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.

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.

An XML representation of the whole cluster configuration and status (cluster options, nodes, resources, constraints and the relationships to each other). The CIB manager (pacemaker-based) keeps the CIB synchronized across the cluster and handles requests to modify it.

A clone is an identical copy of an existing node, used to make deploying multiple nodes simpler.

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.

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.

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.

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.

The ensemble of software technologies and components that make up a cluster.

A type of resource constraint that specifies which resources can or cannot run together on a node.

A resource that should be running on only one node in the cluster is running on several nodes.

Corosync provides reliable messaging, membership and quorum information about the cluster. This is handled by the Corosync Cluster Engine, a group communication system.

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.

The command-line utility crmsh manages the cluster, nodes and resources.

A synchronization tool for replicating configuration files across all nodes in the cluster.

The pacemaker-controld daemon 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.

An unexpected interruption of critical infrastructure caused by nature, humans, hardware failure, or software bugs.

The process by which a function is restored to the normal, steady state after a disaster.

A strategy to recover from a disaster with the minimum impact on IT infrastructure.

DLM coordinates accesses to shared resources in a cluster, for example, managing file locking in clustered file systems to increase performance and availability.

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.

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.

Occurs when a resource or node fails on one machine and the affected resources move to another node.

A named subset of cluster nodes that are eligible to run a resource if a node fails.

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.

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.

Resource groups contain multiple resources that need to be located together, started sequentially and stopped in the reverse order.

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.

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.

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.

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.

The local executor is located between Pacemaker and the resources on each node. Through the pacemaker-execd daemon, Pacemaker can start, stop and monitor resources.

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.

A type of resource constraint that defines the nodes on which a resource can or cannot run.

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.

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 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.

Any server (physical or virtual) that is a member of a cluster.

A type of resource constraint that defines the sequence of actions.

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 pacemakerd daemon launches and monitors all other related daemons.

Parameters determine which instance of a service the resource controls.

A primitive resource is the most basic type of cluster resource.

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 and QNetd participate in quorum decisions. The corosync-qdevice daemon 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 is an arbitrator that runs outside the cluster. The corosync-qnetd daemon provides a vote to the corosync-qdevice daemon on each node to help it participate in quorum decisions.

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.

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.

An administrator tool set for creating disaster recovery images.

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 constraints specify which cluster nodes resources can run on, what order resources load in, and what other resources a specific resource is dependent on.

See also colocation constraint, location constraint and order constraint.

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.

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 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.

The scheduler is implemented as pacemaker-schedulerd. When a cluster transition is needed, pacemaker-schedulerd calculates the expected next state of the cluster and determines what actions need to be scheduled to achieve the next state.

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.

Any component of a cluster that, if it fails, triggers the failure of the entire cluster.

An acronym for shoot the other node in the head. It refers to the fencing mechanism that shuts down a misbehaving node to prevent it from causing trouble in a cluster. In a Pacemaker cluster, STONITH is managed by the fencing subsystem pacemaker-fenced.

The planned moving of resources to other nodes in a cluster. See also failover.

Tells the CRM what capacity a certain resource requires from a node.

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.