Hey! How many blades fit in that rack?

Considering the myriad configurations possible for a fully tricked out rack of blade servers, you'll have to take vendors' claims with a grain of salt.

If you're new to the lingo of blades, one of the terms you'll be hearing a lot over the coming years is "U." Vendors like IBM, Dell, Hewlett-Packard, and RLX--as well as networking vendors-- use "U" to describe the amount of space that their rack-mountable gear takes up in a standard 19-inch rack. In the context of blades, those vendors use the term "U" as a selling point to communicate blade density.

"U" stands for a "unit" of vertical rack space that's approximately 1.75 vertical inches. Although racks come in various heights, when vendors cite the maximum rack density for their blades, they usually cite the number of blades that can fit into a 42U-sized rack. As the measurement implies, such a rack, which typically stands 75 to 80 inches tall (depending on manufacturer), can hold 42 1U devices.

From there, you can calculate the number of devices that a 42U rack can hold based on the number of Us each device takes up. For example, the same rack than can hold 42 1Us can also hold 21 2U-sized devices or 84 ½U-sized devices.

The blades themselves, however, are not the devices that are mounted into the rack. The blades fit into enclosures that are designed in a way that allows them to make efficient use of sharable resources such as cables, networking, power, cooling, and storage. In fact, one of the selling propositions of these enclosures is the redundancy of those resources and how multiple systems gain economies of scale from sharing fault-tolerant measures.

Previously, single-enclosure systems like tower-based servers needed their own redundant power supplies, network connections, fans, and storage devices. But in that approach, applying redundancy to multiple systems turned out to be a costly form of insurance that most companies didn't need.

The more you need to pack into an enclosure, however, the bigger the enclosure needs to be and the less space that's left in that enclosure for the blades themselves. For example, a vendor may have a 6U-sized enclosure that holds a certain number of blades when redundant Gigabit Ethernet and FibreChannel interconnects are included, and more blades if the redundancy is not required. But avoiding those redundancies, especially if the enclosure contains blades that are supporting mission-critical applications, is probably not worth the risk. After all, that's one of the reasons to go with the blade approach in the first place.

Most vendors have a variety of enclosure sizes designed to help IT managers accommodate different scenarios. For example, HP has a 3U enclosure that can hold up to 20 single-processor blades that it claims are designed to power Web servers, firewalls, and gateways or routers. HP also uses a 3U enclosure to hold the redundant power supplies that the rest of the rack's devices rely on. HP has 6U enclosures that are designed to hold the company's mid-tier dual processor/dual drive (2P) blades as well as its forthcoming 4P blades. A 6U enclosure with redundant "end cap switches" (Ethernet switches mounted next to the left and right edges of the enclosure) can hold up to eight of the mid-tier 2P blades.

Considering the myriad configurations that you could come up with for a fully tricked out rack, you have to take some of the vendors' boasting with a grain of salt. For example, vendors will cite the maximum density of a 42U rack as a selling point (even if you would never go for a rack that was full of nothing but servers). For example, if a rack fits 14 3U enclosures and each of HP's 3U enclosure can hold 20 of the company's low end blades, a 42U rack's maximum density--in HP's scheme of things-- is 280 servers. Or, if you max out a 42U rack with seven of HP's 6U multiprocessor configurations (each of which holds eight blades), the maximum 2P density is 56. IBM, on the other hand, says it can fit 84 2P blades into one rack.

But maximum density is not a consideration for most shops and, even if it were, you need to take a closer look at what it means to trick out a 42U rack with nothing but server blades. For example, what about the power supplies? Where will they go? Or what if you decide to put something else in the rack that has nothing to do with your blade configuration, like a network switch that services some client systems?

As always, buyer beware. Read the fine print.

What's your ideal tricked out rack? Share your designs with your fellow readers below using TalkBack, or write to david.berlind@cnet.com.

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