Storage virtualization refers to the process of abstracting logical storage from physical storage. While RAID at the basic level provides this functionality, the term storage virtualization typically includes additional concepts such as data migration and caching. Storage virtualization is hard to
define in a fixed manner due to the variety of ways in which the functionality can be provided.
Typically, it is provided as a feature of:
• Host Based with Special Device Drivers
• Array Controllers
• Network Switches
• Stand Alone Network Appliances
Each vendor has a different approach in this regard. Another primary way that storage virtualization is classified is whether it is in-band or out-of-band. In-band (often called symmetric) virtualization sits between the host and the storage device allowing caching. Out-of-band (often called asymmetric) virtualization makes use of special host based device drivers that first lookup the meta data (indicating where a file resides) and then allows the host to directly retrieve the file from the storage location. Caching at the virtualization level is not possible with this approach.
General benefits of storage virtualization include:
• Migration– Data can be easily migrated between storage locations without interrupting live access to the virtual partition with most technologies.
• Utilization– Similar to server virtualization, utilization of storage devices can be balanced to address over and under utilization.
• Management– Many hosts can leverage storage on one physical device that can be centrally managed.
Some of the disadvantages include:
• Lack of Standards and Interoperability– Storage virtualization is a concept and not a standard. As a result vendors frequently do not easily interoperate.
• Metadata– Since there is a mapping between logical and physical location, the storage metadata and its management becomes key to a working reliable system.
• Backout– The mapping between local and physical locations also makes the backout of virtualization technology from a system a complicated process.