IBM XIV Storage System Copy Services and Migration

Slave peer consistencyWhen the user is changing the slave volume or consistency group to a master volume ormaster consistency group, they may not be in a consistent state. Therefore, the volumes areautomatically restored to the last_replicated snapshot.Changing the master peerWhen a peer role is changed from slave to master, then the mirror automatically becomesinactive because both volumes are a master (Figure 6-25 on page 167). When coupling isinactive, the master volume or consistency group can change roles. After such a change themaster volume or consistency group becomes the slave volume or consistency group(Figure 6-27).Figure 6-27 Original master becomes slaveUnsynchronized master becoming a slave volume or consistency groupWhen a master volume (or consistency group) is inactive, it is also not consistent with theprevious slave. Any changes made after the last replicated snapshot time will be lost whenthe volume (CG) becomes a slave volume (CG). The data will be restored to the lastreplicated snapshot to match the data on the peer volume, which is now the new mastervolume.Upon re-establishing the connection, the primary volume or consistency group (current slavevolume/CG) is updated from the secondary volume/CG (new master volume/CG) with datathat was written to the secondary volume after the last replicated snapshot timestamp.Reconnection when both sides have the same roleSituations where both sides are configured to the same role can only occur when one sidewas changed. The roles must be changed to have one master and one slave (volume orconsistency group). Change the volume roles as appropriate on both sides before the link isresumed.If the link is resumed and both sides have the same role, the coupling does not becomeoperational. The user must use the change role function on one of the volumes and thenactivate the mirroring.Peer reverts to last_replicated snapshot. See 6.5.4, “Mirroring special snapshots” onpage 172.168 IBM XIV Storage System: Copy Services and Migration