Why do we need it?

Migration: When a server has migrated a filesystem to a new location, it needs a way of notifying the client where the new location of the filesystem is.
Replication: The client can ask for the list of replicas (usually at mount time). It can consult that list when it determines that it needs to failover.

Migration: When a server has migrated a filesystem to a new location, it needs a way of notifying the client where the new location of the filesystem is.