CentOS Linux高可用性实战指南

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"《CentOS High Availability》是Mitja Resman撰写的一本关于在CentOS Linux上构建高可用性集群的指南。本书适用于系统工程师和系统管理员，要求读者具有良好的CentOS Linux基础知识和基本的网络经验。书中详细介绍了如何安装、配置和管理运行在CentOS 6和7上的多节点集群，以及如何管理和迁移集群资源，并提供了从故障切换到集群服务配置的实践指导。全书分为两个部分，分别针对CentOS 6和7，涵盖了Corosync集群消息传递软件的安装、CMAN或Pacemaker集群资源管理软件的配置，以及STONITH（隔离设备）和Fencing（节点隔离）的设置等内容。通过阅读本书，读者将具备独立设计、实施和维护CentOS高可用性多节点集群环境的能力。" 这本书的详细章节涵盖了高可用性的概念介绍，然后深入到集群栈及其组件的解析，特别是在CentOS 6和7上的实现。在每个版本的部分，读者将学习如何安装Corosync，以及选择CMAN或Pacemaker作为资源管理器。接下来，读者会接触到集群服务配置、节点管理、节点隔离技术和故障切换测试。对于两节点集群的特殊考虑也在书中有所阐述。例如，在CentOS 6中，读者将学习如何配置和管理资源，如何操作集群节点，以及如何设置Fencing来防止资源冲突。而在CentOS 7部分，读者将接触STONITH（Shoot The Other Node In The Head）技术，这是一种确保高可用性的方法，通过隔离故障节点来避免整个系统的中断。每个章节都包含逐步的实践指导，旨在帮助读者掌握实际操作技巧。《CentOS High Availability》是系统管理员提升在CentOS上构建高可用集群技能的宝贵资源，它提供了详尽的步骤和实例，帮助读者理解和应用集群技术，以确保关键服务的持续运行和稳定性。

[ 1 ]

Getting Started with High

Availability

We live in a fast-paced world and, with all the technology surrounding us, we take

it for granted most of the time. When we set the alarm clock in the evening before

falling asleep, we never give much thought whether the alarm will actually work in

the morning or not; and when we turn the ignition key to drive off to work, we never

stop to think whether the car will fail to start or not. On a normal day, there is hardly

any chance of anything like this happening to us, so we can calmly go to sleep in the

evening. The same applies to visiting our favorite website rst thing in the morning.

We are, in fact, more likely to expect that the car will not start or the bus will be late

than that we will not be able to log in to our Facebook or Gmail account. No wonder,

these sites are is always online and ready to serve information whenever we request it.

Have you ever asked yourself, "How can this be?" We all know we cannot trust

technology implicitly. Sooner or later, it can and it will fail. With such complex

systems and technologies surrounding us, we are actually not aware how many

systems are required to run awlessly so that we can read our e-mails and check our

Facebook walls. How did we become so sure that these complex systems will always

provide what we require?

The answer to the question is high availability. Highly available systems are what

made us all blind, in the belief that services are always there and on and failure is

not an option. As the title of this book suggests, the objective is to familiarize you

with how to achieve high availability, focusing on an actual, practical example of

a three-node cluster conguration on CentOS Linux version 6 and 7. A three-node

cluster is chosen because the number of cluster nodes plays a key role in the cluster

conguration process, which will be explained in more detail later in the book. You

will become familiar with two different software solutions available for achieving

high availability on CentOS Linux.

Getting Started with High Availability

[ 2 ]

In the rst chapter, you will learn about high availability in general. It will start by

laying the foundation and explaining what high availability is, also describing what

system design approaches must be followed to make an IT system highly available.

We will explain the meaning of computer clusters, why we need them, and the

possible computer cluster congurations.

The emphasis of this book is on the following topics:

• A practical, hands-on user guide

• Cluster software installation and configuration

• Cluster resource configuration and management

• Cluster node management

• Cluster failover testing

What is high availability?

The general meaning of the word "availability" is a characteristic of a resource—

either a person or an object that can be accessed or used. Resource availability can be

measured, and therefore, a ratio of the time a resource is accessible or usable to the

time the resource is inaccessible or unusable can be calculated. Adding an adjective

"high" to the word "availability" suggests that the resource should be accessible and

usable most of the time during a given time interval. The term "high availability" is

commonly used in information technology and it describes IT systems with a high

level of availability.

High availability in IT refers to a system that is continuously operational and

available for the delivery of services it provides for end users. The key point when

talking about IT systems is the availability to deliver services to end users, since a

system can be up-and-running from the IT administrator's perspective but can fail to

provide services for end users, which makes it useless. There are a number of factors

that can lead to service downtime, mainly because there are so many different layers

that must work together to provide service availability.

An IT system usually consists of many different components. All of these components

must be continuously available for a desirable length of time. It is needless to say that

it is very important for these highly available systems to be properly designed, well

thought through, and thoroughly tested with the goal of eliminating any possibility

of potential failure. That being said, high availability is a system design approach,

and a service implementation in a way, to provide the highest achievable level of

performance and availability by eliminating all system-wide single points of failure.

Chapter 1

[ 3 ]

Not every system can be marked highly available. It is common practice in IT to

measure and calculate the availability of a system. Monitoring tools such as Nagios,

Zenoss, or Zabbix can be used to provide reports on system availability and also

alerts in the case of system unavailability. The measurements taken must reect the

actual availability of the system to the end user. By measuring and calculating the

availability of a system, we can split them into systems that are classied as highly

available and systems that are not. System availability is commonly expressed as a

percentage of system uptime in a given year.

