Longhorn’s scheduling policy has two stages. The scheduler only goes to the next stage if the previous stage is satisfied. Otherwise, the scheduling will fail.

    If any tag has been set in order to be selected for scheduling, the node tag and the disk tag have to match when the node or the disk is selected.

    The first stage is the node and zone selection stage. Longhorn will filter the node and zone based on the and Replica Zone Level Soft Anti-Affinity settings.

    The Node and Zone Selection Stage

    First, Longhorn will always try to schedule the new replica on a new node with a new zone if possible. In this context, “new” means that a replica for the volume has not already been scheduled to the zone or node, and “existing” refers to a node or zone that already has a replica scheduled to it.

    At this time, if both the Replica Node Level Soft Anti-Affinity and Replica Zone Level Soft Anti-Affinity settings are un-checked, and if there is no new node with a new zone, Longhorn will not schedule the replica.

    Then, Longhorn will look for a new node with an existing zone. If possible, it will schedule the new replica on the new node with an existing zone.

    Last, Longhorn will look for an existing node with an existing zone to schedule the new replica. At this time both Replica Node Level Soft Anti-Affinity and Replica Zone Level Soft Anti-Affinity should be checked.

    Disk Selection Stage

    Once the node and zone stage is satisfied, Longhorn will decide if it can schedule the replica on the disk of the node. Longhorn will check the available disks on the selected node with the matching tag, the total disk space, and the available disk space.

    For example, after the node and zone stage, Longhorn finds Node A satisfies the requirements for scheduling a replica to the node. Longhorn will check all the available disks on this node.

    Settings

    For more information on settings that are relevant to scheduling replicas on nodes and disks, refer to the settings reference: