Managing High Availability Cluster Resources
After configuring High Availability cluster resources, you might need to start, stop, edit or delete them for various reasons. This article describes how to manage cluster resources safely.
Requirements:
An existing SUSE Linux Enterprise High Availability cluster
Existing cluster resources
If you still need to configure cluster resources, see Configuring High Availability Cluster Resources.
1 What are cluster resources? #
In a High Availability cluster, the applications and services that need to be highly available are called resources. Cluster resources can include Web sites, e-mail servers, databases, file systems, virtual machines or any other applications or services that need to be available to users at all times. You can start, stop, monitor and move resources as needed. You can also specify whether resources should run together on the same node, or start and stop in sequential order. If a node fails, the resources running on it fail over (move) to another node instead of being lost.
In SUSE Linux Enterprise High Availability, the cluster resource manager (CRM) is Pacemaker, which manages and coordinates all cluster services. Pacemaker uses resource agents (RAs) to start, stop and monitor resources. A resource agent abstracts the resource and presents its status to the cluster. This means that Pacemaker only interacts with the resource agent, not with the actual application or service. SUSE Linux Enterprise High Availability supports many different resource agents that are designed to manage specific types of resources.
1.1 Primitive resources #
Primitive resources represent the applications or services that need to be highly available.
Configuring a primitive specifies the resource agent and any other information required for
the cluster to manage a resource. To see the supported resource agent classes, run
crm ra classes. To see the resource agents available within a class, run
crm ra list CLASS.
Primitive resources can have the following settings:
- Properties
Resource properties are the basic details required by the cluster: a unique name for the resource, and the resource agent that manages the application or service.
- Parameters
Parameters are specific to each resource agent and determine the details and behavior of an application or service. To show all the parameters available for a resource agent, run
crm ra info RESOURCE_AGENT.- Operations
Operations are actions that the cluster can perform on a resource, such as
start,stopandmonitor. To show the supported operations for a resource agent, runcrm ra info RESOURCE_AGENTand scroll to the bottom of the output.- Meta attributes
Meta attributes change how the cluster treats a resource. To show all the meta attributes available for primitives, run
crm_resource --list-options primitive.- Utilization attributes
If the cluster nodes have utilization attributes configured, you can add these attributes to cluster resources to help with load balancing. For more information, see https://clusterlabs.org/projects/pacemaker/doc/3.0/Pacemaker_Explained/html/utilization.html.
1.2 Resource groups #
Cluster resources might depend on other resources, such as a Web server that requires an IP address and a file system. You can combine these resources into a resource group. Groups contain multiple primitives that need to be located together, started sequentially and stopped in the reverse order.
Groups have the following properties:
- Contents
Groups can only contain primitive resources, not clones or other groups.
- Starting and stopping
Resources start in the order they appear and stop in the reverse order.
- Location
Resources in the group must all run on the same node.
- Dependency
If one of the resources can't run anywhere, then the resources that appear after it in the group can't run anywhere either.
- Meta attributes
The meta attributes
target-role,priority,maintenanceandis-managedcan be added directly to the group. All other attributes are inherited from the group's resources.- Resource stickiness
Stickiness in groups is additive. Every active resource in the group contributes its stickiness value to the group's total.
1.3 Resource clones #
Resource clones can run simultaneously on multiple nodes. This is required for cluster file systems like GFS2, for example. You can clone a primitive or a group if the resource agent supports it. Clones can also have additional clone-specific meta attributes to change their behavior.
Clones can be one of the following types:
- Anonymous clones
Anonymous clones are the simplest type. All instances of the clone behave identically. Therefore, only one instance of an anonymous clone can be active on each node.
- Globally unique clones
Globally unique clones are distinct entities. An instance of the clone running on one node is not necessarily equivalent to another instance on another node.
- Promotable clones
Promotable clones allow the instances to be in one of two operating modes: promoted or unpromoted (also known as primary and secondary, or active and passive). Promotable clones can be either anonymous or globally unique.
To make a clone promotable, the resource agent must support the promote and demote operations. The cluster determines which instance of the clone to promote using one of the following methods (or a combination of both):
If a resource agent supports promotable clones, it might automatically set promotion scores based on its own preference for which instance to promote.
You can manually set promotion rules or preferences by creating location or colocation constraints.
