A server's form factor is its physical shape and how it is housed. The same processor and memory can come in very different packages, and the right choice depends on how many servers you need, how much floor or rack space you have, and how you want power, cooling and networking to be shared. Server+ tests three form factors: tower, rack mount and blade.
A tower server looks like a large desktop computer and stands on the floor or a shelf. It needs no rack, is usually quiet, and often has plenty of internal drive bays and expansion slots. Towers suit small offices or branch sites that need one or two servers and have no server room. Their weaknesses are density and management at scale: ten towers take up a lot of floor space, each has its own power cords and cables, and they are awkward to secure. Some towers can be converted to rack mount with a conversion kit.
A rack mount server is a flat chassis that bolts into a standard 19-inch rack on rails. Sizes are given in rack units: 1U servers are dense but have room for only a few drives and low-profile expansion cards, while 2U and 4U servers trade density for more drive bays, full-height cards, larger fans and sometimes more processors. Each rack server is still a complete, independent machine with its own power supplies, fans and network ports, so a rack full of them needs many power and network cables.
A blade system has two parts. The blade enclosure (also called a chassis) mounts in the rack and provides shared power supplies, cooling fans, a management module and network or storage interconnect modules. Blade servers slide into slots in the front of the enclosure; each blade holds processors, memory and usually a small amount of local storage, but relies on the enclosure for power, cooling and connectivity. Blades give the highest density and far fewer cables, and a single management interface can control every blade. The trade-offs are higher up-front cost for the enclosure, vendor lock-in because blades only fit that vendor's chassis, and a shared component (such as the enclosure's backplane or midplane) that must be designed with redundancy so it is not a single point of failure.
When choosing, match the form factor to the situation. A single file server at a small office points to a tower. A growing data center that needs flexible, independent servers points to rack mount. A site that needs many identical compute nodes in minimal space with centralized management points to blades. Also consider power density: a fully loaded blade enclosure draws a lot of power and produces a lot of heat in a small area, so the rack's circuits and cooling must be sized for it.
Key terms
- Tower server
- A free-standing server in an upright case, suited to small sites without a rack.
- Rack mount server
- A server built to bolt into a 19-inch rack, sized in rack units (1U, 2U, 4U).
- Blade enclosure
- A rack-mounted chassis that supplies shared power, cooling, networking and management to the blade servers inserted into it.
- Blade server
- A thin server module containing CPU and memory that depends on its enclosure for power, cooling and connectivity.
A clinic with one small closet and no rack buys a tower server for file sharing. The hospital's data center, which runs hundreds of virtual machines, uses blade enclosures so that sixteen compute nodes share redundant power supplies and a single management module in a fraction of the rack space.
Check yourself
Which form factor shares power supplies and fans among many servers?
Blade servers, which draw power, cooling and connectivity from the blade enclosure.
Why might you choose a 2U rack server over a 1U server?
A 2U chassis has room for more drives, full-height expansion cards and larger, quieter fans, at the cost of rack density.