Red Hat ENTERPRISE LINUX 5 - VIRTUAL SERVER... Installation guide

Red Hat Enterprise Linux 5
Cluster Suite Overview
Red Hat Cluster Suite for Red Hat Enterprise Linux 5

Cluster Suite Overview
Red Hat Enterprise Linux 5 Cluster Suite Overview
Red Hat Cluster Suite for Red Hat Enterprise Linux 5
Edition 4
Copyright © 2009 Red Hat, Inc.
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Red Hat Cluster Suite Overviewprovides an overview of Red Hat Cluster Suite for Red Hat Enterprise
Linux 5.

iii
Introduction v
1. Document Conventions ................................................................................................... vi
1.1. Typographic Conventions ..................................................................................... vi
1.2. Pull-quote Conventions ........................................................................................ vii
1.3. Notes and Warnings ........................................................................................... viii
2. Feedback ..................................................................................................................... viii
1. Red Hat Cluster Suite Overview 1
1.1. Cluster Basics .............................................................................................................. 1
1.2. Red Hat Cluster Suite Introduction ................................................................................ 2
1.3. Cluster Infrastructure .................................................................................................... 3
1.3.1. Cluster Management .......................................................................................... 4
1.3.2. Lock Management ............................................................................................. 5
1.3.3. Fencing ............................................................................................................. 5
1.3.4. Cluster Configuration System ............................................................................. 9
1.4. High-availability Service Management .......................................................................... 11
1.5. Red Hat GFS ............................................................................................................. 14
1.6. Cluster Logical Volume Manager ................................................................................. 15
1.7. Global Network Block Device ...................................................................................... 19
1.8. Linux Virtual Server .................................................................................................... 20
1.8.1. Two-Tier LVS Topology ................................................................................... 22
1.8.2. Three-Tier LVS Topology ................................................................................. 24
1.8.3. Routing Methods ............................................................................................. 25
1.8.4. Persistence and Firewall Marks ........................................................................ 28
1.9. Cluster Administration Tools ........................................................................................ 29
1.9.1. Conga ............................................................................................................. 29
1.9.2. Cluster Administration GUI ............................................................................... 32
1.9.3. Command Line Administration Tools ................................................................. 35
1.10. Linux Virtual Server Administration GUI ..................................................................... 36
1.10.1. CONTROL/MONITORING .............................................................................. 37
1.10.2. GLOBAL SETTINGS ..................................................................................... 38
1.10.3. REDUNDANCY ............................................................................................. 40
1.10.4. VIRTUAL SERVERS ...................................................................................... 41
2. Red Hat Cluster Suite Component Summary 49
2.1. Cluster Components ................................................................................................... 49
2.2. Man Pages ................................................................................................................ 53
2.3. Compatible Hardware ................................................................................................. 56
A. Revision History 57
Index 59

iv

v
Introduction
This document provides a high-level overview of Red Hat Cluster Suite for Red Hat Enterprise Linux 5
and is is organized as follows:
•Chapter 1, Red Hat Cluster Suite Overview
•Chapter 2, Red Hat Cluster Suite Component Summary
Although the information in this document is an overview, you should have advanced working
knowledge of Red Hat Enterprise Linux and understand the concepts of server computing to gain a
good comprehension of the information.
For more information about using Red Hat Enterprise Linux, refer to the following resources:
•Red Hat Enterprise Linux Installation Guide — Provides information regarding installation of Red
Hat Enterprise Linux 5.
•Red Hat Enterprise Linux Deployment Guide — Provides information regarding the deployment,
configuration and administration of Red Hat Enterprise Linux 5.
For more information about Red Hat Cluster Suite for Red Hat Enterprise Linux 5, refer to the following
resources:
•Configuring and Managing a Red Hat Cluster — Provides information about installing, configuring
and managing Red Hat Cluster components.
•Logical Volume Manager Administration — Provides a description of the Logical Volume Manager
(LVM), including information on running LVM in a clustered environment.
•Global File System: Configuration and Administration — Provides information about installing,
configuring, and maintaining Red Hat GFS (Red Hat Global File System).
•Global File System 2: Configuration and Administration — Provides information about installing,
configuring, and maintaining Red Hat GFS2 (Red Hat Global File System 2).
•Using Device-Mapper Multipath — Provides information about using the Device-Mapper Multipath
feature of Red Hat Enterprise Linux 5.
•Using GNBD with Global File System — Provides an overview on using Global Network Block
Device (GNBD) with Red Hat GFS.
•Linux Virtual Server Administration — Provides information on configuring high-performance
systems and services with the Linux Virtual Server (LVS).
•Red Hat Cluster Suite Release Notes — Provides information about the current release of Red Hat
Cluster Suite.
Red Hat Cluster Suite documentation and other Red Hat documents are available in HTML,
PDF, and RPM versions on the Red Hat Enterprise Linux Documentation CD and online at http://
www.redhat.com/docs/.

