Overview of the High Availability Add-On for Red Hat Enterprise Linux
High Availability Add-On Overview
Red Hat Enterprise Linux 6.2 Beta High Availability Add-OnOverview
Overview of the High Availability Add-On for Red Hat EnterpriseLinuxEdition 2
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High Availability Add-On Overview provides an overview of the High Availability Add-On for Red HatEnterprise Linux 6.
Note: This document is under development, is subject to substantial change, and is provided only as apreview. The included information and instructions should not be considered complete, and should beused with caution.
Introduction v
1. Document Conventions ................................................................................................... vi
1.1. Typographic Conventions ..................................................................................... vi1.2. Pull-quote Conventions ........................................................................................ vii1.3. Notes and Warnings ........................................................................................... viii2. We Need Feedback! ..................................................................................................... viii1. High Availability Add-On Overview 1
1.1. Cluster Basics .............................................................................................................. 11.2. High Availability Add-On Introduction ............................................................................. 21.3. Cluster Infrastructure .................................................................................................... 32. Cluster Management with CMAN 5
2.1. Cluster Quorum ............................................................................................................ 5
2.1.1. Quorum Disks ................................................................................................... 62.1.2. Tie-breakers ...................................................................................................... 63. RGManager 9
3.1. Failover Domains ......................................................................................................... 9
3.1.1. Behavior Examples .......................................................................................... 103.2. Service Policies .......................................................................................................... 10
3.2.1. Start Policy ...................................................................................................... 113.2.2. Recovery Policy ............................................................................................... 113.2.3. Restart Policy Extensions ................................................................................. 113.3. Resource Trees - Basics / Definitions .......................................................................... 12
3.3.1. Parent / Child Relationships, Dependencies, and Start Ordering .......................... 123.4. Service Operations and States .................................................................................... 12
3.4.1. Service Operations .......................................................................................... 123.4.2. Service States ................................................................................................. 133.5. Virtual Machine Behaviors .......................................................................................... 14
3.5.1. Normal Operations ........................................................................................... 143.5.2. Migration ......................................................................................................... 143.5.3. RGManager Virtual Machine Features ............................................................... 153.5.4. Unhandled Behaviors ....................................................................................... 163.6. Resource Actions ....................................................................................................... 16
3.6.1. Return Values .................................................................................................. 164. Fencing 175. Lock Management 23
5.1. DLM Locking Model .................................................................................................... 235.2. Lock States ................................................................................................................ 246. Configuration and Administration Tools 25
6.1. Cluster Administration Tools ........................................................................................ 257. Virtualization and High Availability 27
7.1. VMs as Highly Available Resources/Services ............................................................... 27
7.1.1. General Recommendations .............................................................................. 287.2. Guest Clusters ........................................................................................................... 29
7.2.1. Using fence_scsi and iSCSI Shared Storage ..................................................... 317.2.2. General Recommendations .............................................................................. 31A. Revision History 33
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Introduction
This document provides a high-level overview of the High Availability Add-On for Red Hat EnterpriseLinux 6.
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 agood 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 RedHat Enterprise Linux 6.•Red Hat Enterprise Linux Deployment Guide — Provides information regarding the deployment,configuration and administration of Red Hat Enterprise Linux 6.For more information about this and related products for Red Hat Enterprise Linux 6, refer to thefollowing resources:
•Configuring and Managing the High Availability Add-On Provides information about configuring andmanaging the High Availability Add-On (also known as Red Hat Cluster) for Red Hat EnterpriseLinux 6.•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 2: Configuration and Administration — Provides information about installing,
configuring, and maintaining Red Hat GFS2 (Red Hat Global File System 2), which is included in theResilient Storage Add-On.•DM Multipath — Provides information about using the Device-Mapper Multipath feature of Red HatEnterprise Linux 6.•Load Balancer Administration — Provides information on configuring high-performance systems andservices with the Red Hat Load Balancer Add-On (Formerly known as Linux Virtual Server [LVS]).•Release Notes — Provides information about the current release of Red Hat products.
