Dell MD3620F Synchronous Replication Mode, Primary controller sends the completion status to Host

Dell PowerVault MD3600f/MD3620f Remote Replication Functional Guide

Figure 2: Synchronous Replication Mode

1	Host sends Write Request
2.1	Primary (local) controller at the local site takes in the request, and stores the request in
	the Replication Repository
2.2	Primary (local) controller writes the data into the primary virtual disk
3	Primary (local) Storage controller then sends the write request to the Remote Storage
	controller at the Remote Site
4	The secondary controller then processes the write request
5	Once the write request is processed, Secondary Controller sends acknowledgement
	to Primary Controller
6	Primary controller removes the entry from the Replication Repository
7	Primary controller sends the completion status to Host.

Synchronous replication is usually limited to remote MD36X0f storage arrays located less than 10KM away. This happens to be the fibre channel distances that most equipment supports. The reason for this limitation lies in the fact that the remote write has to complete with each associated local primary write. Extending the distances increases the latency or the time it takes to send the write to the remote MD36X0f storage array. In turn, longer communication time reduces the number of write I/Os that can complete in a given period of time. The benefit is that high IOPs applications with remote replication requirements will not be constrained by the latencies involved in long distance replication.

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