In its simplest configuration, a mirror relationship is between a source volume and a destination volume and data is replicated to the destination volume using Snapshot copies.
Typically, the source volume is a read-write volume that clients can access and modify. The destination volume is a read-only volume that exports a Snapshot copy to clients for read-only access. The only time the source volume is not a read-write volume is in a cascade configuration where the source volume is a destination of one mirror relationship and the source of another mirror relationship.
Snapshot copies are used by the source volume to update destination volumes. Snapshot copies are transferred from the source volume to the destination volume using an automated schedule or manually; therefore, mirror copies are updated asynchronously.