NoteFor information on best practices for deploying and upgrading Red Hat Enterprise Linux clustersusing the High Availability Add-On and Red Hat Global File System 2 (GFS2) refer to the article\"Red Hat Enterprise Linux Cluster, High Availability, and GFS Deployment Best Practices\" on Red1Hat Customer Portal at . https://access.redhat.com/kb/docs/DOC-40821.This document and other Red Hat documents are available in HTML, PDF, and RPM versions on the
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Red Hat Enterprise Linux Documentation CD and online at http://docs.redhat.com/.
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https://access.redhat.com/kb/docs/DOC-40821 http://docs.redhat.com//
v
Introduction
1. Document Conventions
This manual uses several conventions to highlight certain words and phrases and draw attention tospecific pieces of information.
In PDF and paper editions, this manual uses typefaces drawn from the Liberation Fonts set. TheLiberation 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 includesthe Liberation Fonts set by default.
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1.1. Typographic Conventions
Four typographic conventions are used to call attention to specific words and phrases. Theseconventions, 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 highlightkeycaps 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 theshell 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 boldand all distinguishable thanks to context.
Key combinations can be distinguished from keycaps by the hyphen connecting each part of a keycombination. For example:
Press Enter to execute the command.
Press Ctrl+Alt+F2 to switch to the first virtual terminal. Press Ctrl+Alt+F1 toreturn to your X-Windows session.
The first paragraph highlights the particular keycap to press. The second highlights two keycombinations (each a set of three keycaps with each set pressed simultaneously).
If source code is discussed, class names, methods, functions, variable names and returned valuesmentioned 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 fordirectories. 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 MousePreferences. In the Buttons tab, click the Left-handed mouse check box and clickClose to switch the primary mouse button from the left to the right (making the mousesuitable for use in the left hand).
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https://fedorahosted.org/liberation-fonts/
vi
Pull-quote Conventions
To insert a special character into a gedit file, choose Applications → Accessories→ Character Map from the main menu bar. Next, choose Search → Find… from theCharacter Map menu bar, type the name of the character in the Search field and clickNext. 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 theCopy button. Now switch back to your document and choose Edit → Paste from thegedit menu bar.
The above text includes application names; system-wide menu names and items; application-specificmenu names; and buttons and text found within a GUI interface, all presented in proportional bold andall 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 oncircumstance. For example:
To connect to a remote machine using ssh, type ssh username@domain.name ata shell prompt. If the remote machine is example.com and your username on thatmachine is john, type ssh john@example.com.
The mount -o remount file-system command remounts the named file
system. For example, to remount the /home file system, the command is mount -oremount /home.
To see the version of a currently installed package, use the rpm -q packagecommand. It will return a result as follows: package-version-release.
Note the words in bold italics above — username, domain.name, file-system, package, version andrelease. Each word is a placeholder, either for text you enter when issuing a command or for textdisplayed by the system.
Aside from standard usage for presenting the title of a work, italics denotes the first use of a new andimportant 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 svnbooks_tests Desktop1 downloads images notes scripts svgsSource-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{ public static void main(String args[]) throws Exceptionvii
Introduction
{ 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.
NoteNotes are tips, shortcuts or alternative approaches to the task at hand. Ignoring a note shouldhave no negative consequences, but you might miss out on a trick that makes your life easier.ImportantImportant boxes detail things that are easily missed: configuration changes that only apply tothe current session, or services that need restarting before an update will apply. Ignoring a boxlabeled 'Important' will not cause data loss but may cause irritation and frustration.WarningWarnings should not be ignored. Ignoring warnings will most likely cause data loss.2. We Need Feedback!
If you find a typographical error in this manual, or if you have thought of a way to make this manualbetter, we would love to hear from you! Please submit a report in Bugzilla: http://bugzilla.redhat.com/against the product Red Hat Enterprise Linux 6, the component doc-High_Availability_Add-On_Overview and version number: 6.1.
