In an active/standby pair configuration, the primary server is processing, and the back-up servers are standing by in case of a failure on the primary server. The standby systems can be smaller, lower-performance systems, but they must have the processing capability to assure resource availability should the primary server fail.
A standby server can provide backup for more than one active server. For example in the figure above, Server 2 is the standby server in three active/standby resource pairs. The LifeKeeper resource definitions specify the following active/standby paired relationships:
- AppA on Server1 fails over to Server2.
- AppB on Server3 fails over to Server2.
- AppC on Server4 fails over to Server2.
Be aware of these three critical configuration concepts when you are considering configurations with multiple active/standby groups:
- Disk ownership. Different active applications cannot use disk partitions on the same shared disk or LUN from different servers. LifeKeeper applies locks at the disk or LUN level. When the SCSI locks are applied, only one system on the shared SCSI bus can access partitions on the disk or LUN. This requires that applications accessing different partitions on the same disk be active on the same server. In the example, Server 3 has ownership of the AppB disk resources and Server 4 owns the AppC resources.
- Processing capacity. Although it is unlikely that Servers 1, 3 and 4 would fail at the same time, you must take care when designating a standby server to support multiple resource relationships so that the standby server can handle all critical processing should multiple faults occur.
- LifeKeeper administration. In the example, Server 2 provides backup for three other servers. In general it is not desirable to administer the LifeKeeper database on the different logical groups simultaneously. You should first create the resources between the spare and one active system, then between the spare and another active system, and so on.