3.2.5 Multi-standby for providing multiple standby systems
The multi-standby function is used to provide multiple standby servers for a single active server. Use of multiple standby servers protects against system failures while the active system is being recovered. For example, if a failure occurs in the standby system during the hot standby operation, this function enables switchover to be made to another standby system.
The multi-standby function is required in order to use a multi-standby configuration. This subsection explains hot standby using the multi-standby function.
- Organization of this subsection
(1) How to determine the hot standby target
When you use the multi-standby function, you must define the priority order for choosing the hot standby target because there are multiple eligible standby systems. In the event of a failure, HA Monitor determines the hot standby target based on the specified priority order.
(2) In the event of a failure in the active system
When the multi-standby function is being used and a failure occurs on the active server, HA Monitor performs the hot standby operation automatically according to the specified priority order. The following figure shows the hot standby operation in the event of a failure in the active system when the multi-standby function is used.
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![[Figure]](GRAPHICS/F0000049.GIF)
If a failure occurs in the active server, the server is switched over to the available standby server with the highest priority. You must specify in advance in the server environment definition the priorities of the standby servers.
(3) In the event of a failure in the target standby system during hot standby operation
If a failure occurs in the target standby system during the hot standby operation and another standby system is running, HA Monitor switches over to that standby system.
The following figure shows HA Monitor processing in the event of a failure in the target standby system during the hot standby operation.
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![[Figure]](GRAPHICS/F0000050.GIF)
If a failure occurs in standby system 1 while hot standby switching to standby system 1 is underway, HA Monitor switches to standby system 2, because it has the next highest priority. This enables job processing to continue even when a failure occurs in the standby system.
(4) For planned hot standby
When the operator uses an HA Monitor command to perform the hot standby operation, the hot standby target will be the standby system with the highest priority.
If you want to start a standby server on a desired host, terminate the active server and then start the standby server on the other host or terminate the standby servers with a higher priority than the priority of the desired switchover target standby server, and then perform planned hot standby.
(5) For grouped-system switchover
When you use the multi-standby function, you can also perform a grouped-system switchover that switches over multiple servers in the batch mode. In this case, HA Monitor performs the hot standby operation on the system that has the highest priority among all standby systems.
Note the following when you use the multi-standby function, because there are multiple hot standby targets:
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The server start type and priority of all standby servers within the same host must be the same.
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All standby servers must be running. If any standby servers are not running, the switchover targets of the servers in the server group are not consolidated.
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If no_exchange is specified as the switchover type, only the standby server in each of the standby systems with the highest priority is placed in grouped-system switchover wait state.
For details about the server start type and about grouped-system switchover wait state for grouped-system switchover, see 4.4.1 Server switchover type during grouped-system switchover.
(6) Required environment settings
You specify the multistandby operand in the HA Monitor environment settings to use the multi-standby function. You use the standbypri operand in the server environment definition to specify priorities. You must specify a different standby server priority value for each host. Zero (0), which is the highest priority, is set automatically for the active server.
(7) Hardware configuration
When you employ a hot-standby configuration that uses the multi-standby function, your hardware configuration must satisfy the following conditions:
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All hosts must be connected to the monitoring path.
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All hosts must be connected to the reset path.
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Shared disks and LANs must be accessible from all hosts that use them.