If you have a suggestion for improving the documentation, try to be as specific as possible whendescribing it. If you have found an error, please include the section number and some of thesurrounding text so we can find it easily.
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Chapter 1.
High Availability Add-On Overview
The High Availability Add-On is a clustered system that provides reliability, scalability, and availabilityto critical production services. The following sections provide a high-level description of thecomponents and functions of the High Availability Add-On:•Section 1.1, “Cluster Basics”
•Section 1.2, “High Availability Add-On Introduction”•Section 1.3, “Cluster Infrastructure”
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 theservers to simultaneously read and write to a single shared file system. A storage cluster simplifiesstorage 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 ofapplication data and simplifies backup and disaster recovery. The High Availability Add-On providesstorage clustering in conjunction with Red Hat GFS2 (part of the Resilient Storage Add-On).
high availability clusters provide highly available services by eliminating single points of failure and byfailing 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 overcontrol 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 failoverclusters.) The High Availability Add-On provides high availability clustering through its High AvailabilityService Management component, rgmanager.
Load-balancing clusters dispatch network service requests to multiple cluster nodes to balance therequest load among the cluster nodes. Load balancing provides cost-effective scalability because youcan match the number of nodes according to load requirements. If a node in a load-balancing clusterbecomes inoperative, the load-balancing software detects the failure and redirects requests to othercluster nodes. Node failures in a load-balancing cluster are not visible from clients outside the cluster.Load balancing is available with the Load Balancer Add-On.
High-performance clusters use cluster nodes to perform concurrent calculations. A high-performancecluster 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.)
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Chapter 1. High Availability Add-On Overview
NoteThe cluster types summarized in the preceding text reflect basic configurations; your needs mightrequire a combination of the clusters described.Additionally, the Red Hat Enterprise Linux High Availability Add-On contains support forconfiguring and managing high availability servers only. It does not support high-performanceclusters.1.2. High Availability Add-On Introduction
The High Availability Add-On is an integrated set of software components that can be deployedin a variety of configurations to suit your needs for performance, high availability, load balancing,scalability, file sharing, and economy.
The High Availability Add-On consists of the following major components:
•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 toanother in case a node becomes inoperative.•Cluster administration tools — Configuration and management tools for setting up, configuring,and managing a the High Availability Add-On. The tools are for use with the Cluster Infrastructurecomponents, the high availability and Service Management components, and storage.
NoteOnly single site clusters are fully supported at this time. Clusters spread across multiple physicallocations are not formally supported. For more details and to discuss multi-site clusters, pleasespeak to your Red Hat sales or support representative.You can supplement the High Availability Add-On with the following components:
•Red Hat GFS2 (Global File System 2) — Part of the Resilient Storage Add-On, this provides acluster file system for use with the High Availability Add-On. GFS2 allows multiple nodes to sharestorage at a block level as if the storage were connected locally to each cluster node. GFS2 clusterfile system requires a cluster infrastructure.•Cluster Logical Volume Manager (CLVM) — Part of the Resilient Storage Add-On, this providesvolume management of cluster storage. CLVM support also requires cluster infrastructure.•Load Balancer Add-On — Routing software that provides IP-Load-balancing. the Load BalancerAdd-On runs in a pair of redundant virtual servers that distributes client requests evenly to realservers that are behind the virtual servers.
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Cluster Infrastructure
1.3. Cluster Infrastructure
The High Availability Add-On 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 usingthe cluster infrastructure, you can use other components to suit your clustering needs (for example,setting up a cluster for sharing files on a GFS2 file system or setting up service failover). The clusterinfrastructure performs the following functions:•Cluster management•Lock management•Fencing
•Cluster configuration management
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Chapter 2.
Cluster Management with CMAN
Cluster management manages cluster quorum and cluster membership. CMAN (an abbreviation forcluster manager) performs cluster management in the High Availability Add-On for Red Hat EnterpriseLinux. CMAN is a distributed cluster manager and runs in each cluster node; cluster management isdistributed across all nodes in the cluster.
