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"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.
| 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 products like EMC Corp.'s Invista, which only runs on supported switch platforms.
Storage controller-based virtualization
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.
This was first published in September 2008