Key concepts

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As you begin to explore the ONTAP SnapMirror Business Continuity and plan a deployment, it is helpful to become familiar with the key terminology and concepts.


Acronym for the SnapMirror Business Continuity (SM-BC) solution available with ONTAP 9.8 and later.

Consistency group

Beginning with ONTAP 9.10.1, consistency groups have become a first-order management unit. To learn more about consistency groups, refer to Consistency groups overview.

A consistency group (CG) is a collection of FlexVol volumes that provide a write order consistency guarantee for the application workload which needs to be protected for business continuity. The purpose of a consistency group is to take simultaneous crash-consistent Snapshot copies of a collection of volumes at a point in time. In regular deployment, the group of volumes picked to be part of a CG are mapped to an application instance. SnapMirror relationships, also known as a CG relationship, is established between a source CG and a destination CG. The source and destination CGs must contain the same number and type of volumes.


The individual FlexVol volumes that are part of a consistency group.


ONTAP Mediator provides an alternate health path to the peer cluster, with the intercluster LIFs providing the other health path. With the Mediator’s health information, clusters can differentiate between intercluster LIF failure and site failure. When the site goes down, Mediator passes on the health information to the peer cluster on demand, facilitating the peer cluster to fail over. With the Mediator-provided information and the intercluster LIF health check information, ONTAP determines whether to perform an auto failover, if it is failover incapable, continue or stop.

Mediator is one of three parties in the SM-BC quorum, working with the primary cluster and the secondary cluster to reach a consensus. A consensus requires at least two parties in the quorum to agree to an operation.

Out of Sync (OOS)

The application I/O is not replicating to the secondary storage system. The destination volume is not in sync with the source volume because SnapMirror replication is not occuring. If the mirror state is Snapmirrored, this indicates a transfer failure or failure due to an unsupported operation.

Zero RPO

Zero recovery point objective. This is the acceptable amount of data loss from downtime.

Zero RTO

Zero recovery time objective or Transparent Application Failover is achieved by using host multipath I/O (MPIO) software-based path failover to provide non-disruptive access to the storage.

Planned failover

A manual operation to change the roles of copies in a SM-BC relationship. The primary becomes the secondary and the secondary becomes the primary. ALUA reporting also changes.

Automatic unplanned failover (AUFO)

An automatic operation to perform a failover to the mirror copy. The operation requires assistance from Mediator to detect that the primary copy is unavailable.