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## 1.1 Storage virtualization terminology 

Storage virtualization  is a term that is used extensively throughout the storage industry. It can be applied to various technologies and underlying capabilities. In reality, most storage devices technically can claim to be virtualized in one form or another. Therefore, this chapter starts by defining the concept of storage virtualization as it is used in this book. 

IBM describes storage virtualization in the following way: 

GLYPH<SM590000> Storage virtualization is a technology that makes one set of resources resemble another set of resources, preferably with more desirable characteristics. 

GLYPH<SM590000> It is a logical representation of resources that is not constrained by physical limitations and hides part of the complexity of those resources. It also adds or integrates new functions with existing services, and can be nested or applied to multiple layers of a system. 

Storage virtualization is also defined in the Storage Networking Industry Association’s (SNIA) shared storage model version 2, shown in Figure 1-1. 

<img src='content_image/1006559.jpg'>

The model consists of the following layers: 

GLYPH<SM590000> Application : the user of the storage domain 

GLYPH<SM590000> Storage domain , which is split into: 

– File/record/namespace virtualization  and File/record subsystem 

– Block Virtualization 

– Block subsystem 

Applications typically read and write data as vectors of bytes or records. Alternatively, storage presents data as vectors of blocks of a constant size (512 bytes or, in newer devices, 4096 bytes per block). The File/record/namespace virtualization  and File/record subsystem  layers convert records or files required by applications to vectors of blocks, which are the language of the Block Virtualization  layer. The Block Virtualization  layer in turn maps requests of the higher layers to physical storage blocks, provided by Storage devices  in the Block subsystem . 

