Hewlett-Packard became the latest vendor to announce a "mini-data center" housed in a shipping container, which can provide a way for companies to add compute capacity when power and cooling systems in their existing data centers are maxed out.

HP's Performance Optimized Data Center, or POD, will be available in the U.S. by the end of the third quarter and worldwide a few months after that, the company said Wednesday. HP joins Sun Microsystems, Rackable Systems and IBM, among others, who sell similar products.

It sounds like a gimmick, but proponents say the portable data centers can solve real problems. They are customized 20-foot or 40-foot shipping containers that vendors fill with servers and storage gear before shipping them out. Customers plug in a cooling supply, power supply and a network connection, and the mini-data centers are ready to use.

The containers provide a way for resource-constrained facilities to add compute power without having to build a new data center, which is expensive and takes a year or more. They can also be used for disaster recovery, by setting one up on the grounds of a satellite office, for example.

And powerful rack-mount servers, which generate a lot of heat, can be packed more densely in a container because the temperature can be managed more closely in the closed environment.

The HP POD will accommodate 1,800 watts per square foot, compared to about 250 watts per square foot in a normal data center, said Steve Cumings, director of infrastructure with HP's Scalable Computing and Infrastructure group.

HP's 40-foot POD will contain 22 50u server racks and be able to house up to 1,100 1u servers or 12,000 large form-factor hard drives, for a total 12 petabytes of storage, Cumings said. HP will be able to ship the products to customers six weeks after they are ordered, he said.

Sales this year will be "very low," he acknowledged, but HP expects demand to increase next year. "These are a great solution for some things, but they are a complement to traditional data centers. It's not that we expect everyone to suddenly flip over to using containers," Cumings said.

Customers will be able to put other vendors' equipment in the POD, he said, and HP will install and configure the third-party gear alongside its own. An HP subsidiary, EYP Mission Critical Facilities, will provide design and planning services for customers and the PODs will be built to order.

HP hasn't announced pricing, which will vary a lot depending on the payload. Container products from other vendors start from a few hundred thousand dollars and can run into the millions.

The container concept is still new and critics see potential flaws. Some worry about security, although vendors say the boxes are hard to break into and can be housed on private lots. Others worry about the reliability of having a single power or network connection for such a dense load of equipment. There are also mundane issues, like not being able to open a container to service it in heavy rain, unless it's covered up.

Vendors are still figuring out the best way to design the products, too. Sun's Modular Data Center, for example, has server racks along both sides of the container and a narrow aisle down the middle, and is accessed by a door at each end. HP chose to put racks on one side of the container only, with a sliding door behind them to provide access at the back as well as the front. A narrow space behind the racks allows HP to mimic the "hot aisle, cold aisle" configuration in normal data centers, to minimize the retraining required for IT staff, Cumings said.

Microsoft is a big fan: It has said it plans to install more than 200 compact data centers on the ground floor of a new facility in Chicago. It hasn't said yet which vendor will provide them.

Sun's early customers include Hansen Transmissions, a Belgian industrial manufacturer, and Mobile TeleSystems, the Russian mobile operator. The Stanford Linear Accelerator Center in California bought two and has posted a white paper and time-lapse videos showing delivery of the first.