Configuring QDevice and QNetd in an Existing High Availability Cluster
- WHAT?
How to use the CRM Shell to configure QDevice and QNetd in a High Availability cluster that is already installed and running.
- WHY?
QDevice and the arbitrator QNetd participate in quorum calculations in a split-brain scenario. This allows the cluster to sustain more node failures than the standard quorum rules allow.
- EFFORT
Configuring QDevice and QNetd in an existing cluster only takes a few minutes and does not require any downtime for cluster resources.
- GOAL
Help the cluster make quorum calculations more easily. This is recommended for clusters with an even number of nodes, especially two-node clusters.
- REQUIREMENTS
An existing SUSE Linux Enterprise High Availability cluster.
An additional SUSE Linux Enterprise Server to run QNetd.
We recommend having the cluster nodes reach the QNetd server via a different network than the one Corosync uses. Ideally, the QNetd server should be in a separate rack from the cluster, or at least on a separate PSU and not in the same network segment as the Corosync communication channels.
1 What are QDevice and QNetd? #
When communication fails between one or more nodes and the rest of the cluster (a split-brain scenario), a cluster partition occurs. The nodes can only communicate with other nodes in the same partition and are unaware of the separated nodes. A cluster partition has quorum (or is “quorate”) if it has the majority of nodes (or “votes”). This is determined by quorum calculation. Quorum must be calculated so the non-quorate nodes can be fenced.
QDevice and QNetd participate in quorum calculations in a split-brain scenario. QDevice runs on each cluster node and communicates with an arbitrator, QNetd, to provide a configurable number of votes to the cluster. This allows the cluster to sustain more node failures than the standard quorum rules allow. We recommend using QDevice and QNetd for clusters with an even number of nodes, and especially for two-node clusters.
1.1 Components #
- QDevice (
corosync-qdevice) QDevice runs together with Corosync on each cluster node. It communicates with the arbitrator QNetd to provide a configurable number of votes to help with quorum calculation.
- QNetd (
corosync-qnetd) QNetd is an arbitrator that provides a vote to the QDevice service running on the cluster nodes. The QNetd server runs outside the cluster, so you cannot move cluster resources to this server. QNetd can support multiple clusters if each cluster has a unique name.
- Algorithms
QDevice supports different algorithms to determine how votes are assigned. “Fifty-fifty split” is helpful for clusters with an even number of nodes. “Last man standing” is helpful for clusters where only one active node needs to remain quorate.
- Heuristics
QDevice supports a set of commands (or “heuristics”) that run when the cluster services start (or restart), when the cluster membership changes, and when nodes connect to the QNetd server. Optionally, you can also configure the commands to run at regular intervals. The result is sent to QNetd to help with the quorum calculation. Heuristics can be written in any programming language.
- Tiebreaker
This is used as a fallback if the cluster partitions are equal even after the heuristics results are applied. The tie-breaker vote can be configured to go to the node with the lowest node ID, the highest node ID, or a specific node ID.
1.2 Benefits #
Clusters with an even number of nodes can make quorum calculations more easily.
The cluster can sustain more node failures than the standard quorum rules allow.
You can write your own heuristics scripts to affect votes. This is especially useful for complex setups.
Two-node clusters can use diskless SBD if QDevice is also configured.
One QNetd server can provide votes for multiple clusters.
QNetd can work with TLS for client certificate checking.
1.3 For more information #
For more information, see the man pages corosync-qdevice and
corosync-qnetd.
2 Setting up the QNetd server #
QNetd is an arbitrator that provides a vote to the QDevice service running on the cluster nodes. The QNetd server runs outside the cluster, so you cannot move cluster resources to this server. QNetd can support multiple clusters if each cluster has a unique name.
By default, QNetd runs the corosync-qnetd daemon as the user
coroqnetd in the group coroqnetd.
This avoids running the daemon as root.
SUSE Linux Enterprise Server is installed and registered with the SUSE Customer Center.
You have an additional registration code for SUSE Linux Enterprise High Availability.
We recommend having the cluster nodes reach the QNetd server via a different network than the one Corosync uses.
Perform this procedure on a server that is not part of the cluster:
Log in either as the
rootuser or as a user withsudoprivileges.Enable the SUSE Linux Enterprise High Availability extension:
>sudo SUSEConnect -p sle-ha/16.0/x86_64 -r HA_REGCODEInstall the corosync-qnetd package:
>sudo zypper install corosync-qnetdYou do not need to manually start the
corosync-qnetdservice. It starts automatically when you configure QDevice on the cluster.
