In-band appliances
These products, like switch-based virtualization products, perform virtualization within the network. They're located between arrays and servers, and all storage traffic needs to pass through them. While fabric-based virtualization uses wire-speed switching to map and forward storage frames, in-band virtualization appliances require terminating incoming I/Os and initiating new I/Os to the actual storage target based on the information in the mapping table.

"The process of terminating, re-initiating and verifying I/Os adds significant latency to I/O processing," says Brian Garrett, technical director of Milford, MA-based Enterprise Strategy Group's ESG Lab. To compensate for the overhead and performance penalty of having to spawn new I/Os, products like IBM's SVC depend on cache, which adds the complexity of ensuring data integrity and data consistency in the cache, a problem switch-based virtualization products don't have.

IBM SVC is the most prominent product in this category and, through scalable cluster configurations and plenty of cache, it has greatly reduced the performance and scalability concerns that have plagued in-band virtualization products in the past. Relatively low cost, simplicity and a rich feature set have greatly contributed to in-band virtualization being the most widely deployed virtualization architecture today.

"In-band virtualization products like IBM's SVC or DataCore Software Corp.'s SANsymphony have the lowest entry cost; unlike fabric-based virtualization products, they don't require expensive intelligent switches," explains Greg Schulz, founder and senior analyst at StorageIO Group, a technology analyst and consulting firm in Stillwater, MN. Because products like IBM SVC work with any switch, in-band virtualization appliances have another advantage over fabric-based produc...

Storage controller-based virtualization
This architecture, championed by Hitachi Data Systems and used in its Universal Storage Platform V (USP V) storage systems, performs virtualization within the storage controller of the array. A non-Hitachi array can be virtualized by simply plugging it into a Fibre Channel (FC) port on the USP V. To third-party arrays, the USP V presents itself as a Windows server; once the third-party array is discovered by the USP V, it appears to other servers as a Hitachi array. Unlike switch-based virtualization, for companies that have standardized on Hitachi storage and already own USP arrays, the effort to enable virtualization is miniscule and relatively inexpensive. "About 50% of our USP V customers purchase a virtualization license, and with 9,200 USP V units sold in the past three and a half years, we have a significantly higher number of virtualization deployments than all fabric-based installations combined," claims Claus Mikkelsen, chief scientist at Hitachi Data Systems.

For users who are using or have standardized on array-based virtualization, vendor lock-in is high, even more so than for fabric-based virtualization. "You wouldn't buy a Hitachi USP V for virtualization if you're an EMC or NetApp shop; but USP V virtualization would be on top of your list if you had standardized on Hitachi storage," says Schulz.

Having the array and virtualization software from a single vendor has the huge benefit of a single point of support. In stark contrast, fabric-based virtualization products, namely those from EMC and Incipient Inc., require the orchestration of three different vendors (array, switch and virtualization software vendors), which clearly carries the risk of finger pointing if problems arise.

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