System design

IT systems that offer high availability of services must follow a specic system

design approach by which they can provide the most available continuous operation.

The fundamental rule of a high-availability system design approach is to avoid single

points of failure. A single point of failure is a component of a system that could lead

to system and service downtime if it fails. The design should avoid single points of

failure, which makes the system more robust and automatically increases system and

service availability.

A complex IT system providing application services can have a large number of

single points of failure at different levels, but how can we eliminate all of them?

The solution is redundancy. Redundancy means duplication of the system's critical

components. Duplication of devices allows continuous system operation even if

one of the duplicated devices fails. There are two types of redundancy: passive and

active. Passive redundancy means using two or more devices while only one of them

provides its service at certain point in time. The other devices wait to take over in the

case of an unrecoverable failure of the operating device. Active redundancy means

using two or more devices, all providing their service at all times. Even if one of the

devices fails, other devices are continuously providing the service.

Let me try to explain single points of failure and redundancy with a practical

example. Let's say you are hosting a simple website on your personal home

computer. The computer is located at your home, hidden in your storage closet. It

is happily providing a website service for end users. It is always on and users can

access the website any time of the day. If you and I were ignorant, we could say that

you are running a perfect solution with a perfect system design, especially since you

are saving a fair amount of money, not paying for expensive hosting solutions at

your local hosting service. But stop to think for a second and try to count the single

points of failure in the system's design. Running a website on a personal computer

that is not a dedicated server machine has a number of single points of failure to

begin with. Personal computers are not designed to run continuously, mostly due

to the fact that the hardware components of a personal computer are not duplicated

and the redundancy requirements for high availability are not met.

Getting Started with High Availability

[ 4 ]

If the hard drive on your personal computer fails, the system will crash and the

website will experience serious downtime. The same goes for the computer's power

supply. Unexpected failure of any of these components will bring the website down

for anything ranging from an hour to days. The period of the downtime depends on

the availability of the replacement component and the backup solution implemented.

Another major issue with the system design in the provided example is the Internet

Service Provider and the connection to the World Wide Web. Your personal computer

is relying only on a single source to provide its Internet service and connection. If

the Internet Service Provider, for some reason, suddenly experiences huge network

problems and your Internet service goes down, the website will also experience

serious downtime and you will be losing visitors—and possibly money—with every

minute the website is unreachable. Again, the same goes for the electricity supply. You

need to provide redundant components in every possible aspect, not just hardware.

Redundancy must be provided at all layers, including the networking layer, power

supply layer, and also the yet unmentioned application layer.

Nowadays the majority of modern server systems eliminate hardware single points

of failure by duplicating hardware components, but this solution still falls short

of eliminating single points of failure in applications, which is one of the main

reasons for using computer cluster implementation. Application-layer redundancy

is achieved with computer clusters. A computer cluster is a group of computers

running cluster software that enables continuous two-way communication, also

called a heartbeat, between cluster members. A heartbeat provides cluster members

with information on the exact status of any cluster member at any given time.

Practically, this means that any member of the cluster knows the exact number of

the members in the cluster it is joined to and also knows which cluster members are

active or online, in maintenance mode, ofine, and many more aspects.

Computer clusters

A computer cluster is a group of computers joined to work together in order to

provide high availability of some services. The services are usually built with a

number of applications operating in the so-called application layer. As shown in

the example from the previous section, single points of failure are spread across all

layers of a system design, and the application layer is one of the critical layers. It is

usual for an application to encounter an error, or bug, and stop responding or crash.

Such a situation will lead to service downtime and probably nancial loss as well,

so it is necessary to provide redundancy on the application layer also. This is the

reason we need to implement a computer cluster solution into our high-availability

system design.

Chapter 1

[ 5 ]

A computer cluster consists of two or more computers. The computers are connected

to the local area network. The maximum number of computers in a computer cluster

is limited by the cluster software solution implemented but, in general, common

cluster solutions support at least 16 cluster members. It is good practice for the

cluster members to have the same hardware and specications, which means that

the cluster computers consist of components from the same manufacturer and likely

have the same resource specications.

There are two common types of computer clusters:

• Load balancing computer clusters

• High-availability computer clusters

Load balancing computer clusters are used to provide better and higher performance

of services, and are typically used in science for complex scientic measurements

and calculations. Interestingly, the same clusters are also used for websites and web

servers facing extremely high load, which helps improve the overall response of the

website with load distribution to different cluster nodes.

High-availability computer clusters strive to minimize the downtime of the service

provided and not so much to improve the overall performance of the service. The

focus of this book is on high-availability clusters. There are many different cluster

congurations, mainly depending on the number of cluster members and also on the

level of availability you want to achieve.

Some of the different cluster congurations are as follows:

• Active/Active: The Active/Active cluster configuration can be used with

two or more cluster members. The service provided by the cluster is

simultaneously active on all cluster nodes at any given time. The traffic can be

passed to any of the existing cluster nodes if a suitable load balancing solution

has been implemented. If no load balancing solution has been implemented,

the Active/Active configuration can be used to reduce the time it takes to fail

over applications and services from the failed cluster node.

• Active/Passive: The Active/Passive cluster configuration can be used with

two or more cluster members. At a given time, the service is provided

only by the current master cluster node. If the master node fails, automatic

reconfiguration of the cluster is triggered and the traffic is switched to one of

the operational cluster nodes.

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