Downscaling the cluster

Impact

Downscaling can have an adverse effect on the availability and resiliency of the cluster, especially when a member is being removed that runs an OVN central service (OVN SB, OVN NB, OVN Northd).

OVN uses the Raft consensus algorithm for cluster management, which has a fault tolerance of up to (N-1)/2 members. This means that fault resiliency will be lost if a three-node cluster is reduced to two nodes.

Monitoring

You can watch logs on the departing member for indications of removal failures with:

snap logs -f microovn.daemon

Any issues that arise during the removal process will need to be resolved manually.

Remove a cluster member

To remove a cluster member:

microovn cluster remove <member_name>

The value of <member_name> is taken from the Name column in the output of the cluster list command.

Any chassis components (ovn-controller and ovs-vswitchd) running on the member will first be stopped and disabled (prevented from starting). For a member with central components present (microovn.central), the Northbound and Southbound databases will be gracefully removed.

Verification

Upon removal, check the state of OVN services to ensure that the member was properly removed.

# Check status of OVN SB cluster
ovn-appctl -t /var/snap/microovn/common/run/central/ovnsb_db.ctl cluster/status OVN_Southbound

# Check status of OVN NB cluster
ovn-appctl -t /var/snap/microovn/common/run/central/ovnnb_db.ctl cluster/status OVN_Northbound

# Check registered chassis
ovn-sbctl show

Data preservation

MicroOVN will back up selected data directories into the timestamped location /var/snap/microovn/common/backup_<timestamp>/. These backups will include:

• logs

• OVN database files

• OVS database file

• issued certificates and keys