CMAN keeps track of membership by monitoring messages from other cluster nodes. When clustermembership changes, the cluster manager notifies the other infrastructure components, which thentake appropriate action. If a cluster node does not transmit a message within a prescribed amountof time, the cluster manager removes the node from the cluster and communicates to other clusterinfrastructure components that the node is not a member. 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.
CMAN keeps track of cluster quorum by monitoring the count of cluster nodes. If more than half thenodes are active, the cluster has quorum. If half the nodes (or fewer) are active, the cluster does nothave 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 othercluster instance, resulting in corrupted cluster integrity.
2.1. Cluster Quorum
Quorum is a voting algorithm used by CMAN.
A cluster can only function correctly if there is general agreement between the members regardingtheir status. We say a cluster has quorum if a majority of nodes are alive, communicating, and agreeon the active cluster members. For example, in a thirteen-node cluster, quorum is only reached ifseven or more nodes are communicating. If the seventh node dies, the cluster loses quorum and canno longer function.
A cluster must maintain quorum to prevent split-brain issues. If quorum was not enforced, quorum,a communication error on that same thirteen-node cluster may cause a situation where six nodesare operating on the shared storage, while another six nodes are also operating on it, independently.Because of the communication error, the two partial-clusters would overwrite areas of the disk andcorrupt the file system. With quorum rules enforced, only one of the partial clusters can use the sharedstorage, thus protecting data integrity.
Quorum doesn't prevent split-brain situations, but it does decide who is dominant and allowed tofunction in the cluster. Should split-brain occur, quorum prevents more than one cluster group fromdoing anything.
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 througha quorum disk. For quorum via Ethernet, quorum consists of a simple majority (50% of the nodes + 1extra). When configuring a quorum disk, quorum consists of user-specified conditions.
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Chapter 2. Cluster Management with CMAN
NoteBy default, each node has one quorum vote. Optionally, you can configure each node to havemore than one vote.2.1.1. Quorum Disks
A quorum disk or partition is a section of a disk that's set up for use with components of the clusterproject. It has a couple of purposes. Again, I'll explain with an example.
Suppose you have nodes A and B, and node A fails to get several of cluster manager's \"heartbeat\"packets from node B. Node A doesn't know why it hasn't received the packets, but there are severalpossibilities: either node B has failed, the network switch or hub has failed, node A's network adapterhas failed, or maybe just because node B was just too busy to send the packet. That can happen ifyour cluster is extremely large, your systems are extremely busy or your network is flakey.
Node A doesn't know which is the case, and it doesn't know whether the problem lies within itself orwith node B. This is especially problematic in a two-node cluster because both nodes, out of touchwith one another, can try to fence the other.
So before fencing a node, it would be nice to have another way to check if the other node is reallyalive, even though we can't seem to contact it. A quorum disk gives you the ability to do just that.
Before fencing a node that's out of touch, the cluster software can check whether the node is still alivebased on whether it has written data to the quorum partition.
In the case of two-node systems, the quorum disk also acts as a tie-breaker. If a node has access tothe quorum disk and the network, that counts as two votes.
A node that has lost contact with the network or the quorum disk has lost a vote, and therefore maysafely be fenced.
Further information about configuring quorum disk parameters is provided in the chapters on Congaand ccs administration in the Cluster Administration manual.
2.1.2. Tie-breakers
Tie-breakers are additional heuristics that allow a cluster partition to decide whether or not it is quoratein the event of an even-split - prior to fencing. A typical tie-breaker construct is an IP tie-breaker,sometimes called a ping node.
With such a tie-breaker, nodes not only monitor each other, but also an upstream router that is onthe same path as cluster communications. If the two nodes lose contact with each other, the one thatwins is the one that can still ping the upstream router. Of course, there are cases, such as a switch-loop, where it is possible for two nodes to see the upstream router - but each other - causing what iscalled a split brain. That is why, even when using tie-breakers, it is important to ensure that fencing isconfigured correctly.
