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Virtualization: Many Levels of Implementation
Storage virtualization is defined as the presentation to users of a single interface that provides a logical view of data or storage devices different from the way data or storage is physically configured. Virtualization shifts the burden of management from intelligent people to intelligent products. Memory virtualization for operating systems has been around since the late 1960s. Virtual disks first appeared on IBM's Mass Storage System in 1974 then didn't appear again for 20 more years until StorageTek's Iceberg Virtual Array arrived in 1994. Tape device virtualization became popular in 1997, again from IBM and StorageTek.
Virtualization at the host or server level is well under developed and the value of virtual machines has been evident for years. Partitioning a server into several virtual machines makes a single server appear as multiple heterogeneous servers facilitating consolidation efforts. File virtualization at the server level has also seen acceptance as multiple files can be aggregated into one large, easier to manage file system from a single interface.
Storage networking offers lower total cost of management, higher availability, heterogeneous connectivity and, theoretically, more scalable architectures than DAS (Direct Attached Storage). DAS is easy to install and offers higher data transmission speeds as switches and networks are not needed. DAS requires the server to use valuable computing cycles, placing a storage "tax" on performing many I/O operations that are handled outboard in the SAN and NAS systems. The future of an appliance or switch serving as the single interface for virtualization functionality or as the virtualization engine remains an open issue. In-band appliances are located in the data path between the storage devices and the hosts. Data must pass directly through an In-band appliance and requires caching to minimize performance problems. An Out-of-band appliance sits outside the data path and communicates with host systems with network connections requiring an agent on each host. Many storage applications are suited for moving into the fabric or network including virtualization and volume management, local and remote backup and recovery, replication, SRM, HSM, and the long-awaited volume copy services. Several vendors are moving intelligence into the switch and the storage management benefits should be significant. Market acceptance, however, remains cautious.
Storage virtualization equates to further improvements in the storage management gap. With virtualization techniques, the user does not need to know how storage devices are configured, where they are located, or their capacity limits. Thus both the logical and physical characteristics are separated from each other, allowing the application to access the logical image while minimizing any potential disadvantages of the underlying device or storage subsystems. This reduces complexity and management overhead. By separating logical and physical characteristics, physical devices can be added, upgraded or replaced without disrupting the application or server availability. Managing virtual storage pools of over 100 terabytes is now common.
Source: Horison Information Strategies: Storage Navigator
© 2005 Horison
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