Virtualization

Virtualization is a concept that applies to many areas of the information technology industry.

For data storage, virtualization includes the creation of a pool of storage that contains several disk systems. These systems can be supplied from various vendors. The pool can be split into virtual disks (VDisks) that are visible to the host systems that use them. Therefore, VDisks can use mixed back-end storage and provide a common way to manage a storage area network (SAN).

Historically, the term virtual storage has described the virtual memory techniques that have been used in operating systems. The term storage virtualization, however, describes the shift from managing physical volumes of data to logical volumes of data. This shift can be made on several levels of the components of storage networks. Virtualization separates the representation of storage between the operating system and its users from the actual physical storage components. This technique has been used in mainframe computers for many years through methods such as system-managed storage and products like the IBM® Data Facility Storage Management Subsystem (DFSMS). Virtualization can be applied at the following four main levels:

At the server level: Manages volumes on the operating systems servers. An increase in the amount of logical storage over physical storage is suitable for environments that do not have storage networks.
At the storage device level: Uses striping, mirroring and RAIDs to create disk systems. This type of virtualization can range from simple RAID controllers to advanced volume management such as that provided by the IBM TotalStorage® Enterprise Storage Server® (ESS) or by Log Structured Arrays (LSA). The Virtual Tape Server (VTS) is another example of virtualization at the device level.
At the fabric level: Enables storage pools to be independent of the servers and the physical components that make up the storage pools. One management interface can be used to manage different storage systems without affecting the servers. The SAN Volume Controller performs virtualization at the fabric level.
At the file system level: Provides the highest benefit because data is shared, allocated, and protected at the data level rather than the volume level.

Virtualization is a radical departure from traditional storage management. In traditional storage management, storage is attached directly to a host system, which controls storage management. SANs introduced the principle of networks of storage, but storage is still primarily created and maintained at the RAID system level. Multiple RAID controllers of different types require knowledge of, and software that is specific to, the given hardware. Virtualization provides a central point of control for disk creation and maintenance.

One problem area that virtualization addresses is unused capacity. Before virtualization, individual host systems each had their own storage, which wasted unused storage capacity. Using virtualization, storage is pooled so that jobs from any attached system that need large amounts of storage capacity can use it as needed. Virtualization makes it easier to regulate the amount of available storage without having to use host system resources or to turn storage devices off and on to add or remove capacity. Virtualization also provides the capability to move storage between storage systems transparently to host systems.

Types of virtualization

Virtualization can be performed either asymmetrically or symmetrically. Figure 1 provides a diagram of the levels of virtualization.

Asymmetric: A virtualization engine is outside the data path and performs a metadata style service.
Symmetric: A virtualization engine sits in the data path and presents disks to the hosts, but hides the physical storage from the hosts. Advanced functions, such as cache and Copy Services, can therefore be implemented in the engine itself.

Virtualization at any level provides benefits. When several levels are combined, the benefits of those levels can also be combined. For example, you can combine benefits by attaching a RAID controller to a virtualization engine that provides virtual volumes for a virtual file system.

Note: The SAN Volume Controller implements fabric-level virtualization. Within the context of the SAN Volume Controller and throughout this document, virtualization refers to symmetric fabric-level virtualization.

Figure 1. Levels of virtualization