The transaction is complete after the update is applied locally; the update is then made to the remote copy
The remote copy may not include all transactions applied locally
Link failure has no impact on running applications
(1) Write (2) ACK
(3) Write WAN (4) ACK
With asynchronous replication we issue a write to the primary, the primary acknowledges, we now move on. At some point in time later that write goes to the secondary, and we acknowledge. If we are doing this in the subsystem it will track the outstanding writes. If you are doing asynchronous mirroring at the host, it will write to the primary, and it will write gain to the secondary. When it gets the acknowledgement back from the primary, the application moves on but the host still holds the second write whilst it waits for that acknowledgement. It knows how many I/Os it’s going to be out of step. With asynchronous mirroring the secondary will be behind. If the primary failed and you were using asynchronous replication, some data can/will be lost. You cannot guarantee data consistency. Whereas with synchronous replication you can guarantee data synchronicity between the two subsystems.