Chapter 11. Advanced Copy Services 527
/SM590000IBM 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.
/SM590000IBM 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.
/SM590000IBM 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.
/SM590000An 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