Introduction
vi
1. Document Conventions
This manual uses several conventions to highlight certain words and phrases and draw attention to
specific pieces of information.
In PDF and paper editions, this manual uses typefaces drawn from the Liberation Fonts1 set. The
Liberation Fonts set is also used in HTML editions if the set is installed on your system. If not,
alternative but equivalent typefaces are displayed. Note: Red Hat Enterprise Linux 5 and later includes
the Liberation Fonts set by default.
1.1. Typographic Conventions
Four typographic conventions are used to call attention to specific words and phrases. These
conventions, and the circumstances they apply to, are as follows.
Mono-spaced Bold
Used to highlight system input, including shell commands, file names and paths. Also used to highlight
keycaps and key combinations. For example:
To see the contents of the file my_next_bestselling_novel in your current
working directory, enter the cat my_next_bestselling_novel command at the
shell prompt and press Enter to execute the command.
The above includes a file name, a shell command and a keycap, all presented in mono-spaced bold
and all distinguishable thanks to context.
Key combinations can be distinguished from keycaps by the hyphen connecting each part of a key
combination. For example:
Press Enter to execute the command.
Press Ctrl+Alt+F1 to switch to the first virtual terminal. Press Ctrl+Alt+F7 to
return to your X-Windows session.
The first paragraph highlights the particular keycap to press. The second highlights two key
combinations (each a set of three keycaps with each set pressed simultaneously).
If source code is discussed, class names, methods, functions, variable names and returned values
mentioned within a paragraph will be presented as above, in mono-spaced bold. For example:
File-related classes include filesystem for file systems, file for files, and dir for
directories. Each class has its own associated set of permissions.
Proportional Bold
This denotes words or phrases encountered on a system, including application names; dialog box text;
labeled buttons; check-box and radio button labels; menu titles and sub-menu titles. For example:
Choose System → Preferences → Mouse from the main menu bar to launch Mouse
Preferences. In the Buttons tab, click the Left-handed mouse check box and click
Close to switch the primary mouse button from the left to the right (making the mouse
suitable for use in the left hand).
1 https://fedorahosted.org/liberation-fonts/

Pull-quote Conventions
vii
To insert a special character into a gedit file, choose Applications → Accessories
→ Character Map from the main menu bar. Next, choose Search → Find… from
the Character Map menu bar, type the name of the character in the Search field
and click Next. The character you sought will be highlighted in the Character Table.
Double-click this highlighted character to place it in the Text to copy field and then
click the Copy button. Now switch back to your document and choose Edit → Paste
from the gedit menu bar.
The above text includes application names; system-wide menu names and items; application-specific
menu names; and buttons and text found within a GUI interface, all presented in proportional bold and
all distinguishable by context.
Mono-spaced Bold Italic or Proportional Bold Italic
Whether mono-spaced bold or proportional bold, the addition of italics indicates replaceable or
variable text. Italics denotes text you do not input literally or displayed text that changes depending on
circumstance. For example:
To connect to a remote machine using ssh, type ssh [e-mail] at
a shell prompt. If the remote machine is example.com and your username on that
machine is john, type ssh [email protected].
The mount -o remount file-system command remounts the named file
system. For example, to remount the /home file system, the command is mount -o
remount /home.
To see the version of a currently installed package, use the rpm -q package
command. It will return a result as follows: package-version-release.
Note the words in bold italics above — username, domain.name, file-system, package, version and
release. Each word is a placeholder, either for text you enter when issuing a command or for text
displayed by the system.
Aside from standard usage for presenting the title of a work, italics denotes the first use of a new and
important term. For example:
Publican is a DocBook publishing system.
1.2. Pull-quote Conventions
Terminal output and source code listings are set off visually from the surrounding text.
Output sent to a terminal is set in mono-spaced roman and presented thus:
books Desktop documentation drafts mss photos stuff svn
books_tests Desktop1 downloads images notes scripts svgs
Source-code listings are also set in mono-spaced roman but add syntax highlighting as follows:
package org.jboss.book.jca.ex1;
import javax.naming.InitialContext;
public class ExClient
{

