DataKeeper for Windows All Documentation - SIOS

Asynchronous MirroringAsynchronous MirroringIn most cases, SIOS recommends using asynchronous mirroring. With asynchronous mirroring, each write isintercepted and a copy of the data is made. That copy is queued to be transmitted to the target system assoon as the network will allow it. Meanwhile, the original write request is committed to the underlying storagedevice and control is immediately returned to the application that initiated the write. (Note: Certain databaseapplications may send flush commands causing DataKeeper to perform in a synchronous manner. To preventperformance from being impacted in such cases, the registry entry "DontFlushAsyncQueue" may be set.)At any given time, there may be write transactions waiting in the queue to be sent to the target machine. But itis important to understand that these writes reach the target volume in time order, so the integrity of the dataon the target volume is always a valid snapshot of the source volume at some point in time. Should the sourcesystem fail, it is possible that the target system did not receive all of the writes that were queued up, but thedata that has made it to the target volume is valid and usable.The following sequence of events describes what happens when a write request is made to the source volumeof a synchronous mirror.1. Persistent bitmap file on the source system is updated.2. Source system adds a copy of the write to the Asynchronous Write Queue.3. Source system executes the write request to its source volume and returns to the caller.4. Writes that are in the queue are sent to the target system. The target system executes the writerequest on its target volume and then sends the status of the write back to the primary.5. Should an error occur during network transmission or while the target system executes its targetvolume write, the write process on the secondary is terminated. The state of the mirror then changesfrom Mirroring to Paused.DataKeeper for Windows Technical DocumentationPage 13