For more information, see https://clusterlabs.org/projects/pacemaker/doc/3.0/Pacemaker_Explained/html/collective.html#determining-which-instance-is-promoted
1.4 For more information #
Primitive resources: https://clusterlabs.org/projects/pacemaker/doc/3.0/Pacemaker_Explained/html/resources.html
Groups and clones: https://clusterlabs.org/projects/pacemaker/doc/3.0/Pacemaker_Explained/html/collective.html
Resource operations: https://clusterlabs.org/projects/pacemaker/doc/3.0/Pacemaker_Explained/html/operations.html
Utilization attributes: https://clusterlabs.org/projects/pacemaker/doc/3.0/Pacemaker_Explained/html/utilization.html
2 Showing cluster resources #
You can check the status and configuration of existing cluster resources with the
CRM Shell (crmsh) or with the Hawk Web interface.
- CRM Shell
- Hawk
You can perform this procedure on any node in the cluster.
Log in either as the
rootuser or as a user withsudoprivileges.Show the status of the resources:
>sudo crm resource statusShow the configuration of the resources. The basic command shows the full CIB:
>sudo crm configure showYou can limit the output by using modifiers such as an object's name, type or tag. For more information about the available modifiers, run the following command:
>sudo crm configure help show
3 Starting cluster resources #
You can start cluster resources with the CRM Shell (crmsh) or with the
Hawk Web interface.
The application or service that the cluster resource manages must be set up correctly. For example, if you have a resource managing an Apache server, the Apache configuration must be complete before you start the resource, or the start action will fail.
- CRM Shell
- Hawk
You can perform this procedure on any node in the cluster.
Log in either as the
rootuser or as a user withsudoprivileges.Check the current status of the resource:
>sudo crm resource statusStart the resource. If necessary, you can start multiple resources at once:
>sudo crm resource start RESOURCE1 RESOURCE2Check the status again to make sure the resource started successfully:
>sudo crm resource status
4 Stopping cluster resources #
You can stop cluster resources with the CRM Shell (crmsh) or with the
Hawk Web interface.
- CRM Shell
- Hawk
You can perform this procedure on any node in the cluster.
Log in either as the
rootuser or as a user withsudoprivileges.Check the current status of the resource:
>sudo crm resource statusStop the resource. If necessary, you can stop multiple resources at once:
>sudo crm resource stop RESOURCE1 RESOURCE2Check the status again to make sure the resource stopped successfully:
>sudo crm resource status
5 Editing primitive resources #
You can edit primitive resources with the CRM Shell (crmsh) or with the
Hawk Web interface.
- CRM Shell
- Hawk
You can perform this procedure on any node in the cluster.
Log in either as the
rootuser or as a user withsudoprivileges.To add a new monitor operation, use the following command:
>sudo crm configure monitor RESOURCE INTERVAL:TIMEOUTTo add a new parameter, meta attribute or utilization attribute, use one of the following commands:
>sudo crm resource param RESOURCE set PARAMETER VALUE>sudo crm resource meta RESOURCE set ATTRIBUTE VALUE>sudo crm resource utilization RESOURCE set ATTRIBUTE VALUETo delete a parameter, meta attribute or utilization attribute, use one of the following commands:
>sudo crm resource param RESOURCE delete PARAMETER>sudo crm resource meta RESOURCE delete ATTRIBUTE>sudo crm resource utilization RESOURCE delete ATTRIBUTETo change multiple attributes at once, use the following command:
>sudo crm configure edit RESOURCEThis command opens the CIB representation of the resource in a text editor. You can make multiple changes to the resource's configuration, then save and exit.
Don't use this command to rename a resource. The change won't apply to related objects.
To rename a resource, first stop the resource, then use the following command:
>sudo crm configure rename OLD_NAME NEW_NAMEThis also changes the resource's name in related objects such as constraints, groups and clones.
crm configure help editcrm configure help monitorcrm configure help renamecrm resource help metacrm resource help paramcrm resource help util
6 Editing resource groups #
You can edit resource groups with the CRM Shell (crmsh) or with the
Hawk Web interface.
- CRM Shell
- Hawk
You can perform this procedure on any node in the cluster.
Log in either as the
rootuser or as a user withsudoprivileges.To remove a resource from a group, use the following command:
>sudo crm configure modgroup GROUP remove RESOURCETo add a resource to a group, use the following command:
>sudo crm configure modgroup GROUP add RESOURCEThe resource is appended to the end of the group, meaning it starts last and stops first.
Alternatively, you can use
beforeorafterto add the resource to a specific position in the group's order:>sudo crm configure modgroup GROUP add RESOURCE0 before RESOURCE1>sudo crm configure modgroup GROUP add RESOURCE2 after RESOURCE1To change the order of resources that are already in the group, use the following command:
>sudo crm configure edit GROUPThis command opens the CIB representation of the group in a text editor. You can make multiple changes to the group's configuration, then save and exit.
