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What do you mean, "virtualization"?

 

What do you mean, "virtualization"?
Rick Cook

Storage virtualization, which makes managing storage devices in a SAN as easy as managing files in a folder, is getting a lot of attention. However there are at least three different places where storage can be virtualized and, generally speaking, the different types don't mix easily -- or at all.

The basic idea of storage virtualization is to map a logical representation of the storage space onto the physical hardware using something like a set of tables to keep track of what is actually stored where. The administrator of a virtualized storage system doesn't have to worry about things like where a piece of data is located or the capacity of individual storage arrays. As long as there's enough capacity in the system, the data is automatically stored.

While you can perform storage virtualization just about anywhere in the entire storage system, what you can't do very easily is perform virtualization at several levels at once. Storage virtualization can be done at three different levels:

  1. At the server level -- where the capability resides on the storage server(s);
  2. At the fabric level -- where the capabilitiy is in the SAN fabric switches or other equipment, such as SAN appliances;
  3. At the storage-system level -- where virtualization is built into the storage arrays or other storage devices.

All three approaches work, but they don't always play

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well together. In fact they can positively interfere with each other as devices at two or more levels try to do the same job of dynamically mapping storage.

The solution, according to Dan Tanner of the Aberdeen Group may well be a virtualization standard that would allow improved interoperability among the levels. Tanner discusses the problem on the Aberdeen Group's web site. An earlier paper by Tanner, outlining the differences in levels and their effects is available at the Storage Networking Industry Association website.


About the author: 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:

  • One of StorageTek's technical evangelists, Rob Nieboer, has written a useful backgrounder on the subject of different virtualization issues to consider, along with specific questions to ask storage vendors. We've listed highlights of this paper (see "Virtualization: What are you getting for your money?") in recent Storage Management tips. The paper has also generated some buzz from posters in searchStorage's Sound Off discussion forum.
  • If you are confused about the "in-band" vs. "out-of-band" virtualization issue, you might find it useful to spend a half-hour or so with searchStorage's Storage Networking expert, Randy Kerns, who presented insights on these and other virtualization issues in our searchStorage Live Expert Q&A event held in July 2001, "Virtualization in a storage area network (SAN)."
  • Everybody's talking about virtualization. But, if you ask 20 vendors what it means, you may get 20 different answers. Especially when it comes to where it's located. The resources in this "Virtualization" featured topic should help.
  • There are too many separate postings on virtualization in our forums for us to point out just one or two. If you are interested in adding your comments to the threads, or starting your own thread, go to the .8MyMaP4PdMa^0@/searchstorage>main forums page and search for "virtualization." And, happy posting!

This was first published in January 2002

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