Introduction
viii
public static void main(String args[])
throws Exception
{
InitialContext iniCtx = new InitialContext();
Object ref = iniCtx.lookup("EchoBean");
EchoHome home = (EchoHome) ref;
Echo echo = home.create();
System.out.println("Created Echo");
System.out.println("Echo.echo('Hello') = " + echo.echo("Hello"));
}
}
1.3. Notes and Warnings
Finally, we use three visual styles to draw attention to information that might otherwise be overlooked.
Note
Notes are tips, shortcuts or alternative approaches to the task at hand. Ignoring a note
should have no negative consequences, but you might miss out on a trick that makes your
life easier.
Important
Important boxes detail things that are easily missed: configuration changes that only
apply to the current session, or services that need restarting before an update will apply.
Ignoring a box labeled 'Important' won't cause data loss but may cause irritation and
frustration.
Warning
Warnings should not be ignored. Ignoring warnings will most likely cause data loss.
2. Feedback
If you spot a typo, or if you have thought of a way to make this document better, we would love to
hear from you. Please submit a report in Bugzilla (http://bugzilla.redhat.com/bugzilla/) against the
component Documentation-cluster.
Be sure to mention the document's identifier:
Cluster_Suite_Overview(EN)-5 (2010-02-05T09:51)
By mentioning this document's identifier, we know exactly which version of the guide you have.
If you have a suggestion for improving the documentation, try to be as specific as possible. If you have
found an error, please include the section number and some of the surrounding text so we can find it
easily.

Chapter 1.
1
Red Hat Cluster Suite Overview
Clustered systems provide reliability, scalability, and availability to critical production services. Using
Red Hat Cluster Suite, you can create a cluster to suit your needs for performance, high availability,
load balancing, scalability, file sharing, and economy. This chapter provides an overview of Red Hat
Cluster Suite components and functions, and consists of the following sections:
•Section 1.1, “Cluster Basics”
•Section 1.2, “Red Hat Cluster Suite Introduction”
•Section 1.3, “Cluster Infrastructure”
•Section 1.4, “High-availability Service Management”
•Section 1.5, “Red Hat GFS”
•Section 1.6, “Cluster Logical Volume Manager”
•Section 1.7, “Global Network Block Device”
•Section 1.8, “Linux Virtual Server”
•Section 1.9, “Cluster Administration Tools”
•Section 1.10, “Linux Virtual Server Administration GUI”
1.1. Cluster Basics
A cluster is two or more computers (called nodes or members) that work together to perform a task.
There are four major types of clusters:
• Storage
• High availability
• Load balancing
• High performance
Storage clusters provide a consistent file system image across servers in a cluster, allowing the
servers to simultaneously read and write to a single shared file system. A storage cluster simplifies
storage administration by limiting the installation and patching of applications to one file system.
Also, with a cluster-wide file system, a storage cluster eliminates the need for redundant copies of
application data and simplifies backup and disaster recovery. Red Hat Cluster Suite provides storage
clustering through Red Hat GFS.
High-availability clusters provide continuous availability of services by eliminating single points
of failure and by failing over services from one cluster node to another in case a node becomes
inoperative. Typically, services in a high-availability cluster read and write data (via read-write
mounted file systems). Therefore, a high-availability cluster must maintain data integrity as one cluster
node takes over control of a service from another cluster node. Node failures in a high-availability
cluster are not visible from clients outside the cluster. (High-availability clusters are sometimes
referred to as failover clusters.) Red Hat Cluster Suite provides high-availability clustering through its
High-availability Service Management component.

