Updates are applied to local and remote copies before the transaction is complete
Local and remote copies are always in step Causes application to wait for remote write to complete
Error handling and recovery processes are needed to cope with link failures
(1) Write
(2) Write WAN (4) ACK (3) ACK
Mirroring can be performed synchronously or asynchronously. With synchronous replication both the primary and the secondary must acknowledge before we move on. We are guaranteed to be in step. We will have exactly the same data. The downside is that it will result in application latency. This will depend on the distance and the network architectures. Which is why we say for synchronous – use optical extension only and up to a maximum of 125km. It will consume SAN bandwidth because every write has to be done twice either by the host or by a virtualization layer or by the storage. We write and we write again.
It can replicate corrupt data, in fact for synchronous replication it will replicate corrupt data. At that point we are going to have to use this in conjunction with point in time copies. Synchronous replication can protect us against hardware and site failure with no loss of data.