Don't use this command to rename a group. The change won't apply to related objects.
To rename a group, use the following command:
>sudo crm configure rename OLD_NAME NEW_NAMEThis also changes the group's name in related objects such as constraints and clones.
crm configure help editcrm configure help modgroupcrm configure help rename
7 Cleaning up failed resources #
The cluster automatically restarts resources if they fail, but each failure increases the resource's fail count. If the fail count reaches the resource's migration threshold, the current node becomes ineligible to run the resource. The cluster either moves the resource to an eligible node or stops the resource if no eligible node is available.
By default, fail counts are not automatically reset. This procedure explains how to reset the fail count manually.
You can clean up failed resources with the CRM Shell (crmsh) or with the Hawk Web interface.
If the resource is part of a group, the other resources in the group are also cleaned up.
- CRM Shell
- Hawk
You can perform this procedure on any node in the cluster.
Log in either as the
rootuser or as a user withsudoprivileges.Show the status of the cluster, including fail counts:
>sudo crm status failcountsIf a resource has a fail count, it appears in the
Migration Summarysection.Clean up the resource on all nodes:
>sudo crm resource cleanup RESOURCEAlternatively, you can clean up the resource on a specific node:
>sudo crm resource cleanup RESOURCE NODE
You can configure a cluster resource's fail count to reset automatically by adding the
failure-timeout meta attribute to the resource.
crm resource help cleanup
8 Deleting cluster resources #
You can delete a resource from the cluster configuration with the CRM Shell (crmsh)
or with the Hawk Web interface.
If the resource has the status
FAILED, you must clean it up before you can delete it.
- CRM Shell
- Hawk
You can perform this procedure on any node in the cluster.
Log in either as the
rootuser or as a user withsudoprivileges.Check the current status of the resource:
>sudo crm resource statusIf the resource is
Started, you can stop it with the following command:>sudo crm resource stop RESOURCEDelete the resource. If necessary, you can delete multiple resources at once:
>sudo crm configure delete RESOURCE1 RESOURCE2
crm configure help delete
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HA glossary #
- active/active, active/passive #
How resources run on the nodes. Active/passive means that resources only run on the active node, but can move to the passive node if the active node fails. Active/active means that all nodes are active at once, and resources can run on (and move to) any node in the cluster.
- arbitrator #
An arbitrator is a machine running outside the cluster to provide an additional instance for cluster calculations. For example, QNetd provides a vote to help QDevice participate in quorum decisions.
- CIB (cluster information base) #
An XML representation of the whole cluster configuration and status (cluster options, nodes, resources, constraints, etc.). The CIB manager (
pacemaker-based) keeps the CIB synchronized across the cluster nodes and handles requests to modify it.- clone #
In the context of a cluster resource, a clone is a resource that can be active on multiple nodes. Any resource can be cloned if its resource agent supports it.
- cluster #
A high-availability cluster is a group of servers (physical or virtual) designed primarily to secure the highest possible availability of data, applications and services. Not to be confused with a high-performance cluster, which shares the application load to achieve faster results.
- Cluster LVM (Cluster logical volume manager) #
The term Cluster LVM indicates that LVM is being used in a cluster environment. This requires configuration adjustments to protect the LVM metadata on shared storage.
- cluster offline upgrade #
If a new product version includes major changes that are backward incompatible, the cluster requires a cluster offline upgrade. You must stop the cluster services on all nodes and upgrade the whole cluster before bringing the nodes back online.
For minor version upgrades, see cluster rolling upgrade.
- cluster partition #
A cluster partition occurs when communication fails between one or more nodes and the rest of the cluster. The nodes are split into partitions but are still active. They can only communicate with nodes in the same partition and are unaware of the separated nodes. This is known as a split brain scenario.
- cluster rolling upgrade #
In a cluster rolling upgrade, one cluster node at a time is upgraded while the rest of the cluster is still running. Stop the cluster services on a node, upgrade it, then bring it back online to join the cluster. Continue one by one until all cluster nodes are upgraded.
For major version upgrades, see cluster offline upgrade.
- cluster services #
The services that run the cluster. Typically this refers to Pacemaker and Corosync, but might also include SBD and QDevice if those services are used.
- cluster stack #
The ensemble of software technologies and components that make up a cluster.
- colocation constraint #
A type of resource constraint that specifies which resources can or cannot run together on a node.
- concurrency violation #
A resource that should be running on only one node in the cluster is running on several nodes.