Chapter 1. Red Hat Cluster Suite Overview
2
Load-balancing clusters dispatch network service requests to multiple cluster nodes to balance the
request load among the cluster nodes. Load balancing provides cost-effective scalability because you
can match the number of nodes according to load requirements. If a node in a load-balancing cluster
becomes inoperative, the load-balancing software detects the failure and redirects requests to other
cluster nodes. Node failures in a load-balancing cluster are not visible from clients outside the cluster.
Red Hat Cluster Suite provides load-balancing through LVS (Linux Virtual Server).
High-performance clusters use cluster nodes to perform concurrent calculations. A high-performance
cluster allows applications to work in parallel, therefore enhancing the performance of the applications.
(High performance clusters are also referred to as computational clusters or grid computing.)
Note
The cluster types summarized in the preceding text reflect basic configurations; your
needs might require a combination of the clusters described.
1.2. Red Hat Cluster Suite Introduction
Red Hat Cluster Suite (RHCS) is an integrated set of software components that can be deployed
in a variety of configurations to suit your needs for performance, high-availability, load balancing,
scalability, file sharing, and economy.
RHCS consists of the following major components (refer to Figure 1.1, “Red Hat Cluster Suite
Introduction”):
• Cluster infrastructure — Provides fundamental functions for nodes to work together as a cluster:
configuration-file management, membership management, lock management, and fencing.
• High-availability Service Management — Provides failover of services from one cluster node to
another in case a node becomes inoperative.
• Cluster administration tools — Configuration and management tools for setting up, configuring, and
managing a Red Hat cluster. The tools are for use with the Cluster Infrastructure components, the
High-availability and Service Management components, and storage.
• Linux Virtual Server (LVS) — Routing software that provides IP-Load-balancing. LVS runs in a pair
of redundant servers that distributes client requests evenly to real servers that are behind the LVS
servers.
You can supplement Red Hat Cluster Suite with the following components, which are part of an
optional package (and not part of Red Hat Cluster Suite):
• GFS (Global File System) and GFS2 (Global File System 2) — Provides a cluster file system for use
with Red Hat Cluster Suite. GFS/GFS2 allows multiple nodes to share storage at a block level as if
the storage were connected locally to each cluster node.
• Cluster Logical Volume Manager (CLVM) — Provides volume management of cluster storage.

Cluster Infrastructure
3
Note
When you create or modify a CLVM volume for a clustered environment, you must
ensure that you are running the clvmd daemon. For further information, refer to
Section 1.6, “Cluster Logical Volume Manager”.
• Global Network Block Device (GNBD) — An ancillary component of GFS/GFS2 that exports block-
level storage to Ethernet. This is an economical way to make block-level storage available to Red
Hat GFS.
For a lower level summary of Red Hat Cluster Suite components and optional software, refer to
Chapter 2, Red Hat Cluster Suite Component Summary.
Figure 1.1. Red Hat Cluster Suite Introduction
Note
Figure 1.1, “Red Hat Cluster Suite Introduction” includes GFS, CLVM, and GNBD, which
are components that are part of an optional package and not part of Red Hat Cluster
Suite.
1.3. Cluster Infrastructure
The Red Hat Cluster Suite cluster infrastructure provides the basic functions for a group of computers
(called nodes or members) to work together as a cluster. Once a cluster is formed using the cluster

Chapter 1. Red Hat Cluster Suite Overview
4
infrastructure, you can use other Red Hat Cluster Suite components to suit your clustering needs (for
example, setting up a cluster for sharing files on a GFS file system or setting up service failover). The
cluster infrastructure performs the following functions:
• Cluster management
• Lock management
• Fencing
• Cluster configuration management
Note
The maximum number of nodes supported in a Red Hat Cluster is 16.
1.3.1. Cluster Management
Cluster management manages cluster quorum and cluster membership. CMAN (an abbreviation
for cluster manager) performs cluster management in Red Hat Cluster Suite for Red Hat Enterprise
Linux 5. CMAN is a distributed cluster manager and runs in each cluster node; cluster management is
distributed across all nodes in the cluster (refer to Figure 1.2, “CMAN/DLM Overview”).
CMAN keeps track of cluster quorum by monitoring the count of cluster nodes. If more than half the
nodes are active, the cluster has quorum. If half the nodes (or fewer) are active, the cluster does not
have quorum, and all cluster activity is stopped. Cluster quorum prevents the occurrence of a "split-
brain" condition — a condition where two instances of the same cluster are running. A split-brain
condition would allow each cluster instance to access cluster resources without knowledge of the other
cluster instance, resulting in corrupted cluster integrity.
Quorum is determined by communication of messages among cluster nodes via Ethernet. Optionally,
quorum can be determined by a combination of communicating messages via Ethernet and through
a quorum disk. For quorum via Ethernet, quorum consists of 50 percent of the node votes plus 1. For
quorum via quorum disk, quorum consists of user-specified conditions.
Note
By default, each node has one quorum vote. Optionally, you can configure each node to
have more than one vote.
CMAN keeps track of membership by monitoring messages from other cluster nodes. When cluster
membership changes, the cluster manager notifies the other infrastructure components, which then
take appropriate action. For example, if node A joins a cluster and mounts a GFS file system that
nodes B and C have already mounted, then an additional journal and lock management is required for
node A to use that GFS file system. If a cluster node does not transmit a message within a prescribed
amount of time, the cluster manager removes the node from the cluster and communicates to other
cluster infrastructure components that the node is not a member. Again, other cluster infrastructure
components determine what actions to take upon notification that node is no longer a cluster member.
For example, Fencing would fence the node that is no longer a member.

