242 Implementing the IBM Storwize V7000 with IBM Spectrum Virtualize V8.2.1
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.
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.
7.1.1  Operations on volumes
The following operations can be performed on a volume:
/SM590000Volumes can be created and deleted. 
/SM590000Volumes can be resized (expanded or shrunk).
/SM590000Volumes can be migrated at run time to another MDisk or storage pool.
/SM590000Volumes can be mirrored, or mirrored volumes can be split.
/SM590000Point-in-time volume can be created by using FlashCopy. Multiple snapshots and quick 
restore from snapshots (reverse FlashCopy) are supported.
7.1.2  Volume characteristics
Volumes have the following characteristics or attributes:
/SM590000Volumes can be configured as managed or in image mode.
/SM590000Volumes can have extents allocated in striped or sequential mode.
/SM590000Volumes 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.
/SM590000Volumes 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.
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. 
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.