BPC, Articles and Glossary

Rack Mounted Servers

Definition: A rack mount server is a computer built into a special case, instead of a standalone case like a desktop or tower case, that can be slotted into an industry standard 19-inch equipment rack.

The 19-inch rack format has been around for a long time, having originated as a rack for mounting railway signalling equipment in the USA. The four vertical posts at the front and back of the rack have mounting holes drilled every one-and-a-quarter inches, from top to bottom. This allows equipment of all sorts to be mounted one on top of each other, using less space than if they were all in individual cases.

For servers to use this rack format they need to be in cases that will fit the standard dimensions. The cases need to have mounting brackets at the front (sometimes at the back too) with holes drilled in the correct places to match up with the holes on the posts.

Rack Mount Servers in Business Environments

When talking about rack mount servers most people have a vision of a huge computer room with mile upon mile of computing equipment of all sorts. This of course is where most rack mount servers are to be found but small and medium-sized businesses can also make use of them. It's not only seven foot tall racks that use the rack mount format. There are many other smaller cabinets, floor mounted or wall mounted, that can take a couple of servers, a network switch, access point and a cabling patch panel as well, centralising all the computing equipment in one place.

Once a company begins to grow to the point where they have a few small servers dotted around the offices, with fans on permanently next to people trying to work and make phone calls, a rack mount cabinet can be just the right move. Racks of all shapes and sizes nearly always come with removable doors and side panels. A closed cabinet can be best for an office environment because as well as keeping the noise in, the computing and network equipment can be locked, securing it from tampering, either deliberate or accidental.

Server Configuration for Rack Mounting

The server components that go into rack mount cases are not a great deal different from those that would be found in a tower or desktop case. A server should be specified according to the work that it needs to do rather than the format of the case it is going to be put in. Many major computer suppliers provide servers in rack mount format or they can be built into a case for bespoke needs. The real difference when it comes to specifying components is that the space constraints of a rack mount case need to be taken into account.

Servers are available in many different heights and the lower the height the more computing you can fit into the available space. But this also means that the cost of each server goes up as you will need specialist components to fit in the available space. The standard unit of measurement for rack height is a rack unit, normally referred to as a 'U', which equates to just under 1.75 inches, the height that covers three rack post holes. So a 5U server, which used to be the default size, would be just under 8.75 inches, more than enough to accommodate a normal PC motherboard along with a complete collection of expansion slots and cards and a number of disk drives and optical drives. The measurements are always around 1/32^nd of an inch less than the full 'U' height to allow room to manoeuvre equipment in and out of the racks.

Dealing With Smaller Rack Heights

Increasingly servers have become smaller and now 1U  servers are commonplace. With 1U servers the motherboards are customised, having the expansion slots raised and pivoted ninety degrees so that expansion boards can run parallel to the motherboard. It is also not unusual for the voltages delivered around the boards to be lower than usual, around 3.3v rather than 5V. This is because lower voltages mean less power consumed and thus less heat being produced in a confined space. Alternatively servers have all the usual components mounted on the motherboard to make expansion cards unnecessary. Take care when ordering expansion boards and other accessories that they are for the correct voltage and will fit in the case. Many expansion boards come in half height formats to help them fit into smaller cases. Also the hard disk space will be in a separate case, accessed over the network.

The width and height of a rack mount server case are the critical dimensions; depth is to some extent less important. There are many rack depths available but servers are usually mounted on drawer-style rails so that they can be slid out for easy maintenance. This also means that only one person needs to be there. If a server is bolted directly to the rack posts at the back and the front then a second person is vital when removing or installing a server to avoid it being dropped.

Precautions when Moving to Rack Mount Servers

If you are considering moving to a rack mount server environment, perhaps to house a growing collection of equipment in a closed cabinet, there are three things that need to be considered as well as those we have already talked about. The first is to assess the power requirements and make sure that you have an adequate supply.

You should also consider having a backup power source and a backup of critical data to another location as your data and computing facilities are now all going to be in one place. Finally make sure that you have adequate cooling for all the equipment. Rack mounted fans are available to draw cooling air through a cabinet but air conditioning might be necessary.