## 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. 

## Fibre Channel connectivity 

You must remember several considerations when you use Fibre Channel technology for the intersystem network: 

GLYPH<SM590000> Redundancy 

GLYPH<SM590000> Basic topology and problems 

GLYPH<SM590000> Switches and ISL oversubscription 

GLYPH<SM590000> Distance extensions options 