Lock Management
5
Figure 1.2. CMAN/DLM Overview
1.3.2. Lock Management
Lock management is a common cluster-infrastructure service that provides a mechanism for other
cluster infrastructure components to synchronize their access to shared resources. In a Red Hat
cluster, DLM (Distributed Lock Manager) is the lock manager. As implied in its name, DLM is a
distributed lock manager and runs in each cluster node; lock management is distributed across all
nodes in the cluster (refer to Figure 1.2, “CMAN/DLM Overview”). GFS and CLVM use locks from the
lock manager. GFS uses locks from the lock manager to synchronize access to file system metadata
(on shared storage). CLVM uses locks from the lock manager to synchronize updates to LVM volumes
and volume groups (also on shared storage).
1.3.3. Fencing
Fencing is the disconnection of a node from the cluster's shared storage. Fencing cuts off I/O from
shared storage, thus ensuring data integrity. The cluster infrastructure performs fencing through the
fence daemon, fenced.
When CMAN determines that a node has failed, it communicates to other cluster-infrastructure
components that the node has failed. fenced, when notified of the failure, fences the failed node.
Other cluster-infrastructure components determine what actions to take — that is, they perform any
recovery that needs to done. For example, DLM and GFS, when notified of a node failure, suspend
activity until they detect that fenced has completed fencing the failed node. Upon confirmation that
the failed node is fenced, DLM and GFS perform recovery. DLM releases locks of the failed node;
GFS recovers the journal of the failed node.
The fencing program determines from the cluster configuration file which fencing method to use.
Two key elements in the cluster configuration file define a fencing method: fencing agent and fencing
device. The fencing program makes a call to a fencing agent specified in the cluster configuration
file. The fencing agent, in turn, fences the node via a fencing device. When fencing is complete, the
fencing program notifies the cluster manager.
Red Hat Cluster Suite provides a variety of fencing methods:

Chapter 1. Red Hat Cluster Suite Overview
6
• Power fencing — A fencing method that uses a power controller to power off an inoperable node.
• Fibre Channel switch fencing — A fencing method that disables the Fibre Channel port that
connects storage to an inoperable node.
• GNBD fencing — A fencing method that disables an inoperable node's access to a GNBD server.
• Other fencing — Several other fencing methods that disable I/O or power of an inoperable node,
including IBM Bladecenters, PAP, DRAC/MC, HP ILO, IPMI, IBM RSA II, and others.
Figure 1.3, “Power Fencing Example” shows an example of power fencing. In the example, the fencing
program in node A causes the power controller to power off node D. Figure 1.4, “Fibre Channel Switch
Fencing Example” shows an example of Fibre Channel switch fencing. In the example, the fencing
program in node A causes the Fibre Channel switch to disable the port for node D, disconnecting node
D from storage.
Figure 1.3. Power Fencing Example

Fencing
7
Figure 1.4. Fibre Channel Switch Fencing Example
Specifying a fencing method consists of editing a cluster configuration file to assign a fencing-method
name, the fencing agent, and the fencing device for each node in the cluster.
The way in which a fencing method is specified depends on if a node has either dual power supplies
or multiple paths to storage. If a node has dual power supplies, then the fencing method for the
node must specify at least two fencing devices — one fencing device for each power supply (refer
to Figure 1.5, “Fencing a Node with Dual Power Supplies”). Similarly, if a node has multiple paths to
Fibre Channel storage, then the fencing method for the node must specify one fencing device for each
path to Fibre Channel storage. For example, if a node has two paths to Fibre Channel storage, the
fencing method should specify two fencing devices — one for each path to Fibre Channel storage
(refer to Figure 1.6, “Fencing a Node with Dual Fibre Channel Connections”).

