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 presented to upper layers. The  resultant decoupling of logical storage space representation and its characteristics 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 ,  implemented by IBM as IBM Spectrum Virtualize software 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 able to virtualize both its internal 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)  identified by SCSI logical unit numbers (LUNs).  Note:  Although formally logical units and logical unit numbers are different entities, in  practice the term LUN is often used to refer to a logical disk, that is, an LU. While most applications do not directly access storage, but work with files or records, the  operating system (OS) of a host must convert these abstractions to the language of storage,  which is vectors of storage blocks identified by logical block addresses within an LU. In IBM  Spectrum Virtualize, each of the externally visible LUs is internally represented by a volume,  which is an amount of storage taken out of a storage pool.  Storage pools in turn are made out of managed disks (MDisks), which are LUs presented to  the storage system by external virtualized storage, or arrays made out of internal disks. LUs  presented to IBM Spectrum Virtualize by external storage usually correspond to RAID arrays  configured on that storage. For more information about volumes, storage pools, and external  virtualized storage systems, see Chapter 6, “Storage pools” on page 213 and Chapter 7,  “Volumes” on page 263. The hierarchy of objects, from a file system block down to a physical  block on a physical drive, is shown in Figure 1-2.  You can use storage virtualization to manage the mapping between logical blocks within an  LU presented to a host, and blocks on physical drives. This mapping can be as simple, or as  complicated, as required by a given use case. A logical block can be mapped to one physical  block or, for increased availability, multiple blocks physically stored on different physical  storage systems, possibly in different geographical locations.  Chapter 1. Introduction to storage virtualization  3 Figure 1-2 Block-level virtualization overview