GLYPH<SM590000> IBM Spectrum Virtualize does not require a control network or fabric to be installed to  manage Global Mirror. For intercluster Global Mirror, the system maintains a control link  between the two systems. This control link is used to control the state and to coordinate  the updates at either end. The control link is implemented on top of the same FC fabric  connection that the system uses for Global Mirror I/O. GLYPH<SM590000> IBM Spectrum Virtualize implements a configuration model that maintains the Global  Mirror configuration and state through major events, such as failover, recovery, and  resynchronization, to minimize user configuration action through these events. GLYPH<SM590000> IBM Spectrum Virtualize implements flexible resynchronization support, enabling it to  resynchronize volume pairs that experienced write I/Os to both disks, and to  resynchronize only those regions that changed. GLYPH<SM590000> An optional feature for Global Mirror is a delay simulation to be applied on writes that are  sent to auxiliary volumes. It is useful in intracluster scenarios for testing purposes. Colliding writes The Global Mirror algorithm requires that only a single write is active on a volume. I/Os that  overlap an active IO are sequential, which is called  colliding writes.  If another write is  received from a host while the auxiliary write is still active, the new host write is delayed until  the auxiliary write is complete. This rule is needed if a series of writes to the auxiliary must be  tried again, which is called  reconstruction . Conceptually, the data for reconstruction comes  from the master volume.  If multiple writes are allowed to be applied to the master for a sector, only the most recent  write gets the correct data during reconstruction. If reconstruction is interrupted for any  reason, the intermediate state of the auxiliary is inconsistent. Applications that deliver such  write activity do not achieve the performance that Global Mirror is intended to support. A  volume statistic is maintained about the frequency of these collisions.  An attempt is made to allow multiple writes to a single location to be outstanding in the Global  Mirror algorithm. There is still a need for master writes to be sequential, and the intermediate  states of the master data must be kept in a non-volatile journal while the writes are  outstanding to maintain the correct write ordering during reconstruction. Reconstruction must  never overwrite data on the auxiliary with an earlier version. The volume statistic that is  monitoring colliding writes is now limited to those writes that are not affected by this change. Figure 11-91 shows a colliding write sequence example. Figure 11-91 Colliding writes example Chapter 11. Advanced Copy Services  527