The EIA 310-D standard rack is 19 inches wide for servers, This has been in use for many decades as data centers galore can then use commodity machines for various purposes.
EIA racks units are 1¾ inches. So generally servers are 1U, 2U or 4U however some blade boxes may be as large as 10U. The rack itself is usually 42U tall which affords room for many machines with a network box on top to wire up the machines. The 4 posts are drilled at 1U intervals so that servers can be bolted to the frame. 42U racks are just about 6 feet tall. Servers front bezels are 1/32″ less to allow space for several servers to be fit into the rack.
Server racks generally are able to cope with 29 inch deep servers. The best racks have 4 posts which make them much more sturdy and much easier to use Rails can be installed to make it easier to remove servers for repair, Storage servers may have 2 or more rows of disks inside so rails make it possible to swap disks.
- 1U server with up to 4 processors
- 2U server usually with 2½” SSD drives mounted on the front
- 4U server with 16 front 3½” hard disks
- 4U Server with 60 internal hard disks
Over the years IBM and others built blade servers galore. IBM offered many CPU choices for every legacy machine they had. Many others did the same. More recently workloads for web servers have been so extreme that several blade servers have been run in parallel.
The photo is the HP c7000 blade server which can handle a very high workload. These servers are expensive but they can deliver some substantial performance numbers when needed.