Chapter 1. Introduction to storage virtualization 3
File/record/namespace virtualization and File/record subsystem layers translate records or 
files that are required by applications to vectors of blocks, which are the language of the Block 
Virtualization layer. Block Virtualization layer, in turn, maps requests of the higher layers to 
physical storage blocks, which are provided by Storage devices in the Block subsystem.
Each of the layers in the storage domain abstracts away complexities of the lower layers and 
hides them behind an easy-to-use, standard interface that is presented to upper layers. The 
resultant decoupling of logical storage space representation and its characteristics that are 
visible to servers (storage consumers) from underlying complexities and intricacies of storage 
devices is a key concept of storage virtualization.
The focus of this publication is block-level virtualization at the block virtualization layer, 
which is implemented by IBM as IBM Spectrum Virtualize software that is running on IBM 
SAN Volume Controller and the IBM Storwize family. The IBM SAN Volume Controller is 
implemented as a clustered appliance in the storage network layer. The IBM Storwize family 
is deployed as modular storage that can virtualize its internally and externally attached 
storage.
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). 
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. 
Figure 1-2   Block-level virtualization overview
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.