text: **7.1 Introduction to volumes**
  
  A volume is a logical disk that the system presents to attached hosts. For an IBM Spectrum 
  Virtualize system cluster, the volume that is presented to a host is internally represented as a 
  virtual disk (VDisk). A VDisk is an area of usable storage that was allocated out of a pool of 
  storage that is managed by IBM Spectrum Virtualize cluster. The term*virtual*is used 
  because the volume that is presented does not necessarily exist on a single physical entity. 
  
  **Note:**Volumes are made out of extents that are allocated from a storage pool. Storage 
  pools group managed disks (MDisks), which are Redundant Arrays of Independent Disks 
  (RAIDs) from internal storage, or logical units that are presented to and virtualized by IBM 
  Spectrum Virtualize system. Each MDisk is divided into sequentially numbered extents 
  (0 based indexing). The extent size is a property of a storage pool, and is used for all 
  MDisks comprising the storage pool. 
  
  MDisks are not directly visible to or used by host systems. 
  
  A volume is presented to hosts by an I/O Group, and within that group has a preferred node; 
  that is, a node that by default serves I/O requests to that volume. 