With thin provisioning, more than 100% of storage capacity can be allocated to applications, but capacity remains available because it won't be consumed all at once.

You can play it safe by allocating small volumes that never exceed the physical storage, or allocate as much as you want to each application, then monitor your systems closely, says Themis Tokkaris, systems engineer at Truly Nolen Pest Control in Tucson, Ariz. It's best if you can find a happy balance between those two extremes.

"You have to monitor your pool so you don't run out of space, because that would really crash everything," Tokkaris says.

How server virtualization fits in

A common question is whether it makes sense to virtualize storage if you're not also using server virtualization. The short answer is yes -- though it's true you won't get as much flexibility as IT shops that virtualize both servers and storage.

"If you virtualize both, then you have the maximum flexibility when deploying new applications," says Chris Saul, IBM's storage-virtualization marketing manager.

Nevertheless, there are benefits to just virtualizing storage.

Improved disaster recovery, availability and data migrations can all be gained without having virtual servers, says product marketing manager Augie Gonzalez of storage virtualization vendor DataCore Software. In addition, storage virtualization by itself can provide thin provisioning, as well as the simplified management structure that comes with pooling storage devices and managing them from a central console.

On the flip side, virtualizing servers without virtualizing storage is problematic. It doesn't make sense to have multiple virtual servers on a physical machine that aren't able to share data, says Enterprise Strategy Group (ESG) analyst Mark Peters.

"You can gain tremendous benefits from storage virtualization, even without server virtualization. It's harder the other way around," Peters says. (Compare server products.)

Virtualization in a heterogeneous environment

Given that virtualization is designed to combine multiple storage devices, it's not immediately obvious why it makes sense to virtualize your storage if it all comes from a single vendor.

There are compelling reasons, however, says storage analyst Arun Taneja. "A lot of people think storage virtualization has a prerequisite of heterogeneity, that it only comes into play when storage from three companies is involved," he says. "I say, forget it, it has value even if you are stuck with a single vendor."

The storage market is more proprietary than just about any other IT space, and this creates problems even if you have just one storage vendor, Taneja says.

Say you're an EMC customer with two Symmetrix DMX boxes, and "you just want to combine the power of those two boxes and manage it as one," Taneja says. "[Without storage virtualization] you can't do it. That's how ridiculous the world of storage is."