7.1 Introduction to volumes 7.1.1 Operations on volumes 7.1.2 Volume characteristics 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. IBM Spectrum Virtualize uses 512 bytes block size for volumes that are presented to hosts. The following operations can be performed on a volume: Note:  With V7.4 and later, it is possible to prevent accidental deletion of volumes if they  recently performed any I/O operations. This feature is called  Volume protection , and it  prevents active volumes, or host mappings from being deleted inadvertently. This process  is done by using a global system setting. For more information, see section 7.8.9, “Volume  delete protection” on page 303 and the “Enabling volume protection” topic in  IBM  Knowledge Center. Volumes have the following characteristics or attributes: GLYPH<SM590000> Volumes can be configured to have single data copy, or be mirrored to make them  resistant to disk subsystem failures or to improve the read performance. GLYPH<SM590000> Volumes can be created as fully allocated or thin-provisioned. A conversion from a fully  allocated to a thin-provisioned volume and vice versa can be done at run time. GLYPH<SM590000> Volumes can have extents allocated in striped or sequential mode. GLYPH<SM590000> Volumes can be configured as managed or in image mode. GLYPH<SM590000> Point-in-time volume can be created by using FlashCopy. Multiple snapshots and quick  restore from snapshots (reverse FlashCopy) are supported. GLYPH<SM590000> Volumes can be mirrored, or mirrored volumes can be split. GLYPH<SM590000> Volumes can be migrated at run time to another MDisk or storage pool. GLYPH<SM590000> Volumes can be resized (expanded or shrunk). GLYPH<SM590000> Volumes can be created and deleted.  242  Implementing the IBM Storwize V7000 with IBM Spectrum Virtualize V8.2.1