- Corosync #
Corosync provides reliable messaging, membership and quorum information about the cluster. This is handled by the Corosync Cluster Engine, a group communication system.
- CRM (cluster resource manager) #
The management entity responsible for coordinating all non-local interactions in a High Availability cluster. SUSE Linux Enterprise High Availability uses Pacemaker as the CRM. It interacts with several components: local executors on its own node and on the other nodes, non-local CRMs, administrative commands, the fencing functionality, and the membership layer.
crmsh(CRM Shell) #The command-line utility
crmshmanages the cluster, nodes and resources.- Csync2 #
A synchronization tool for replicating configuration files across all nodes in the cluster.
- DC (designated coordinator) #
The
pacemaker-controlddaemon is the cluster controller, which coordinates all actions. This daemon has an instance on each cluster node, but only one instance is elected to act as the DC. The DC is elected when the cluster services start, or if the current DC fails or leaves the cluster. The DC decides whether a cluster-wide change must be performed, such as fencing a node or moving resources.- disaster #
An unexpected interruption of critical infrastructure caused by nature, humans, hardware failure, or software bugs.
- disaster recovery #
The process by which a function is restored to the normal, steady state after a disaster.
- Disaster Recovery Plan #
A strategy to recover from a disaster with the minimum impact on IT infrastructure.
- DLM (Distributed Lock Manager) #
DLM coordinates accesses to shared resources in a cluster, for example, managing file locking in clustered file systems to increase performance and availability.
- DRBD #
DRBD® is a block device designed for building High Availability clusters. It replicates data on a primary device to secondary devices in a way that ensures all copies of the data remain identical.
- existing cluster #
The term existing cluster is used to refer to any cluster that consists of at least one node. An existing cluster has a basic Corosync configuration that defines the communication channels, but does not necessarily have resource configuration yet.
- failover #
Occurs when a resource or node fails on one machine and the affected resources move to another node.
- failover domain #
A named subset of cluster nodes that are eligible to run a resource if a node fails.
- fencing #
Prevents access to a shared resource by isolated or failing cluster members. There are two classes of fencing: resource-level fencing and node-level fencing. Resource-level fencing ensures exclusive access to a resource. Node-level fencing prevents a failed node from accessing shared resources and prevents resources from running on a node with an uncertain status. This is usually done by resetting or powering off the node.
- GFS2 #
Global File System 2 (GFS2) is a shared disk file system for Linux computer clusters. GFS2 allows all nodes to have direct concurrent access to the same shared block storage. GFS2 has no disconnected operating mode, and no client or server roles. All nodes in a GFS2 cluster function as peers. GFS2 supports up to 32 cluster nodes. Using GFS2 in a cluster requires hardware to allow access to the shared storage, and a lock manager to control access to the storage.
- group #
Resource groups contain multiple resources that need to be located together, started sequentially and stopped in the reverse order.
- Hawk (HA Web Konsole) #
A user-friendly Web-based interface for monitoring and administering a High Availability cluster from Linux or non-Linux machines. Hawk can be accessed from any machine that can connect to the cluster nodes, using a graphical Web browser.
- heuristics #
QDevice supports using a set of commands (heuristics) that run locally on start-up of cluster services, cluster membership change, successful connection to the QNetd server, or optionally at regular times. The result is used in calculations to determine which partition should have quorum.
- knet (kronosnet) #
A network abstraction layer supporting redundancy, security, fault tolerance, and fast fail-over of network links. In SUSE Linux Enterprise High Availability 16, knet is the default transport protocol for the Corosync communication channels.
- local cluster #
A single cluster in one location (for example, all nodes are located in one data center). Network latency is minimal. Storage is typically accessed synchronously by all nodes.
- local executor #
The local executor is located between Pacemaker and the resources on each node. Through the
pacemaker-execddaemon, Pacemaker can start, stop and monitor resources.- location #
In the context of a whole cluster, location can refer to the physical location of nodes (for example, all nodes might be located in the same data center). In the context of a location constraint, location refers to the nodes on which a resource can or cannot run.
- location constraint #
A type of resource constraint that defines the nodes on which a resource can or cannot run.
- meta attribute #
Parameters that tell the CRM (cluster resource manager) how to treat a specific resource. For example, you might define a resource's priority or target role.
- metro cluster #
A single cluster that can stretch over multiple buildings or data centers, with all sites connected by Fibre Channel. Network latency is usually low. Storage is frequently replicated using mirroring or synchronous replication.