Other types of tie-breakers include where a shared partition, often called a quorum disk, providesadditional details. clumanager 1.2.x (Red Hat Cluster Suite 3) had a disk tie-breaker that allowedoperation if the network went down as long as both nodes were still communicating over the sharedpartition.6
Tie-breakers
More complex tie-breaker schemes exist, such as QDisk (part of linux-cluster). QDisk allows arbitraryheuristics to be specified. These allow each node to determine its own fitness for participation in thecluster. It is often used as a simple IP tie-breaker, however. See the qdisk(5) manual page for moreinformation.
CMAN has no internal tie-breakers for various reasons. However, tie-breakers can be implementedusing the API. This API allows quorum device registration and updating. For an example, look at theQDisk source code.
You might need a tie-breaker if you:
•Have a two node configuration with the fence devices on a different network path than the path usedfor cluster communication•Have a two node configuration where fencing is at the fabric level - especially for SCSI reservationsHowever, if you have a correct network and fencing configuration in your cluster, a tie-breaker onlyadds complexity, except in pathological cases.
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Chapter 3.
RGManager
RGManager manages and provides failover capabilities for collections of cluster resources calledservices, resource groups, or resource trees. These resource groups are tree-structured, and haveparent-child dependency and inheritance relationships within each subtree.
How RGManager works is that it allows administrators to define, configure, and monitor cluster
services. In the event of a node failure, RGManager will relocate the clustered service to another nodewith minimal service disruption. You can also restrict services to certain nodes, such as restrictinghttpd to one group of nodes while mysql can be restricted to a separate set of nodes.
There are various processes and agents that combine to make RGManager work. The following listsummarizes those areas.
•Failover Domains - How the RGManager failover domain system works•Service Policies - Rgmanager's service startup and recovery policies
•Resource Trees - How rgmanager's resource trees work, including start/stop orders and inheritance•Service Operational Behaviors - How rgmanager's operations work and what states mean•Virtual Machine Behaviors - Special things to remember when running VMs in a rgmanager cluster•ResourceActions - The agent actions RGManager uses and how to customize their behavior fromthe cluster.conf file.•Event Scripting - If rgmanager's failover and recovery policies do not fit in your environment, you cancustomize your own using this scripting subsystem.
3.1. Failover Domains
A failover domain is an ordered subset of members to which a service may be bound. Failoverdomains, while useful for cluster customization, are not required for operation.
The following is a list of semantics governing the options as to how the different configuration optionsaffect the behavior of a failover domain.
•preferred node or preferred member: The preferred node was the member designated to run agiven service if the member is online. We can emulate this behavior by specifying an unordered,unrestricted failover domain of exactly one member.•restricted domain: Services bound to the domain may only run on cluster members which are alsomembers of the failover domain. If no members of the failover domain are available, the service isplaced in the stopped state. In a cluster with several members, using a restricted failover domaincan ease configuration of a cluster service (such as httpd), which requires identical configuration onall members that run the service. Instead of setting up the entire cluster to run the cluster service,you must set up only the members in the restricted failover domain that you associate with thecluster service.•unrestricted domain: The default behavior, services bound to this domain may run on all clustermembers, but will run on a member of the domain whenever one is available. This means that if aservice is running outside of the domain and a member of the domain comes online, the service willmigrate to that member.•ordered domain: The order specified in the configuration dictates the order of preference ofmembers within the domain. The highest-ranking member of the domain will run the service
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Chapter 3. RGManager
whenever it is online. This means that if member A has a higher-rank than member B, the servicewill migrate to A if it was running on B if A transitions from offline to online.