The QNetd server is ready to accept connections from a QDevice client
(corosync-qdevice). Further configuration is handled
by crmsh when you connect QDevice clients.
3 Connecting QDevice to the QNetd server #
QDevice runs together with Corosync on each cluster node. It communicates with the arbitrator QNetd to provide a configurable number of votes to help with quorum calculation.
This procedure explains how to configure QDevice after the cluster is already installed and running, not during the initial cluster setup.
The setup script checks if a cluster restart is required and whether it is safe to do so automatically. If any non-STONITH resources are running, the script warns you to restart the cluster services manually. This allows you to put the cluster into maintenance mode first to avoid resource downtime. However, be aware that the resources will not have cluster protection while in maintenance mode.
An existing High Availability cluster is already running.
The latest corosync-qdevice package is installed on all nodes.
The latest corosync-qnetd package is installed on the QNetd server.
To connect to the QNetd server as a
sudouser: The user must have passwordlesssudopermission.To connect to the QNetd server as the
rootuser: Passwordless SSH authentication must be configured between the nodes and the QNetd server.
Perform this procedure on only one cluster node:
Log in either as the
rootuser or as a user withsudoprivileges.Run the QDevice stage of the cluster setup script:
>sudo crm cluster init qdeviceConfirm with
ythat you want to configure QDevice and QNetd.Enter the IP address or host name of the QNetd server, with or without a user name:
If you include a non-
rootuser name, a later step will prompt you for the user's password and the script will configure passwordless SSH authentication from the nodes to the QNetd server.If you omit a user name, the script defaults to the
rootuser, so passwordless SSH authentication must already be configured for the nodes to access the QNetd server.
For the remaining fields, you can accept the default values or change them as required:
Accept the proposed port (
5403) or enter a different one.Choose the algorithm that determines how votes are assigned. The default is
ffsplit.ffsplit(“fifty-fifty split”): If the cluster splits into two even partitions, one of the partitions gets the vote based on the results of heuristics checks and other factors. This algorithm is helpful for clusters with an even number of nodes.lms(“last man standing”): If only one remaining node can still communicate with the QNetd server, that node gets the vote. This algorithm is helpful for clusters where only one active node needs to remain quorate.
Choose the method to use when a tie-breaker is required. The default is
lowest.lowest: The node with the lowest node ID gets the vote.highest: The node with the highest node ID gets the vote.Alternatively, you can enter a specific node ID. The designated node always gets the vote.
Choose whether to enable TLS for client certificate checking. The default is
on.off: TLS is not required and should not be tried.on: Attempt to connect with TLS, but connect without TLS if it is not available.required: TLS is mandatory, so QDevice exits with an error if TLS is not available.
Enter heuristics commands to assist in quorum calculation, or leave the field blank to skip this step.
You can enter one command, multiple commands separated by semicolons, or the path to a script file. The commands can be written in any programming language.
If you enter heuristics commands, you must also select the mode of operation. The default is
sync.sync: QDevice runs heuristics when the cluster services start (or restart), when the cluster membership changes, and when nodes connect to the QNetd server.on: QDevice runs heuristics in the same scenarios assyncand also at regular intervals.
If required, the script prompts you for the password of the QNetd server, then configures passwordless SSH authentication between the cluster nodes and the QNetd server.
The script configures QDevice on the nodes and completes the QNetd server's configuration, including generating CA and server certificates and starting the
corosync-qnetdservice. The script also checks whether a cluster restart is required and whether it is safe to do so automatically. If any non-STONITH resources are running, the script warns you to restart the cluster services manually.If you need to restart the cluster services manually, follow these steps to avoid resource downtime:
Put the cluster into maintenance mode:
>sudo crm maintenance onIn this state, the cluster stops monitoring all resources. This allows the services managed by the resources to keep running while the cluster restarts. However, be aware that the resources will not have cluster protection while in maintenance mode.
Restart the cluster services on all nodes:
>sudo crm cluster restart --allCheck the status of the cluster:
>sudo crm statusThe nodes will have the status
UNCLEAN (offline), but will soon change toOnline.When the nodes are back online, put the cluster back into normal operation:
>sudo crm maintenance off
4 Checking the QDevice and QNetd setup #
Use the crm corosync status command to check the cluster's quorum status
and the status of QDevice and QNetd. You can run this command from any node in the cluster.
