GMCV peak 60-minute period example For a cycle period of 60 minutes, the peak 60-minute period is made up of one 10-minute  period of 4500 writes per second, and five 10-minute periods of 1500 writes per second. The  average write for the 60-minute cycle period can be expressed as follows: (4500 + 5 x 1500) / 6 = 2000 writes/sec for a 60-minute cycle period The minimum bandwidth that is required for a cycle period of 60 minutes is as follows: 2000 x 0.0625 = 125 Mbps + 20% resync allowance + 5 Mbps heartbeat = 155 Mbps  dedicated plus any safety margin plus growth Now consider whether the business does not have aggressive RPO requirements and does  not want to provide dedicated bandwidth for Global Mirror. But the network is available and  unused at night, so Global Mirror can use that. There is an element of risk here, which is if the  network is unavailable for any reason, GM/CV cannot keep running during the day until it  catches up. Therefore, you would need to allow a much higher resync allowance in your  replication window, for example, 100 percent.  A GM/CV replication based on daily point-in-time copies at 8 PM each night, and replicating  until 8 AM at the latest would probably require at least the following bandwidth: (9000 + 70 x 1500) / 72 = 1584 x 0.0625 = 99 Mbps + 100% + 5 Mbps heartbeat = 203 Mbps at night plus any safety margin plus growth, non-dedicated, time-shared  with daytime traffic Global Mirror with Change Volumes provides a way to maintain point-in-time copies of data at  a secondary site where insufficient bandwidth is available to replicate the peak workloads in  real time. Another factor that can reduce the bandwidth that is required for Global Mirror with Change  Volumes is that it only sends one copy of a changed grain, which might have been rewritten  many times within the cycle period. Remember that these are examples. The central principle of sizing is that you need to know  your data write rate, which is the number of write I/Os and the average size of those I/Os. For  Metro Mirror and Global Mirror, you need to know the peak write I/O rates. For GM/CV, you  need to know the average write I/O rates.  GMCV bandwidth:  In the above samples, the bandwidth estimation for the GMCV is  based on the assumption that the write operations occurs in such a way that a change  volume grain (that has a size of 256 KB) is completely changed before it is transferred to  the remote site. In the real life, this situation is unlikely to occur.  Usually only a portion of a grain is changed during a GMCV cycle, but the transfer process  always copies the whole grain to the remote site. This behavior can lead to an unforeseen  processor burden in the transfer bandwidth that, in the edge case, can be even higher than  the one required for a standard Global Mirror. You must remember several considerations when you use Fibre Channel technology for the  intersystem network: Chapter 5. Copy services  219 GLYPH<SM590000> Redundancy GLYPH<SM590000> Basic topology and problems GLYPH<SM590000> Switches and ISL oversubscription GLYPH<SM590000> Distance extensions options Fibre Channel connectivity