•unordered domain: The default behavior, members of the domain have no order of preference;any member may run the service. Services will always migrate to members of their failover domainwhenever possible, however, in an unordered domain.•failback: Services on members of an ordered failover domain should fail back to the node that it wasoriginally running on before the node failed, which is useful for frequently failing nodes to preventfrequent service shifts between the failing node and the failover node.Ordering, restriction, and nofailback are flags and may be combined in almost any way (ie, ordered+restricted, unordered+unrestricted, etc.). These combinations affect both where services start afterinitial quorum formation and which cluster members will take over services in the event that the servicehas failed.
3.1.1. Behavior Examples
Given a cluster comprised of this set of members: {A, B, C, D, E, F, G}.
Ordered, restricted failover domain {A, B, C}
With nofailback unset: A service 'S' will always run on member 'A' whenever member 'A' is onlineand there is a quorum. If all members of {A, B, C} are offline, the service will not run. If the serviceis running on 'C' and 'A' transitions online, the service will migrate to 'A'.
With nofailback set: A service 'S' will run on the highest priority cluster member when a quorum isformed. If all members of {A, B, C} are offline, the service will not run. If the service is running on'C' and 'A' transitions online, the service will remain on 'C' unless 'C' fails, at which point it will failover to 'A'.
Unordered, restricted failover domain {A, B, C}
A service 'S' will only run if there is a quorum and at least one member of {A, B, C} is online. Ifanother member of the domain transitions online, the service does not relocate.Ordered, unrestricted failover domain {A, B, C}
With nofailback unset: A service 'S' will run whenever there is a quorum. If a member of the
failover domain is online, the service will run on the highest-priority member, otherwise a memberof the cluster will be chosen at random to run the service. That is, the service will run on 'A'whenever 'A' is online, followed by 'B'.
With nofailback set: A service 'S' will run whenever there is a quorum. If a member of the failoverdomain is online at quorum formation, the service will run on the highest-priority member of thefailover domain. That is, if 'B' is online (but 'A' is not), the service will run on 'B'. If, at some laterpoint, 'A' joins the cluster, the service will not relocate to 'A'.
Unordered, unrestricted failover domain {A, B, C}
This is also called a \"Set of Preferred Members\". When one or more members of the failoverdomain are online, the service will run on a nonspecific online member of the failover domain. Ifanother member of the failover domain transitions online, the service does not relocate.
3.2. Service Policies
RGManager has three service recovery policies which may be customized by the administrator on aper-service basis.
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Start Policy
NoteThese policies also apply to virtual machine resources.3.2.1. Start Policy
RGManager by default starts all services when rgmanager boots and a quorum is present. Thisbehavior may be altered by administrators.
•autostart (default) - start the service when rgmanager boots and a quorum forms. If set to '0', thecluster will not start the service and instead place it in to the disabled state.
3.2.2. Recovery Policy
The recovery policy is the default action rgmanager takes when a service fails on a particular node.There are three available options, defined in the following list.
•restart (default) - restart the service on the same node. If no other recovery policy is specified, thisrecovery policy is used. If restarting fails, rgmanager falls back to relocate the service.•relocate - Try to start the service on other node(s) in the cluster. If no other nodes successfully startthe service, the service is then placed in the stopped state.•disable - Do nothing. Place the service in to the disabled state.
•restart-disable - Attempt to restart the service, in place. Place the service in to the disabled state ifrestarting fails.
3.2.3. Restart Policy Extensions
When the restart recovery policy is used, you may additionally specify a maximum threshold for howmany restarts may occur on the same node in a given time. There are two parameters available forservices called max_restarts and restart_expire_time which control this.
The max_restarts parameter is an integer which specifies the maximum number of restarts beforegiving up and relocating the service to another host in the cluster.
The restart_expire_time parameter tells rgmanager how long to remember a restart event.
The use of the two parameters together creates a sliding window for the number of tolerated restarts ina given amount of time. For example:
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Chapter 3. RGManager
NoteYou must specify both parameters together; the use of either parameter by itself is undefined.3.3. Resource Trees - Basics / Definitions
The following illustrates the structure of a resource tree, with a correpsonding list that defines eacharea.