A Secondary in asynchronous mode might be out of date for various reasons, such as network outages or a surge of writes which exceed available network bandwidth. As the Secondary falls behind, the data to be sent to the Secondary starts accumulating in the write-buffer space on the Primary. If the Secondaries in asynchronous mode cannot keep up with the application write rate, VVR might need to free the Primary kernel buffer, so that incoming write requests are not delayed.

Secondary hosts that fall behind in this manner are serviced by reading back the writes from the Primary SRL. In this case, the writes are sent from the Read Back Buffer, rather than from the Primary buffer as described earlier. The read back process continues until the Secondary catches up with the Primary; at this point, the process of sending writes to the Secondary reverts back to sending from the kernel buffer, instead of sending by reading back from the SRL.