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Configuring QDevice and QNetd in an Existing High Availability Cluster
SUSE Linux Enterprise High Availability 16.0

Configuring QDevice and QNetd in an Existing High Availability Cluster

Publication Date: 24 Oct 2025
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.

Requirements
 

  • 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:

  1. Log in either as the root user or as a user with sudo privileges.

  2. Enable the SUSE Linux Enterprise High Availability extension: 

    > sudo SUSEConnect -p sle-ha/16.0/x86_64 -r HA_REGCODE

  3. Install the corosync-qnetd package: 

    > sudo zypper install corosync-qnetd

    You do not need to manually start the corosync-qnetd service. 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.

Important
Important: Cluster restart might be required

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.

Requirements
 

  • 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 sudo user: The user must have passwordless sudo permission.

  • To connect to the QNetd server as the root user: Passwordless SSH authentication must be configured between the nodes and the QNetd server.

Perform this procedure on only one cluster node:

  1. Log in either as the root user or as a user with sudo privileges.

  2. Run the QDevice stage of the cluster setup script: 

    > sudo crm cluster init qdevice

  3. Confirm with y that you want to configure QDevice and QNetd.

  4. Enter the IP address or host name of the QNetd server, with or without a user name:

    • If you include a non-root user 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 root user, 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:

  5. Accept the proposed port (5403) or enter a different one.

  6. 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.

  7. 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.

  8. 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.

  9. 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 as sync and also at regular intervals.

  10. 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-qnetd service. 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.

  11. If you need to restart the cluster services manually, follow these steps to avoid resource downtime:

    1. Put the cluster into maintenance mode: 

      > sudo crm maintenance on

      In 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.

    2. Restart the cluster services on all nodes: 

      > sudo crm cluster restart --all

    3. Check the status of the cluster: 

      > sudo crm status

      The nodes will have the status UNCLEAN (offline), but will soon change to Online.

    4. 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).

Example 1: Showing the cluster's quorum status
 
> sudo crm corosync status quorum
1 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) or NA (not alive)

Shows the connectivity status between QDevice and Corosync.

V (vote) or NV (non vote)

Shows if the node has a vote. V means that both nodes can communicate with each other. In a split-brain scenario, one node would be set to V and the other node would be set to NV.

MW (master wins) or NMW (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.

Example 2: Showing the status of QDevice
 
> sudo crm corosync status qdevice
1 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:    [...]
Example 3: Showing the status of QNetd
 
> sudo crm corosync status qnetd
1 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).

  1. Log in either as the root user or as a user with sudo privileges.

  2. Put the cluster into maintenance mode: 

    > sudo crm maintenance on

    In 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.

  3. Stop the cluster services on all nodes: 

    > sudo crm cluster stop --all

  4. Open the Corosync configuration file: 

    > sudo crm corosync edit

  5. Change the required setting in the quorum section, then save and close the file.

  6. Copy the new configuration to all nodes: 

    > sudo crm corosync push

  7. Start the cluster service on all nodes: 

    > sudo crm cluster start --all

  8. Check the status of the cluster: 

    > sudo crm status

    The nodes will have the status UNCLEAN (offline), but will soon change to Online.

  9. When the nodes are back online, put the cluster back into normal operation: 

    > sudo crm maintenance off

  10. Verify that the change was successful: 

    > sudo crm corosync status qnetd