• The virtualization engine is an out-of-band appliance or intelligent switch that passes the virtual volume descriptions to the application hosts and is operating system-independent.

• The virtualization engine is an in-band appliance located in the data path, meaning every I/O request passes through that device and is operating system-independent.

• Virtualization is provided in the storage architecture by disk array or tape controller microcode and is operating system-independent.

Comparing the Options for Storage Virtualization

Storage virtualization is extensively used in mainframes with Enterprise Systems Connection (ESCON) and Fibre Connectivity (FICON) subsystem controllers and in open systems in conjunction with SANs. An ongoing challenge the open systems SAN administrator faces is a lack of visibility regarding I/O behavior. This makes it difficult to:

• Optimize performance and utilization of storage assets

• Understand how I/O is navigating the infrastructure

• Identify where the congestion is concentrated.

Mainframes, in contrast, use control unit-based instrumentation and the host-based Resource Measurement Facility (RMF) to provide a much higher level of visibility to I/O performance. Being proprietary in the mainframe market doesn’t present the same degree of interoperability problems as it does for open systems since there are few equipment suppliers and a small number of storage management software suppliers compared to open systems. The interoperability issues involve fewer entities.

Many of the disk array and tape controller storage virtualization capabilities described in the following section are now available to mainframes and nonmainframe systems. The integrated virtual tape solution isn’t as mature for open systems as it is for mainframes; it doesn’t offer the capability for intelligent virtual volume management for tape cartridges.

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