Comparison of RS secondary servers and HDR secondary servers

An RS secondary server is similar in many ways to an HDR secondary server. Logs are sent to the RS secondary server in much the same way as a primary server sends logs to an HDR secondary server. However, the RS secondary server is designed to function entirely within an asynchronous communication framework so that its affect on the primary server is minimized. Neither transaction commits nor checkpoints are synchronized between the primary server and RS secondary servers. Any transaction committed on the primary server is not guaranteed to be committed at the same time on the RS secondary server.

In a high-availability cluster, the log of the HDR secondary server must be ahead of the logs of any RS secondary servers. If the HDR secondary server becomes offline, the primary server continues to send logs to the RS secondary servers. However, when the HDR secondary comes back online, HCL Informix® stops sending logs to RS secondary servers and prioritizes sending logs to the HDR secondary server until its log replay is ahead of the RS secondary server. This prioritization of the HDR secondary server logs is required because the HDR secondary server is the first failover choice in the cluster. If the RS secondary server logs are ahead of the HDR secondary server logs when a failover occurs, then the RS secondary server cannot synchronize with the new primary server.

While an RS secondary server is similar to an HDR secondary server, there are several things that an HDR secondary server supports that an RS secondary server does not support:
  • SYNC mode
  • DRAUTO parameter
  • Synchronized checkpoints

For HDR, RSS, and SDS secondary servers in a high-availability cluster, logical logging on temporary tables must always be disabled by setting the TEMPTAB_NOLOG configuration parameter to 1.

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