Storage Virtualization

There are several forms of organizing storage.

DAS (Direct Attached Storage, also known as SAS, Server Attached Storage) – a classic way of organizing storage, such that a drive (disk array under a RAID-controller) is either located inside the server box or is assembled as a standalone device connected to one or more servers.

NAS (Network Attached Storage) – these are in fact autonomous file servers having a RAID-array on board (sometimes additional storage media can be connected to extend NAS volume). Usually integrated Web-server or SSH-console is used to control NAS.

SAN (Storage Area Network) – the most versatile solution for storage virtualization at an enterprise of any scale. SAN is a standalone network separated from the main enterprise network, which is commonly based on a Fibre Channel or iSCSI. Fibre Channel (FC) connects storage media and servers to each other. Lately Internet SCSI (iSCSI) is often used as internal SAN interface for the reason it is less expensive and allows virtually unlimited distances between network nodes. Data transfer between SAN and servers connected to it goes on block level, similar to DAS.

Data center virtualization technologies

There are two main approaches to virtualization technologies on SAN level: in-band storage virtualization and out-of-band storage virtualization. In the case of in-band virtualization, a special device is set up between servers and storage modules, which incorporates rules of converting physical storage resources to logical ones. As for out-of-band virtualization device, it is connected directly to server or switch/router and manages the information about data transfer between application and server on the way to storage media connected to SAN.

Server level virtualization
Historically, server-side storage virtualization was the first solution. Logical volume managers first made their appearance on mainframes, later on Unix-servers, and in later years they were introduced on Windows platform. They provide for virtualization by means of displaying physical devices as logical ones having so called logical numbers (LUN) and divided into logical disk groups or logical volumes. This allows applications to mount logical volumes without establishing a dependency on a certain physical device.

Storage subsystem level virtualization
Storage Virtualization Managers, which create Virtual Volumes, are used in mainframes since 1990s and to date. Another approach is called “SAN-in-a-box”; it is based on an integrated solution such that one stand is shared by drives, control systems and switches. This solution satisfies the goals of virtualization, however only within the “box”.

Storage network level virtualization
Any storage network consists of two component groups: functional (servers, drives) and infrastructural (adapters, hubs, switches). In order to implement storage network level virtualization, the third component group should be added, which may be called a controlling group. Devices that make the controlling group are called SAN-add-ons or SAN-servers. These are computing devices connected to SAN or installed on data transfer routes, which are responsible for topology and implement abstraction of the data from its physical location.