4. Enabling High Availability¶

High availability keeps Acronis Software-Defined Infrastructure services operational even if the node they are located on fails. In such cases, services from a failed node are relocated to healthy nodes according to the Raft consensus algorithm. High availability is ensured by:

• Metadata redundancy. For a storage cluster to function, not all but just the majority of MDS servers must be up. By setting up multiple MDS servers in the cluster you will make sure that if an MDS server fails, other MDS servers will continue controlling the cluster.
• Data redundancy. Copies of each piece of data are stored across different storage nodes to ensure that the data is available even if some of the storage nodes are inaccessible.
• Monitoring of node health.

To achieve complete high availability of the storage cluster and its services, we recommended that you do the following:

1. deploy three or more metadata servers,
2. enable management node HA, and
3. enable HA for the specific service.

On top of highly available metadata services and enabled management node HA, Acronis Software-Defined Infrastructure provides additional high availability for the following services:

• Admin panel. If the management node fails or becomes unreachable over the network, an admin panel instance on another node takes over the panel’s service so it remains accessible at the same dedicated IP address. The relocation of the service can take several minutes. Admin panel HA is enabled manually (see Enabling Management Node High Availability).
• iSCSI service. If the active path to volumes exported via iSCSI fails (e.g., a storage node with active iSCSI targets fails or becomes unreachable over the network), the active path is rerouted via targets located on healthy nodes. Volumes exported via iSCSI remain accessible as long as there is at least one path to them.
• S3 service. If an S3 node fails or becomes unreachable over the network, name server and object server components hosted on it are automatically balanced and migrated between other S3 nodes. S3 gateways are not automatically migrated; their high availability is based on DNS records. You need to maintain the DNS records manually when adding or removing S3 gateways. High availability for S3 service is enabled automatically after enabling management node HA and creating an S3 cluster from three or more nodes. An S3 cluster of three nodes may lose one node and remain operational.
• Backup gateway service. If a backup gateway node fails or becomes unreachable over the network, other nodes in the backup gateway cluster continue to provide access to the chosen storage backend. Backup gateways are not automatically migrated; their high availability is based on DNS records. You need to maintain the DNS records manually when adding or removing backup gateways. High availability for backup gateway is enabled automatically after creating a backup gateway cluster from two or more nodes. Access to the storage backend remains until at least one node in the backup gateway cluster is healthy.
• NFS shares. If a storage node fails or becomes unreachable over the network, NFS volumes located on it are migrated between other NFS nodes. High availability for NFS volumes on a storage node is enabled automatically after creating an NFS cluster.

Also take note of the following:

1. Creating the compute cluster prevents (and replaces) the use of the management node backup and restore feature.
2. If nodes to be added to the compute cluster have different CPU models, consult the section “Setting Virtual Machines CPU Model” in the Administrator’s Command Line Guide.