IBM XIV Storage System Copy Services and Migration

►►A disaster that makes the whole of the local site and data unavailable.In this scenario, the standby (inactive) servers at the secondary site are activated andattached to the secondary XIV to continue normal operations. This requires changing therole of the slave peers to become master peers.After the primary site is recovered, the data at the secondary site can be mirrored back tothe primary site. This most likely requires a full initialization of the primary site because thelocal volumes may not contain any data. See 6.1, “Asynchronous mirroring configuration”on page 150, for details related to this process.When initialization completes the peer roles can be switched back to master at the primarysite and the slave at secondary site. The servers are then redirected back to the primarysite. See 6.2, “Role reversal” on page 166, for details related to this process.A disaster that breaks all links between the two sites but both sites remain runningIn this scenario the primary site continues to operate as normal. When the links arereestablished the data at the primary site can be resynchronized with the secondary site.Only the changes since the previous last_replicated snapshot are sent to the remote site.6.5 Mirroring processThis section explains the overall asynchronous mirroring process, from initialization tooutgoing operations. The asynchronous mirroring process generates snapshots of the masterat user-configured intervals and synchronizes these snapshots with the slave (see “Snapshotlife-cycle” on page 172).6.5.1 Initialization processThe mirroring process starts with an initialization phase:1. Read requests are served from the master. Upon each write operation to the master, themaster writes the data locally (primary site) and acknowledges the write operation.2. Before any actual synchronization of a master can commence, a most_recent snapshot ofthe master is created. This snapshot determines the scope of replication for theinitialization phase and the data to be replicated can be determined.170 IBM XIV Storage System: Copy Services and Migration