- network device bonding #
Network device bonding combines two or more network interfaces into a single bonded device to increase bandwidth and/or provide redundancy. When using Corosync, the bonded device is not managed by the cluster software. Therefore, the bonded device must be configured on every cluster node that might need to access it.
- node #
Any server (physical or virtual) that is a member of a cluster.
- order constraint #
A type of resource constraint that defines the sequence of actions.
- Pacemaker #
Pacemaker is the CRM (cluster resource manager) in SUSE Linux Enterprise High Availability, or the “brain” that reacts to events occurring in the cluster. Events might be nodes that join or leave the cluster, failure of resources, or scheduled activities such as maintenance, for example. The
pacemakerddaemon launches and monitors all other related daemons.- parameters (instance attributes) #
Parameters determine which instance of a service the resource controls.
- primitive #
A primitive resource is the most basic type of cluster resource.
- promotable clone #
Promotable clones are a special type of clone resource that can be promoted. Active instances of these resources are divided into two states: promoted and unpromoted (also known as “active and passive” or “primary and secondary”).
- QDevice #
QDevice and QNetd participate in quorum decisions. The
corosync-qdevicedaemon runs on each cluster node and communicates with QNetd to provide a configurable number of votes, allowing a cluster to sustain more node failures than the standard quorum rules allow.- QNetd #
QNetd is an arbitrator that runs outside the cluster. The
corosync-qnetddaemon provides a vote to thecorosync-qdevicedaemon on each node to help it participate in quorum decisions.- quorum #
A cluster partition is defined to have quorum (be quorate) if it has the majority of nodes (or “votes”). Quorum distinguishes exactly one partition. This is part of the algorithm to prevent several disconnected partitions or nodes (“split brain”) from proceeding and causing data and service corruption. Quorum is a prerequisite for fencing, which then ensures that quorum is unique.
- RA (resource agent) #
A script acting as a proxy to manage a resource (for example, to start, stop or monitor a resource). SUSE Linux Enterprise High Availability supports different kinds of resource agents.
- ReaR (Relax and Recover) #
An administrator tool set for creating disaster recovery images.
- resource #
Any type of service or application that is known to Pacemaker, for example, an IP address, a file system, or a database. The term resource is also used for DRBD, where it names a set of block devices that use a common connection for replication.
- resource constraint #
Resource constraints specify which cluster nodes a resource can run on, what order the resources should start in, and which other resources a specific resource is dependent on.
See also colocation constraint, location constraint and order constraint.
- resource set #
As an alternative format for defining location, colocation or order constraints, you can use resource sets, where primitives are grouped together in one set. When creating a constraint, you can specify multiple resources for the constraint to apply to.
- resource template #
To help create many resources with similar configurations, you can define a resource template. After being defined, it can be referenced in primitives or in certain types of constraints. If a template is referenced in a primitive, the primitive inherits all operations, instance attributes (parameters), meta attributes and utilization attributes defined in the template.
- SBD (STONITH Block Device) #
SBD provides a node fencing mechanism through the exchange of messages via shared block storage. Alternatively, it can be used in diskless mode. In either case, it needs a hardware or software watchdog on each node to ensure that misbehaving nodes are really stopped.
- scheduler #
The scheduler is implemented as
pacemaker-schedulerd. When a cluster transition is needed,pacemaker-schedulerdcalculates the expected next state of the cluster and determines what actions need to be scheduled to achieve the next state.- split brain #
A scenario in which the cluster nodes are divided into two or more groups that do not know about each other (either through a software or hardware failure). STONITH prevents a split-brain scenario from badly affecting the entire cluster. Also known as a partitioned cluster scenario.
The term split brain is also used in DRBD but means that the nodes contain different data.
- SPOF (single point of failure) #
Any component of a cluster that, if it fails, triggers the failure of the entire cluster.
- stickiness #
Resource stickiness is a meta attribute that determines how much a resource prefers to stay on its current node.
- STONITH #
Another term for the fencing mechanism that shuts down a misbehaving node to prevent it from causing trouble in a cluster. In a Pacemaker cluster, node fencing is managed by the fencing subsystem
pacemaker-fenced.- switchover #
The planned moving of resources to other nodes in a cluster. See also failover.
- utilization #
Tells the CRM what capacity a certain resource requires from a node.
- watchdog #
SBD (STONITH Block Device) needs a watchdog on each node to ensure that misbehaving nodes are really stopped. SBD “feeds” the watchdog by regularly writing a service pulse to it. If SBD stops feeding the watchdog, the hardware enforces a system restart. This protects against failures of the SBD process itself, such as becoming stuck on an I/O error.