Chapter 1. Red Hat Cluster Suite Overview
8
Figure 1.5. Fencing a Node with Dual Power Supplies

Cluster Configuration System
9
Figure 1.6. Fencing a Node with Dual Fibre Channel Connections
You can configure a node with one fencing method or multiple fencing methods. When you configure
a node for one fencing method, that is the only fencing method available for fencing that node. When
you configure a node for multiple fencing methods, the fencing methods are cascaded from one
fencing method to another according to the order of the fencing methods specified in the cluster
configuration file. If a node fails, it is fenced using the first fencing method specified in the cluster
configuration file for that node. If the first fencing method is not successful, the next fencing method
specified for that node is used. If none of the fencing methods is successful, then fencing starts again
with the first fencing method specified, and continues looping through the fencing methods in the order
specified in the cluster configuration file until the node has been fenced.
1.3.4. Cluster Configuration System
The Cluster Configuration System (CCS) manages the cluster configuration and provides configuration
information to other cluster components in a Red Hat cluster. CCS runs in each cluster node and
makes sure that the cluster configuration file in each cluster node is up to date. For example, if a

Chapter 1. Red Hat Cluster Suite Overview
10
cluster system administrator updates the configuration file in Node A, CCS propagates the update
from Node A to the other nodes in the cluster (refer to Figure 1.7, “CCS Overview”).
Figure 1.7. CCS Overview
Other cluster components (for example, CMAN) access configuration information from the
configuration file through CCS (refer to Figure 1.7, “CCS Overview”).

High-availability Service Management
11
Figure 1.8. Accessing Configuration Information
The cluster configuration file (/etc/cluster/cluster.conf) is an XML file that describes the
following cluster characteristics:
• Cluster name — Displays the cluster name, cluster configuration file revision level, and basic fence
timing properties used when a node joins a cluster or is fenced from the cluster.
• Cluster — Displays each node of the cluster, specifying node name, node ID, number of quorum
votes, and fencing method for that node.
• Fence Device — Displays fence devices in the cluster. Parameters vary according to the type of
fence device. For example for a power controller used as a fence device, the cluster configuration
defines the name of the power controller, its IP address, login, and password.
• Managed Resources — Displays resources required to create cluster services. Managed resources
includes the definition of failover domains, resources (for example an IP address), and services.
Together the managed resources define cluster services and failover behavior of the cluster
services.
1.4. High-availability Service Management
High-availability service management provides the ability to create and manage high-availability
cluster services in a Red Hat cluster. The key component for high-availability service management
in a Red Hat cluster, rgmanager, implements cold failover for off-the-shelf applications. In a Red
Hat cluster, an application is configured with other cluster resources to form a high-availability cluster
service. A high-availability cluster service can fail over from one cluster node to another with no

Chapter 1. Red Hat Cluster Suite Overview
12
apparent interruption to cluster clients. Cluster-service failover can occur if a cluster node fails or if a
cluster system administrator moves the service from one cluster node to another (for example, for a
planned outage of a cluster node).
To create a high-availability service, you must configure it in the cluster configuration file. A cluster
service comprises cluster resources. Cluster resources are building blocks that you create and
manage in the cluster configuration file — for example, an IP address, an application initialization
script, or a Red Hat GFS shared partition.
You can associate a cluster service with a failover domain. A failover domain is a subset of cluster
nodes that are eligible to run a particular cluster service (refer to Figure 1.9, “Failover Domains”).
Note
Failover domains are not required for operation.
A cluster service can run on only one cluster node at a time to maintain data integrity. You can specify
failover priority in a failover domain. Specifying failover priority consists of assigning a priority level to
each node in a failover domain. The priority level determines the failover order — determining which
node that a cluster service should fail over to. If you do not specify failover priority, a cluster service
can fail over to any node in its failover domain. Also, you can specify if a cluster service is restricted
to run only on nodes of its associated failover domain. (When associated with an unrestricted failover
domain, a cluster service can start on any cluster node in the event no member of the failover domain
is available.)
In Figure 1.9, “Failover Domains”, Failover Domain 1 is configured to restrict failover within that
domain; therefore, Cluster Service X can only fail over between Node A and Node B. Failover Domain
2 is also configured to restrict failover with its domain; additionally, it is configured for failover priority.
Failover Domain 2 priority is configured with Node C as priority 1, Node B as priority 2, and Node D as
priority 3. If Node C fails, Cluster Service Y fails over to Node B next. If it cannot fail over to Node B, it
tries failing over to Node D. Failover Domain 3 is configured with no priority and no restrictions. If the
node that Cluster Service Z is running on fails, Cluster Service Z tries failing over to one of the nodes
in Failover Domain 3. However, if none of those nodes is available, Cluster Service Z can fail over to
any node in the cluster.