To round out our hardware selection phase, it's time to decide just what kind of case to order from our server vendor. This is the final protective element that hosts the motherboard, drives, and power supplies necessary to keep everything running. And like always, we place a heavy emphasis on redundancy.

For the purposes of this section, we will concentrate primarily on 1U and 2U rack-mounted servers. Why not 4U or larger? Our goal is to obtain at least two of everything, with similar or matching specifications in every possible scenario. The idea is to scale horizontally in order to more easily replace a failed component or server. As the size of the chassis increases, its cost, complexity, and resource consumption also rise. In this delicate balancing act, it's safer to err toward two smaller systems with respectable capabilities, than one giant server that's twice as powerful.

This recipe will provide a quick set of calculations we can perform to estimate how many server chassis we may need for a PostgreSQL cluster.

Since the server chassis and motherboard are generally a package deal, it's a good idea to refer to the Managing motherboards recipe. We will be using a very similar process to choose a server case. This time, we will focus on adequate room for hard drives and redundant power supplies.

Now it's time to do some more research. Follow these steps:

1. Refer to the final list of servers from our motherboard selection.
2. For our ideal count of active (not replacement) hard drives, remove any choice that doesn't have enough drive slots. Use this list if it's not immediately obvious:

• The maximum number of 2.5" drives in a 2U server is 24.
• The maximum number of 3.5" drives in a 2U server is 12.
• The maximum number of 2.5" drives in a 1U server is 8.
• The maximum number of 3.5" drives in a 1U server is 4.

3. Remove from consideration any chassis that does not support dual power supplies. This should happen rarely in server-class systems.
4. As the list dwindles, give higher priority to cases with more fans or lower average operating temperatures.

This time, our job was much easier than considering motherboard constraints, as the drives themselves determine most of our decisions.

Hot-swappable hard drives are slightly larger than their standard brethren due to the swap enclosure. Yet cases exist than can hold up to 24 hot-swappable drives across the front. If we need that many storage devices, we save space by taking advantage of cases that can accommodate them. We also need to remember to reserve two drives for the operating system in a RAID-1, separate from our PostgreSQL storage. We can't diagnose problems on a server that is unable to boot.

If we need more drives than are available in any configuration, we should consider Direct Attached Storage (DAS), Network Attached Storage (NAS), or Storage Area Network (SAN). Some vendors supply drive extension cages specifically to provide more hot-swap bays for specific server models. While we want to conserve space when possible, these are relatively inexpensive and much smaller than a NAS or SAN if we haven't progressed to requiring such a device.

The dual power-supply requirement is not negotiable. Many data centers provide two power rails per server rack. The intent is to deliver two separate sources of power to the server in case the server's power supply fails, or power is cut to one of the sources. Sometimes, these power sources even have separate generators. We're not the only ones interested in redundancy; data centers want to avoid outages too.

The final, more optional element involves investigating the case itself. Many server cases have several fans inside and along the rear, and are very loud as a consequence. This won't matter when the server is in the data center, but the number of fans and the shape of the airflow will directly affect the server temperature. Higher temperatures decrease system stability. It's not uncommon for vendors to list the maximum operating temperatures of each case, so try to gravitate toward the cooler ones if all else is equal.

We use the word vendor frequently, and there's a reason for that. Short of outright accusing bare cases and motherboards of being faulty, they are simply not stable enough for our use. There are some great cases available that, in many ways, exceed the capabilities provided by established server providers.

We don't suggest the smaller vendors for a few reasons. Larger companies often have replacement policies for each server component, including the case and motherboard. Building a system ourselves may provide more satisfaction, but vendors presumably spend time testing for compatibility and failure conditions. They produce manuals hundreds of pages long detailing the viable parts, configurations, and failure conditions of the entire unit.

However, we could just as easily argue that redundant servers increase failure tolerance, as there's always an available backup. Bare cases and motherboards are usually cheaper, and user-serviceable besides. That is a completely valid path, and if your risk assessment suggests it's viable, try it out. Our particular recommendations and biases should not limit your choices.