Communications requirements for geographic mirroring

When you are implementing an IBM i high-availability solution that uses geographic mirroring, you should plan communication lines so that geographic mirroring traffic does not adversely affect system performance.

The following is recommended:
  • Geographic mirroring can generate heavy communications traffic. If geographic mirroring shares the same IP connection with another application, for example clustering, then geographic mirroring might be suspended, which results in synchronization. Likewise, clustering response might be unacceptable, which results in partitioned nodes. Geographic mirroring should have its own dedicated communication lines. Without its own communication line, geographic mirroring can contend with other applications that use the same communication line and affect user network performance and throughput. This also includes the ability to negatively affect cluster heartbeat monitoring, resulting in a cluster partition state. Therefore, it is recommended that you have dedicated communication lines for both geographic mirroring and clusters. Geographic mirroring supports up to four communications lines.

    Geographic mirroring distributes changes over multiple lines for optimal performance. The data is sent on each of the configured communication lines in turn, from 1 to 4, over and over again. Four communication lines allow for the highest performance, but you can obtain relatively good performance with two lines.

    If you use more than one communication line between the nodes for geographic mirroring, it is best to separate those lines into different subnets, so that the usage of those lines is balanced on both systems.

  • If your configuration is such that multiple applications or services require the use of the same communication line, some of these problems can be alleviated by implementing Quality of Service (QoS) through the TCP/IP functions of IBM i. The IBM i quality of service (QoS) solution enables the policies to request network priority and bandwidth for TCP/IP applications throughout the network.
  • Ensure that throughput for each data port connection matches. This means that the speed and connection type should be the same for all connections between system pairs. If throughput is different, performance will be gated by the slowest connection.
  • Consider the delivery method for a geographic mirroring ASP session. Before 7.1, the mirroring uses synchronous communication between the production and mirror copy systems. This delivery method is best for low latency environments. In 7.1, asynchronous support was added, which means asynchronous communications is used between the production and mirror copy systems. This delivery method is best for high latency environments. This delivery method will consume more system resources on the production copy node than synchronous delivery.
  • Consider configuring a virtual private network for TCP/IP connections for the following advantages:
    • Security of data transmission by encrypting the data
    • Increased reliability of data transmission by sending greater redundancy