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Which mapping is for you?

There are good and bad things about in-band and out-of-band mapping for virtual storage.


Which mapping is for you?
Rick Cook

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 (

For a paper supporting in-band metadata, see:

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.

Additional Resources:
1. What are the pros and cons of asymmetrical SANs?
LAN-free operation of a SAN usually isn't completely LAN free. Learn how asymmetrical SANs can correct this inefficiency but at the same time create management complications.

2. Is Cisco's 5420 storage router a good option?
I am in the process of researching a storage solution for my firm. No high I/O applications. Is the 5420 storage router from Cisco a solution worth consideration? Read what SAN expert Christopher Poelker has to say about this product as well as traditional NAS devices.

3. How can I troubleshoot SAN connection issues?
As storage requirements increase and companies find themselves trying to support multiple "SAN islands" of data, trying to troubleshoot connection issues becomes more complex. Here are a few tips that may make troubleshooting these complex SAN environments a bit easier.

4. Why would I want to virtualize storage?
Storage virtualization allows administrators to manage storage more efficiently by reducing underutilization. However reaping the benefits of virtualization requires paying attention to the way the enterprise uses storage.

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