The main difference between synchronouse and asynchronous volume replication is that synchronouse replication need to wait for the destination server in any write operation. The process goes like this :
1. Write data are buffered on the source system,
2. Source system sends the data to destination system and waits,
3. Destination system writes the data to disk,
4. When the data are on the disk (if RAID has cache enabled then when the RAID cached it) the destination inform the souce server about it,
5. The source server write the buffered data to the disk.
In result synchronous volume replication limits the write speed by speed of network and both source and destination disk speeds.
In case of asynchronous replication the data are directly written on the source server disk and send to the destinatlion system right away but doens't wait for the process to complete. In result the writing is limited only by speed of the source disk but the replication itselves is still limited by network speed and the destination disks.
Asynchronous replication gives also possiblity setup start/stop schedule. For example if you have limited network speed and you don't want to overload it with the replication you can set an schedule for nights or/and weekends. But you need to be aware that it could not make it to copy all changes over that time and in result the disk will be inconsistant.
Both of the replication modes are based on the DRBD mechanism. The difference is that synchronous replication uses DRBD Protocol C and the asynchronous replication uses DRBD Protocol A.
The main drawback of the asynchronous volume replication is the size of the TCP send buffer size which is only 2MB and cannot be increased. If this buffer becomes overloaded the asynchronous volume replications starts to work in a very similar way to the synchronous volume replications. As a result the write speed is limited by the replication link and the speed of the secondary node.
For more details about DRDB Protocols take a look at the following webpage:
Starting from DSS V6 b5087 we abandoned the idea of using asynchronous volume replication.
However if asynchronous volume replication tasks were created in build b4786 and then the system were updated, those task will still run as asynchronous. But there is NO possibility to create any new asynchronous volume replication tasks with version after 4786.