Interpreting Iperf results

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Interpreting Iperf results

Any results obtained with Iperf detail the bandwidth that was available specifically when the test was run. Available bandwidth can be influenced by many factors, such as the following examples:

  • The time of day and the day of the week. Networks are expected to be busier during peak hours or when backups are running.

  • Applications running on the nodes that are being tested. A high load may prevent Iperf and network stacks from getting sufficient CPU time, which skews results.

  • Routing in use. Sometimes different network paths get used at different times of the day.

Because of these influencing factors, you should run Iperf a number of times across the day to get a feel for what is normally achievable bandwidth and to know if there are any periods where bandwidth is higher or lower than normal.

The available bandwidth can also be influenced by physical factors in the network, such as the following examples:

  • Asymmetric routing. With asymmetric routing, data takes a different route through the network from client to server compared to server to client. Network traffic might be significantly faster in one direction.

  • Data compression. Data can be compressed depending on the given ports and protocols.

You should always test the available bandwidth in both directions between the nodes to ensure that the available bandwidth is comparable depending on which node is used as the server. Otherwise, you can experience significant changes in Veritas Volume Replicator (VVR) throughput if the direction of replication is reversed after migration or fail over between sites. Likewise, you should test the available bandwidth on the same ports that is used by VVR with comparable options, such as protocol and packet size. This ensures that Iperf-generated traffic is subject to the same compression and other factors as the VVR traffic.

When using the TCP protocol, data is sent on the VVR heartbeat port, which is port 4145 by default, whereas when using the UDP protocol, data is sent on anonymous UDP ports by default. This makes it very difficult to determine the exact port numbers that are in use for data transmission. If VVR throughput is very poor, but Iperf shows significantly improved data throughput on a named UDP port or range of ports, you should configure VVR to use the same ports as a test to see if VVR shows an increase in throughput.

For further information on displaying and modifying VVR ports, see the vrport(1M) manual page.

See Examples of using Iperf to diagnose network issues

See Using Iperf to diagnose network issues

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