It's not a matter of if you're going to virtualize your way out of server sprawl, but when. Thing is, if you don't bring your storage infrastructure up to par in tandem, it's akin to building a server superhighway that bottoms out on a dirt road.

Specifying a storage area network as part of your server virtualization program is more than just a complementary add-on. It brings critical performance options that don't exist with other storage strategies, as well as opening the door to new space allocation and disaster-recovery benefits built into virtualization platforms from Citrix, Virtual Iron, VMware, and others.

This may not be an easy sell--the server group is typically a separate entity from the storage and disaster-recovery teams, so it's easy to see how a company's virtualization and SAN initiatives can branch off in separate directions. And, in lean economic times, getting both new technologies funded is a challenge; we help with an ROI analysis (see story, "ROI Analysis: Virtualization"). But this is one political and budget battle worth fighting.

NEED MY SPACE
Virtualization is a great way to increase hardware utilization, in terms of both CPU and memory. However, the same can't be said for virtualization's effect on conventional storage systems. For example, VMware and other virtual machine players demand large initial space allocations on the storage array per virtual server, oftentimes more than for a physical server, to provide room to write the server state to the storage disk if the physical server on which the VM resides were to have problems. This tendency toward overallocation makes for both low storage utilization rates and higher costs, drastically reducing the benefits of a virtualized server deployment.

The answer is intelligent, or thin, provisioning, embraced by SAN manufacturers including EqualLogic, Hitachi, NetApp, and Xiotech, which lets IT present a logical volume of any size to the virtual server, while committing capacity only when data is written. Say your database team swears it needs 500 GB for an application. You can provision the full 500 GB, but only actual usage will be committed, saving critical disk space. This even gives you the ability to overcommit beyond current capacity.

What happens when your overcommits catch up? You shouldn't get caught unaware because SAN manufacturers provide management software that sets thresholds, raises alarms, and can even automatically reprovision space as real utilization grows. However, it's not like overbooking a flight, then sending some folks home if too many show up--if your storage grows to the full capacity allocated, you'll have to settle up on your promises. That may mean adding more storage to that SAN or grouping in another storage array.