HP P6000 Continuous Access Implementation Guide (T3680-96431, August 2012)

• Fiber with WDM supports up to 500 kilometers.
• FC-to-IP gateways support the longest distances.
For detailed descriptions of supported link technologies, see “Planning the remote replication
fabric” (page 23).
NOTE: Regardless of the transmission technology, HP P6000 Continuous Access does not take
into account the type of media used for the intersite network connection. Acceptable media is
determined by the quality of the link as described in Part IV, “SAN extension and bridging” of the
HP SAN Design Reference Guide.
Recovery point objective
The RPO is the amount of data loss that the business can tolerate as a result of a disaster or other
unplanned event requiring failover. RPO is measured in time, and ranges from no time (zero) to
hours, or in some instances even days. An RPO of zero means no completed transaction can be
lost and requires synchronous replication. Note that synchronous replication mode may require
more bandwidth than asynchronous. For descriptions of synchronous and asynchronous write
modes, see “Choosing a write mode” (page 20).
Bandwidth
When you cannot adjust the distance between sites, you may be able to improve performance by
increasing bandwidth.
Consider a low-bandwidth link and a high-bandwidth link that are moving writes containing identical
amounts of data from site A to site B. See Figure 4 (page 18). The writes move through low- and
high-bandwidth links at the same speed, so the leading edges of both writes arrive simultaneously
at site B. The upper link in Figure 4 (page 18) is one-third the bandwidth of the lower link, so the
bits are three times longer. Because the bits are longer in the low-bandwidth link, the data takes
more time to unload than the data in the high-bandwidth link. The same advantage applies to
loading data into a high-bandwidth link compared to a low-bandwidth link.
Figure 4 Bandwidth and I/O rate
1. Site A
2. T3 link (44.5 Mb/s)
3. OC3 link (155 Mb/s)
4. Site B
Bandwidth capacity and peak loads
With synchronous replication, the intersite link must accommodate the peak write rate of your
applications. With asynchronous replication, the intersite link must support the peak average write
18 Designing a remote application solution