CentOS Linux高可用集群实战指南: 企业级开源操作系统的HA技术

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《Packt.CentOS High Availability.2015》是一本专为IT专业人士设计的技术指南，它深入探讨了如何在企业级开源操作系统CentOS Linux上构建和利用高可用性集群技术。这本书由Mitja Resman撰写，由Packt Publishing出版，版权所有，未经事先书面许可，禁止任何形式的复制、存储或传输，除非用于嵌入在评论或关键文章中。本书的核心内容聚焦于以下几个方面： 1. **CentOS简介**：作为企业级的开源操作系统，CentOS以其稳定性和安全性著称，本书将其作为实现高可用性集群的理想平台，展示了其在企业环境中的广泛应用价值。 2. **高可用性集群技术**：书中详细讲解了各种高可用性（HA）方法，包括负载均衡、故障转移、冗余服务和数据一致性策略，以确保系统的持续运行和业务连续性。 3. **安装与配置**：通过一步步的实践教程，读者可以学习如何在CentOS环境中安装和配置HA组件，如Heartbeat、Corosync、 Pacemaker和DRBD等，这些工具是实现高可用性集群的关键要素。 4. **故障检测与恢复**：作者介绍了如何设置监控系统，以便及时发现和处理潜在的故障，并且提供了应对不同故障场景的解决方案。 5. **案例研究与最佳实践**：书中包含真实世界案例，展示了实际项目中如何应用高可用性集群技术，以及如何避免常见问题并优化性能。 6. **法律声明**：尽管作者和出版社努力确保信息的准确性，但本书不承担任何责任，因为提供的信息是“按现状销售”，不提供任何形式的保修，也不保证商标信息的准确性。 7. **版权和许可**：版权所有，任何复制或传播必须获得书面许可，反映了出版行业的知识产权保护原则。《Packt.CentOS High Availability.2015》是一本适合希望提升CentOS集群管理技能，尤其是对高可用性有需求的企业IT管理员和系统架构师的实用参考书籍。随着云计算和数据中心的发展，掌握这种技术对于维护业务运行至关重要。然而，由于出版日期是2015年，书中可能部分信息已过时，读者在应用时还需结合最新的技术和安全标准进行调整。

[ 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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