Book Image

PostgreSQL 9 High Availability Cookbook

By : Shaun Thomas
Book Image

PostgreSQL 9 High Availability Cookbook

By: Shaun Thomas

Overview of this book

Table of Contents (17 chapters)
PostgreSQL 9 High Availability Cookbook
About the Author
About the Reviewers

Selecting a chassis

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 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.

Getting ready

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.

How to do it...

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:

    • Maximum 2.5" drives in a 2U server is 24

    • Maximum 3.5" drives in a 2U server is 8

    • Maximum 2.5" drives in a 1U server is 8

    • Maximum 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.

How it works...

This time, our job was much easier than considering motherboard constraints. This time, drives determine most of our decision.

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-swap drives across the front when stacked vertically. 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 can't boot.


Some cases reserve mounts inside, or at the rear, for operating-system drives. They are harder to replace, but make more room for storage dedicated to PostgreSQL. Here, operating system drives are treated as operating overhead without sacrificing case functionality.

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.

Regarding the dual power supplies, this is not negotiable. Many data centers provide two power rails per server rack. The intent is to provide 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 last, more optional element involves investigating the case itself. Many server cases have several fans inside and along the rear, and as a consequence, are very loud. 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 maximum operating temperatures of each case, so try to gravitate toward the cooler ones if all else is equal.

There's more...

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 viable parts, configurations, and failure conditions of the entire unit.

However, one 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 risk assessment suggests it's viable, give it a try. The advice we give is by no means set in stone.