Which mapping is for you?
Storage virtualization constructs a pool of storage and then divides that pool up into logical units irrespective of the actual underlying physical devices. To make this work, the system must translate logical storage addresses into physical ones and provide the information to the storage devices.
One of the differences between storage virtualization vendors' products is how this information is transmitted. One group provides the metadata about the virtual to logical translation through a separate data path ("out of band", or "asymmetric") and the other group sends the metadata on the same path as the data itself ("in band" or "symmetric"). Typically out-of-band architectures provide the metadata from a separate controller to an intelligent host bus adapter or a device driver that issues the I/O commands directly to the storage device. In-band architectures usually put the intelligence for translating in a controller (physical or virtual) that sits in the data path and translates the logical to the physical I/O requests.
Which approach is better depends on whom you talk with. Theoretically an out-of-band architecture should be more scalable because it avoids putting another box in the data path. However out-of- band mapping is new and inherently more complex than the in-band approach. That raises the specter of incompatabilities between the various components that distribute the intelligence over the network.
For a paper listing the advantages of out-of-band metadata, see: "The Value of Storage Virtualization" at FalconStor's web site (www.falconstor.com).
For a paper supporting in-band metadata, see: http://data.fibrechannel-europe.com/technology/whitepapers/210801_4.html
Rick Cook has been writing about mass storage since the days when the term meant an 80K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last twenty years he has been a freelance writer specializing in storage and other computer issues.
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