Blade servers were created a few years ago as a solution for the cabling and integration management issues of clustered systems. They're built using identical server modules mounted as plug-in "blades." Typically, a couple of HDDs are included in each blade.
A set of blades is mounted in a blade chassis, with redundant power supplies. Intra-cluster networking is provided by a built-in Ethernet switch. These configurations make for a compact, ready-to-use cluster.
The blade server concept began with the idea of having low-cost, small-form-factor servers in a cabinet. Configurations of 12 servers in a 3U cabinet looked like the sweet spot, and the aim was to cut space, power, and acquisition cost for ISPs servicing the LAMP market.
The concept of inexpensive small-blade servers got lost in the post-9/11 crash of the industry. But blades survived as larger modules with more memory. These units can't be described as inexpensive. Some were even made of anodized gold to add a bit of class (and price).
The blade server continues today with a reasonable market size. Increasingly, densely packed server designs have nibbled away at the market, and the advent of Google-class CSPs and containerized data centers has renewed the emphasis on smaller footprints and lower costs.
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