"Packaging" refers to the container into which a computer is installed. Packaging ranges from cases that sit vertically beside a desk to high-density rack-mounted units. As space is frequently a key design constraint, choice of packaging is instrumental in determining a maximum cluster size. The decision can affect both the type of storage a cluster will use and the overall cluster storage density.
Desktop cases are the most common example of packaging. Also referred to as pizza box cases, they are typically twelve to sixteen inches wide, four to six inches tall and twelve to sixteen inches deep. The earliest clusters were build out of these. Another common type of packaging are the tower cases that many consumer-level computers and built from. They typically stand one to three feet tall, six inches wide, and one to two feet deep. Because of the large size, cooling is usually not a serious problem with this type of machine. On the other hand, shape and design of desktops make rack mounting relatively difficult, and leads to a lower density than can be achieved with other designs. Laptops are occasionally used to construct low-profile clusters. These lead to small clusters, but are typically low-performance as well.
Rack-mounted cases are low-profile cases usually marketed to businesses for use in locations with large numbers of machines. These cases are designed to be mounted into a rack unit about six feet tall, and are almost universally nineteen inches wide. The machines are typically mounted on sliding rails, making service on an individual node a matter of sliding it out of the rack. These cases range in size from one to four rack units tall. One rack unit is 1.75 inches. Some manufacturers have even managed to fit complete machines with commodity parts into cases less than one rack unit in volume. Rack-mounted machines provide high machine density and good serviceability. However, because of the high density, care must be taken to provide adequate cooling.
The final option in node packaging is blade servers. These are machines that have been packed into cases as tightly as feasible. In many cases, common parts such as power supplies are shared between machines. This configuration provides extremely high machine density. The disadvantage is that blade server hardware is still somewhat specialized, and nodes, similarly, are not necessarily expandable.
Packaging is clearly an important decision point when choosing a cluster. Typically, this decision involves considering space constraints, along with cost and serviceability concerns. Generally, desktop machines are the least expensive, followed by rack mount machines, and then blade servers.