In a duplicated checkpoint dump file configuration, the same checkpoint dump information is collected in both files for system A and system B. If recovery from the file for system A fails at OpenTP1 recovery, the file for system B can be used to recover OpenTP1; this eliminates the need for the user to return to old generations of checkpoint dump files.

When a checkpoint dump file is duplicated, one-system operation can be enabled or disabled.

When one-system operation is available in a dual system configuration, the file group can be used when at least one of the two physical files is open. Therefore, if an error occurs on either of systems A and B while collecting checkpoint dump, the other system continues the dump collection. When the dump collection is completed, the file group becomes valid.

When one-system operation is unavailable in dual system configuration, the file group can be used only when both of the physical files are open. If either of the physical files cannot be opened, the file group is placed in reserved status and cannot be used. However, if an error occurs while reading from system A, the read is continued using system B. After the read is completed, the file group is reserved.

The following table shows the difference in the file the group status at error between when one-system operation is available and when one-system operation is unavailable.

Table 4-16 Difference in file group status at error between when one-system operation is available and when unavailable

Error type		One-system operation available	One-system operation unavailable
Open error at full recovery	The error occurred on either of the systems.	The other system is opened and the processing is continued.	When the other system is overwritable, it is opened (when it becomes non-overwritable, the file group is placed in the reserved status). When the other system is non-overwritable, the file group is placed in the reserved status.
	The error occurred on both of the systems.	The file group is placed in reserved status.	The file group is placed in reserved status.
An error occurred while collecting (writing) checkpoint dump.	The error occurred on either of the systems.	The other system continues the dump collection.	The file group is placed in reserved status and the dump is collected in another file group.
	The error occurred on both of the systems.	The file group is placed in reserved status and the dump is collected in another file group.	This error does not occur when one-system operation is disabled. Take an action when either of the systems failed.
An error occurred while reading checkpoint dump.	The error occurred on system A.	The checkpoint dump is read from system B.	After the read from system B is completed, the file group is placed in reserved status. If system B is non-overwritable, the file group is placed in reserved status when it becomes overwritable.
	The error occurred on system B.	The file group is placed in reserved status and the checkpoint dump is read from the previous generation.	The file group is placed in reserved status and the checkpoint dump is read from the previous generation.

Between when one-system operation is available and when unavailable, there is a difference in online allocation, disconnection, open, and close of only one system.

The following table shows the difference in online operation for only one system.

Table 4-17 Difference in online operation for only one system

Operation	One-system operation available	One-system operation unavailable
Allocating only one system in online mode	Possible	Possible
Disconnecting only one system from online processing	Possible. However, the file group cannot be used when disconnected.	Possible. However, the file group cannot be used when disconnected.
Opening only one system	Possible when both systems are allocated.	Impossible
Closing only one system	Possible when overwritable. Impossible when non-overwritable.	Impossible