Five sizing strategies

Refer to the following typical sizing strategies as you determine an approach to sizing bandwidth. This is not a complete list of sizing strategies, and your approach might combine several strategies.

• Size bandwidth to peak workload This approach results in the smallest difference between the data in the P-VOL and S-VOL. Identify peak workload on the production volumes, and then add extra capacity to accommodate packet loss and protocol overhead. RPO is at or near zero when bandwidth is sized to peak workload.
• Size bandwidth to peak workload rolling average The rolling average is less than peak but more than average. This guarantees that at some point data will accumulate in the journal, but most of the time it will not. Your system can afford to journal for the planned amount of time and still maintain RPO.
• Size bandwidth to typical workload When bandwidth is sized to typical write-workload and an extended peak workload is experienced, excess write-data is written to journal. This excess data is delayed for subsequent transmission to the secondary site until network capacity becomes available. The amount of differential data is proportional to the amplitude and duration of the workload surge.
• Size bandwidth to average or mean workload If you cannot determine a typical workload, sizing should be to the average or mean workload with a small compensation for network overhead. In this scenario, excess data in the journals will be completely emptied to the S-VOL only occasionally. If bandwidth is sized below average write-workload, the journals never fully drain and eventually overflow.
• Alternate pair status between suspend and resync You can size bandwidth and journal size for cases such as data migration in which data consistency is not required. In this strategy, you can alternate the pair status between suspend and resync in order to process point-in-time copies in batches. When pairs are suspended, journals are not used to queue write operations. Instead, a bitmap is used to track the changed cylinders on the disks. For access patterns that favor multiple writes to a relatively small region of disk, this technique can provide especially efficient transfer of data: multiple writes to one region are not sent each and every time, and only the last update before resync is sent. The disadvantage of this strategy is that it does not guarantee I/O consistency on the secondary storage system until the resync is complete.