Asynchronous mode of replication avoids adding the network latency to each write by sending the data to the Secondary after the write is completed to the application. The obvious disadvantage of this is that there is no immediate guarantee that a write that appears complete to the application has actually been replicated. A more subtle effect of asynchronous mode is that while application throughput remains mostly unaffected, overall replication performance may slow down.

In asynchronous mode, the Primary kernel memory buffer fills up if the network bandwidth or the Secondary cannot keep up with the incoming write rate. For VVR to provide memory for incoming writes and continue their processing, it must free the memory held by writes that have been written to the Primary data volume but not yet sent to the Secondary. When VVR is ready to send the unsent writes that were freed, the writes must first be read back from the SRL. Hence, in synchronous mode the data is always available in memory, while in asynchronous mode VVR might have to frequently read back the data from the SRL. Consequently, replication performance might suffer because of the delay of the additional read operation. VVR does not need to read back from the SRL if the network bandwidth and the Secondary always keep up with the incoming write rate, or if the Secondary only falls behind for short periods during which the accumulated writes are small enough to fit in the VVR kernel buffer. In a shared environment, VVR always reads back from the SRL when replicating in asynchronous mode. You can tune the size of kernel buffers for VVR and VxVM to meet your requirements.

See the Symantec Storage Foundation and High Availability Solutions Tuning Guide for more information on VVR buffer space.

If VVR reads back from the SRL frequently, striping the SRL over several disks using mid-sized stripes (for example, 10 times the average write size), could improve performance. To determine whether VVR is reading back from the SRL, use the vxstat command. In the output, note the number of read operations on the SRL.