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There are two types of block-level virtualization--symmetric and asymmetric. Symmetric virtualization can be either embedded or appliance-based. In symmetric virtualization, control and data traffic share the same I/O path, while in asymmetric virtualization, control and data traffic is segregated into separate channels or paths. Most control path processing is performed by an appliance or a meta data server that sits outside the data path.

In embedded virtualization, virtualization is managed by an ASIC or array processor that controls a switch, port or its subset. All control and data traffic flows through the port the hosts are plugged into. Appliance-based virtualization devices are designed to enable all communication to pass through a single appliance plugged into the fabric. This appliance is responsible for all virtualization and data migration/mobility functions, with no dependencies on the existing fabric.

In semi-embedded or hybrid virtualization, the control traffic is segregated and routed via either a separate out-of-band network or in-band through IP over Fibre Channel (FC). Data, however, flows in-band via the fabric.

IBM Corp.'s SAN Volume Controller provides symmetric virtualization, while EMC's Invista employs hybrid virtualization--virtualization that's a combination of asymmetric

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and symmetric approaches. The latter requires intelligent switches to be installed in the SAN. Switch vendors are also catching up on fabric virtualization where the entire abstraction is performed in the network.

Selecting the type of virtualization that suits your environment is a complex process that requires careful evaluation and selection. Symmetric embedded and appliance- or array-based virtualization seem to be the most prevalent, but this doesn't imply that other methods don't merit evaluation.

This was first published in June 2006

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