Chapter 11. Advanced Copy Services 525
11.6.11  Asynchronous remote copy
Global Mirror is an asynchronous remote copy technique. In asynchronous remote copy, the 
write operations are completed on the primary site and the write acknowledgment is sent to 
the host before it is received at the secondary site. An update of this write operation is sent to 
the secondary site at a later stage, which provides the capability to perform remote copy over 
distances that exceed the limitations of synchronous remote copy.
The Global Mirror function provides the same function as Metro Mirror remote copy, but over 
long-distance links with higher latency without requiring the hosts to wait for the full round-trip 
delay of the long-distance link.
Figure 11-89 shows that a write operation to the master volume is acknowledged back to the 
host that is issuing the write before the write operation is mirrored to the cache for the 
auxiliary volume.
Figure 11-89   Global Mirror write sequence
The Global Mirror algorithms maintain a consistent image on the auxiliary. They achieve this 
consistent image by identifying sets of I/Os that are active concurrently at the master, 
assigning an order to those sets, and applying those sets of I/Os in the assigned order at the 
secondary. As a result, Global Mirror maintains the features of Write Ordering and Read 
Stability.
The multiple I/Os within a single set are applied concurrently. The process that marshals the 
sequential sets of I/Os operates at the secondary system. Therefore, the process is not 
subject to the latency of the long-distance link. These two elements of the protocol ensure 
that the throughput of the total system can be grown by increasing system size while 
maintaining consistency across a growing data set.
Limit: When a local fabric and a remote fabric are connected for Global Mirror purposes, 
the ISL hop count between a local node and a remote node must not exceed seven hops.