Q: What are the main uses of storage virtualization?
A: One of the key things it does obviously is volume management. This function used to be provided by the arrays themselves, but with storage virtualization you can perform it in a centralized way across multiple distinct physical arrays. For example, you can combine disk capacity from several arrays into a single virtual volume. You can do replication from one array to another for disaster recovery or off-line backup. You can migrate data from older arrays that are being retired to newly purchased equipment without interrupting data access for your applications. Virtualization also makes it easier to use whatever type of storage hardware makes sense for your application. If youre doing testing and development with throw-away data, you can store snapshots or copies of production data on cheap SATA (serial ATA) disks rather than expensive Fiber Channel gear.

Q: Does storage virtualization really reduce administrative overhead the way the vendors claim it does?
A: No, not necessarily. What it does is move the work from one place, the individual arrays, to another place, the centralized console of the virtualization product. But administrators still have to perform the same day-to-day chores such as volume management, data replication and migration. However, virtualization can certainly make your life much easier if youre running a mixed-vendor storage environment. In that case the only thing you have to do on each vendors specific console is initial configuration, then after that you do everything from the centralized console.

Q: Whats the difference between in-band and out-of-band virtualization?
A: In-band means the control functionality you have taken out of the array still sits on the data path out in the network. Out-of-band means that control function sits somewhere outside the data path. But really all the virtualization solutions have to be in-band to some degree, since they involve moving data around. It makes more sense to distinguish between appliance-based and switch-based approaches.