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24 PS Series Asynchronous Replication Best Practices and Sizing Guide | BP1012
Effects of TCP window
Figure 11 shows the effect of increasing the size of the TCP window from 72K to 2MB when replicating across
different WAN links speeds with 20ms or 50ms link latency. Based on these results, the following conclusions
can be made:
 Higher connection latency decreases maximum replication throughput. As expected, when
increasing the latency from 20ms to 50ms, the time to complete the replication data transfer
increased significantly for all test cases.
 Larger TCP window sizes can increase replication throughput for high speed, high latency
networks. When increasing the size of the TCP window from 72K to 2MB, an 18.9 percent average
decrease was measured in the replication time for 20ms link latency, and a 19.5 percent average
decrease in replication time for 50ms link latency.
 For very low latency connections, the beneficial effect of increasing TCP window size will be
minimal. The same tests were run with zero added latency across the replication link (results not
shown in Figure 11). For zero added latency, there was no measurable difference in throughput
performance between 72K and 2MB TCP window sizes.
Note: Changing the TCP window size setting on PS Series controllers to non-optimal values can have
negative side effects. The ability to change this setting is currently not supported using the PS Series Group
Manager. Customers interested in changing TCP window size settings should contact PS Series Technical
Support for assistance.
138.5
138.9
141.5
343.2
343.7
346.1
112.0
112.3
115.4
276.8
276.1
279.1
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10Gb 1Gb OC3
Replication time (minutes)
Link speed
Effect of TCP window
72K 20ms 72K 50ms 2MB 20ms 2MB 50ms