Facing strong concerns about control and security, the cloud-computing trend has drifted somewhat\u2014 away from the notion that all computing resources can be had from outside, and toward a vision of a data center magically transformed for easy connections to internal and external IT resources.\nSales of cloud-related technology are growing at 26 percent per year\u2014six times the rate of IT spending overall, though they made up only about 5 percent of total IT revenue this year, according to IDC's Cloud Services Overview report. Defining what constitutes cloud-related spending is difficult, the report acknowledges, though it estimates global spending of $17.5 billion on cloud technologies in 2009 will grow to $44.2 billion by 2013.\nHybrid or internal clouds will be the rule, however; even in 2013, only about 10 percent of that spending will go specifically to public clouds, IDC predicts.\n[For expert advice on proving the value of your cloud plan to the business, see CIO.com's recent article 8 Ways to Measure Cloud ROI. ]\nHybrid cloud infrastructure isn't radically different from existing data-center best practices, except that all the pieces are supposed to fit neatly together using Internet-age interoperability standards rather than homegrown kludge, according to according to Chris Wolf, analyst at The Burton Group.\nAs you prepare spending plans that line up with a move to the cloud, consider these four items as key for your list.\n1. Application Integration\nSurprise: Software integration isn't the first thing most companies think about when building a cloud, but it's the most important one, according to Bernard Golden, CEO at cloud consulting firm HyperStratus, and CIO.com blogger.\nIntegration means more than just batch-processing chunks of data being traded between applications once or twice per day the way that was done on mainframes, according to Tom Fisher, vice president of cloud computing at SuccessFactors.com, a business-application SaaS provider in San Mateo, Calif.\nBeing able to provision and manage user identities from a single location across a range of applications is critical, especially for companies that have never been in the software-providing business before and don't view their IT as a primary product, he says.\n"What you're looking for is take your schema and map it to PeopleSoft or another application so you can get more functional integration," Fisher says. "You're passing messages back and forth to each other with proper error-handling agreement so you can be more responsive. It's still not real time integration, but in most cases you don't really need that."\n2. Security\nThe second critical factor in building a useful cloud is the ability to federate\u2014securely connect without completely merging\u2014two networks, Golden says.\nThat requires layers of security, including multifactor authentication, identity brokers, access management and, in some cases, an external service provider who can provide that high a level of administrative control, according to Nico Popp, VP of product development at Verisign, which is considering adding a cloud-based cloud security service.\nWhat it really requires is technology that doesn't yet exist, according to Wolf: an Information Authority that can act as a central repository for security data and control of applications, data and platforms within the cloud. Today, it's possible to assemble that function out of some of the pieces Popp mentions, but there is no single technology able to span all the platforms necessary to provide real control of even an internally housed cloud environment, Wolf says.\n3. Virtual I\/O\nHaving to squeeze data for a dozen VMs through a couple of NICs will keep you from scaling your VM cluster to cloud proportions, according to an IT manager at a large digital mapping firm.\n"When you're in the dev\/test stage, having eight or 10 [Gigabit Ethernet] cables per box is an incredible labeling issue; beyond that, forget it," says the IT manager. "Moving to virtual I\/O is a concept shift\u2014you can't touch most of the connections anymore\u2014but you're moving stuff across a high-bandwidth backplane and you can reconfigure the SAN connections or the LANs without having to change cables."\nVirtual I\/O servers, such as the Xsigo I\/O Director servers that his company uses, can run 20Gbit\/sec through a single cord and as many as 64 cords to a single server, connecting to a backplane with a total of 1,560Gbit\/sec of bandwidth.\nConcentrating so much bandwidth in one device saves space, power and cabling, he says, keeps network performance high and ultimately saves money on network gear.\n"It becomes cost effective pretty quickly," he says of the Xsigo servers, which start around $28,000 through resellers such as Dell. "You end up getting three, four times the bandwidth at a quarter the price."\n4. Storage\nWe mentioned it before, but storage continues to be the weak point\u2014the hole into which one pours money\u2014of both the virtualization and cloud-building world.\n"Storage is going to continue to be one of the big costs of virtualization," Golden says. "Even if you turn 90 percent of your servers into images, you still have to store them somewhere."\nFollow everything from CIO.com on Twitter @CIOonline.