Another feature to investigate is how, or if, the bandwidth between the two storage systems is managed. Most storage systems leave it to the network to manage the bandwidth and quality of service (QoS) on the network pipe connecting the storage systems, while the storage system focuses on filling the pipe with data.
EqualLogic's PS Series, for example, relies primarily on network switches and routers to manage QoS. To fill the network pipe, the PS Series storage system manipulates TCP/IP packets to create larger window sizes so it can put more data into the pipe. Compellent's Storage Center uses a combination of both. Rather than leaving everything to the network, which can manage only bandwidth, Compellent's Storage Center permits administrators to prioritize which specific volumes are sent. If multiple volumes are being replicated and bandwidth availability is at a premium, the volumes with the highest priorities are replicated first to increase the likelihood they'll transmit successfully before lower priority volumes are sent.
Asynchronous software on midrange systems will meet the requirements of most average business applications. You can expect relatively good success on storage systems with low to average write I/O loads and applications that have RPOs that can exceed 30 minutes. Companies running applications with a large number of write I/Os and RPOs approaching zero should only consider deploying high-end storage systems from EMC, HDS and IBM that
This was first published in September 2007