Logical volume management
Volume managers differ but some basic concepts exist across most versions.  The volume manager starts with physical volumes or PVs, which can be hard disk partitions, RAID devices or SAN LUNs.  PVs are split into small chunks of even size (a default of 4 MB on HP-UX) called physical extents or PEs.  The PEs are then pooled into a volume group or VG.