate with its clients and presents storage space in form of SCSI logical units (LUs), 
  which are identified by SCSI logical unit numbers (LUNs). 
  
  **Note:**Although LUs and LUNs are different entities, the term LUN in practice is often used 
  to refer to a logical disk; that is, an LU. 
  
  Although most applications do not directly access storage but work with files or records, the 
  operating system of a host must translate these abstractions to the language of storage; that 
  is, vectors of storage blocks that are identified by logical block addresses within an LU. 
  
  Inside IBM Spectrum Virtualize, each of the externally visible LUs is internally represented by 
  a volume, which is an amount of storage that is taken out of a storage pool. Storage pools in 
  turn are made out of managed disks (MDisks); that is, are LUs that are presented to the 
  storage system by external virtualized storage or arrays that consist of internal disks. LUs that 
  are presented to IBM Spectrum Virtualize by external storage usually correspond to RAID 
  arrays that are configured on that storage. 
  
  The hierarchy of objects, from a file system block down to a physical block on a physical drive, 
  is shown in Figure 1-2. 