The following examples show a cluster with two nodes (alice and
bob) and a QNetd server (charlie).
>sudo crm corosync status quorum1 alice member 2 bob member Quorum information ------------------ Date: [...] Quorum provider: corosync_votequorum Nodes: 2 Node ID: 2 Ring ID: 1.e Quorate: Yes Votequorum information ---------------------- Expected votes: 3 Highest expected: 3 Total votes: 3 Quorum: 2 Flags: Quorate Qdevice Membership information ---------------------- Nodeid Votes Qdevice Name 1 1 A,V,NMW alice 2 1 A,V,NMW bob (local) 0 1 Qdevice
The Membership information section shows the following status codes:
A(alive) orNA(not alive)Shows the connectivity status between QDevice and Corosync.
V(vote) orNV(non vote)Shows if the node has a vote.
Vmeans that both nodes can communicate with each other. In a split-brain scenario, one node would be set toVand the other node would be set toNV.MW(master wins) orNMW(not master wins)Shows if the master_wins flag is set. By default, the flag is not set, so the status is
NMW.NR(not registered)Shows that the cluster is not using a quorum device.
>sudo crm corosync status qdevice1 alice member 2 bob member Qdevice information ------------------- Model: Net Node ID: 1 HB interval: 10000ms Sync HB interval: 30000ms Configured node list: 0 Node ID = 1 1 Node ID = 2 Heuristics: Disabled Ring ID: 1.e Membership node list: 1, 2 Quorate: Yes Quorum node list: 0 Node ID = 2, State = member 1 Node ID = 1, State = member Expected votes: 3 Last poll call: [...] Qdevice-net information ---------------------- Cluster name: hacluster QNetd host: charlie:5403 Connect timeout: 8000ms HB interval: 8000ms VQ vote timer interval: 5000ms TLS: Supported Algorithm: Fifty-Fifty split Tie-breaker: Node with lowest node ID KAP Tie-breaker: Enabled Poll timer running: Yes (cast vote) State: Connected TLS active: Yes (client certificate sent) Connected since: [...] Echo reply received: [...]
>sudo crm corosync status qnetd1 alice member 2 bob member Cluster "hacluster": Algorithm: Fifty-Fifty split (KAP Tie-breaker) Tie-breaker: Node with lowest node ID Node ID 1: Client address: ::ffff:192.168.1.185:45676 HB interval: 8000ms Configured node list: 1, 2 Ring ID: 1.e Membership node list: 1, 2 Heuristics: Undefined (membership: Undefined, regular: Undefined) TLS active: Yes (client certificate verified) Vote: ACK (ACK) Node ID 2: Client address: ::ffff:192.168.1.168:55034 HB interval: 8000ms Configured node list: 1, 2 Ring ID: 1.e Membership node list: 1, 2 Heuristics: Undefined (membership: Undefined, regular: Undefined) TLS active: Yes (client certificate verified) Vote: No change (ACK)
5 Changing the QDevice or QNetd configuration #
Use this procedure to change the configuration of QDevice or QNetd (for example, to
change the tie-breaker method from lowest to highest).
Log in either as the
rootuser or as a user withsudoprivileges.Put the cluster into maintenance mode:
>sudo crm maintenance onIn this state, the cluster stops monitoring all resources. This allows the services managed by the resources to keep running even when you stop the cluster services.
Stop the cluster services on all nodes:
>sudo crm cluster stop --allOpen the Corosync configuration file:
>sudo crm corosync editChange the required setting in the
quorumsection, then save and close the file.Copy the new configuration to all nodes:
>sudo crm corosync pushStart the cluster service on all nodes:
>sudo crm cluster start --allCheck the status of the cluster:
>sudo crm statusThe nodes will have the status
UNCLEAN (offline), but will soon change toOnline.When the nodes are back online, put the cluster back into normal operation:
>sudo crm maintenance offVerify that the change was successful:
>sudo crm corosync status qnetd
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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 and the relationships to each other). The CIB manager (
pacemaker-based) keeps the CIB synchronized across the cluster and handles requests to modify it.- clone #
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.
- 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 logical volume manager (Cluster LVM) #
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 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 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 attributes (resource options) #
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 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.
- 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.
- STONITH #
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.- 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.