14 QDevice and QNetd #

In comparison to calculating quora among cluster nodes, the QDevice-and-QNetd approach has the following benefits:

A setup with QDevice/QNetd consists of the following components and mechanisms:

Before setting up QDevice and QNetd, you need to prepare the environment as follows:

The QNetd server runs outside the cluster and can support multiple clusters (as long as each cluster has a unique name). As such, you cannot move resources to this server.

The QNetd server is almost “state free”. Usually, you do not need to change anything in the configuration file /etc/sysconfig/corosync-qnetd. By default, the corosync-qnetd service runs the daemon as user coroqnetd in the group coroqnetd. This avoids running the daemon as root.

To create a QNetd server, proceed as follows:

Your QNetd server is ready to accept connections from a QDevice client (corosync-qdevice). Further configuration is handled by crmsh when you connect QDevice clients.

After you set up your QNetd server, you can set up and run the clients. You can connect the clients to the QNetd server during the installation of your cluster, or you can add them later. This procedure documents how to add them later.

To complete the QNetd configuration, the clients must have root SSH access to the QNetd server.

The script configures QDevice on the nodes and completes the QNetd configuration on the QNetd server, including generating CA and server certificates, starting the QNetd daemon, and updating the cluster's quorum configuration.

Important: SBD_WATCHDOG_TIMEOUT for diskless SBD and QDevice

If you use QDevice with diskless SBD, the SBD_WATCHDOG_TIMEOUT value must be greater than QDevice's sync_timeout value, or SBD will time out and fail to start.

The default value for sync_timeout is 30 seconds. Therefore, in the file /etc/sysconfig/sbd, make sure that SBD_WATCHDOG_TIMEOUT is set to a greater value, such as 35.

If you need additional control over how votes are determined, use heuristics. Heuristics are a set of commands that are executed in parallel.

For this purpose, the command crm cluster init qdevice provides the option --qdevice-heuristics. You can pass one or more commands (separated by semicolons) with absolute paths.

For example, if your own command for heuristic checks is located at /usr/sbin/my-script.sh you can run it on one of your cluster nodes as follows:

# crm cluster init qdevice --qnetd-hostname=charlie \
     --qdevice-heuristics=/usr/sbin/my-script.sh \
     --qdevice-heuristics-mode=on

The command or commands can be written in any language such as Shell, Python, or Ruby. If they succeed, they return 0 (zero), otherwise they return an error code.

You can also pass a set of commands. Only when all commands finish successfully (return code is zero), have the heuristics passed.

The --qdevice-heuristics-mode=on option lets the heuristics commands run regularly.

You can query the quorum status on one of your cluster nodes as shown in Example 14.1, “Status of QDevice”. It shows the status of your QDevice nodes.

# corosync-quorumtool1
 Quorum information
------------------
Date:             ...
Quorum provider:  corosync_votequorum
Nodes:            2 2
Node ID:          3232235777 3
Ring ID:          3232235777/8
Quorate:          Yes 4

Votequorum information
----------------------
Expected votes:   3
Highest expected: 3
Total votes:      3
Quorum:           2
Flags:            Quorate Qdevice

Membership information
----------------------
    Nodeid      Votes    Qdevice Name
 3232235777         1    A,V,NMW 192.168.1.1 (local) 5
 3232235778         1    A,V,NMW 192.168.1.2 5
         0          1            Qdevice

If you query the status of the QNetd server, you get a similar output to that shown in Example 14.2, “Status of QNetd server”:

# corosync-qnetd-tool -lv1
Cluster "hacluster": 2
    Algorithm:          Fifty-Fifty split 3
    Tie-breaker:        Node with lowest node ID
    Node ID 3232235777: 4
        Client address:         ::ffff:192.168.1.1:54732
        HB interval:            8000ms
        Configured node list:   3232235777, 3232235778
        Ring ID:                aa10ab0.8
        Membership node list:   3232235777, 3232235778
        Heuristics:             Undefined (membership: Undefined, regular: Undefined)
        TLS active:             Yes (client certificate verified)
        Vote:                   ACK (ACK)
    Node ID 3232235778:
        Client address:         ::ffff:192.168.1.2:43016
        HB interval:            8000ms
        Configured node list:   3232235777, 3232235778
        Ring ID:                aa10ab0.8
        Membership node list:   3232235777, 3232235778
        Heuristics:             Undefined (membership: Undefined, regular: Undefined)
        TLS active:             Yes (client certificate verified)
        Vote:                   No change (ACK)

For additional information about QDevice and QNetd, see the man pages of corosync-qdevice(8) and corosync-qnetd(8).