. Flexibility"> Workhorse vs. Flexibility Big companies and Internet Service Providers (ISPs) alike want the reliable consolidated servers that come with guarantees of no downtime for a companys operations, even when a piece of equipment fails.IBM recently gave its venerable System 390 mainframe a new look as the Z900, with improved Internet capabilities. HP in December launched its Superdome server with up to 64 processors. And for two years Sun has been making gains on the competition with its Starfire Enterprise 10000, which can be powered by as many as 64 processors.At some point, commodity servers may challenge these established lines, but for now, their redundancies and fail-safe features make these muscle-bound veterans the engines of the Internet for large enterprise. At the same time, however, Internet start-ups and network-based hosting services, such as application service providers (ASPs) or ISPs, are looking for small, light machines that can be added quickly and easily to accommodate growth spurts. And with PC sales slowing, Compaq and Dell have entered the so-called thin-server market with a vengeance but so have HP, IBM and Sun, not to mention a host of newcomers, including Advantech Technologies; Cobalt Networks, recently acquired by Sun; Network Appliance; RLX Technologies, a new company formed last week by a group of former Compaq executives; and VA Linux Systems. Intel may make its own foray into the thin-server space with its acquisition last August of Ziatech, a maker of next-generation ultracompact servers. Thin servers are, as their name suggests, small, light, flexible boxes based on off-the-shelf Intel processors. Their "thin-ness" is measured in increments of 1.75-inch heights denoted by the letter "u," for "unit". There are 42 units in the vertical computer racks that are quickly replacing the tower servers that used to make up the bulk of low-end sales. That means users can stack up to 42 1u servers or up to 21 2u servers in a standard rack. By buying these thin clients only as needed, at prices ranging from $2,600 to $5,000, users can cheaply add server power as needed to meet customer traffic. "The fastest-growing part of the server market is the 1u and 2u space," said Michael Lambert, senior vice president at Dells Enterprise Systems Group. That segment represented only 94,000 units in 1999. By 2004, it is expected to constitute more than half of servers sold some 701,000, according to figures from International Data Corp. Internet businesses, ASPs and other network-based services like thin servers because they make it easy to add computer power and they take up little floor space when stacked in a rack. Most service providers establish their computing resources at a colocation service provider, where they are charged by how much space they occupy. In return, they get broad bandwidth access to the Internet. The ability of companies such as Compaq and Dell to capture customers online and mass-produce thin servers to individual orders makes them potentially formidable competitors to the established server makers. "Enterprises and dot-coms alike are demanding new solutions from their server vendors," said Mark Melenovsky, research supervisor for Intel architecture servers at IDC. Through 2000, demand for thin servers doubled between the first and fourth quarters, from 15 percent to 30 percent, he said.