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Vendors that provide combined-path architectures offer virtualization software that operates in appliances, FC director blades or array-based configurations. Combined-path configurations handle the processing of I/O and storage service functions such as LUN masking and LUN discovery in the same logical configuration.
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A key differentiator among appliance configurations is whether they're implemented in a clustered or N+1 configuration. Clustered configurations such as IBM's SAN Volume Controller (SVC) operate as one logical entity that keeps the content of the cache of each node in the cluster in sync at all times, as opposed to an N+1 configuration that will be briefly out of sync when a management change occurs. The upside of the N+1 approach is that each node has no interdependencies with other nodes. If an error occurs in the core operating system or virtualization software of a node that causes a failure, that error won't replicate to other nodes in the N+1 configuration as would occur in a clustered configuration.
IBM's and FalconStor's virtualization software can operate on a blade that's inserted into an FC director. While delivering the same software management functions, there are some differences in how virtualization runs on an FC director than on an appliance:
- IBM SVC and FalconStor IPStor software can only be implemented on a Cisco MDS 9000 or Maxxan MXV500 director switch.
- FC director blades eliminate the need for another appliance and centralize the physical management of the devices.
- The software on the FC director blade can interact with software on its appliance counterpart on other SANs, regardless of the FC directors present in those SANs, to enable advanced storage management functions such as asynchronous replication.
Conversely, HDS took its existing proven line of microcode from its 9900 series of arrays and carried it over to its new TagmaStore and NSC55 platforms. HDS also mimicked the approach of other vendors by presenting a Windows image to other vendor's storage arrays to discover and virtualize LUNs on those arrays. By doing that, HDS circumvents possible interoperability issues between its array and those of other vendors because most storage vendors certify that their systems operate with Windows.
Using the same microcode on both platforms allows users to have the same storage management console for volume management across the enterprise. It also supports most major multipathing drivers such as EMC's PowerPath, IBM's MultiPath IO and Symantec Corp.'s/Veritas' Dynamic Multi-Pathing (DMP) in addition to HDS' own Hitachi Dynamic Link Manager. The biggest advantage their approach offers over any other virtualization option is the minimal amount of change users will encounter when implementing HDS TagmaStore or NSC55, assuming they're not using the advanced functions on other vendors' storage arrays.
This was first published in December 2005