IBM SC34-7012-01 manual Disaster recovery tier 4 active secondary site

Site One Site Two

VTAM

3745 3745

Channel extender

3990	ESCON
	3990
Approach	Recovery
Workload may be shared	Continuous transmission of data
Site one backs up site two	Dual online for critical data
and the reverse	Network switching capability
Critical applications and	Recovery in minutes to hours
data are online
Switch network
Recover other applications

Figure 22. Disaster recovery tier 4: active secondary site

Tier 4 closes the gap between the point-in-time backups and current online processing recovery. Under a tier 4 recovery plan, site one acts as a backup to site two, and site two acts as a backup to site one.

Tier 4 duplicates the vital data of each system at the other's site. You must transmit image copies of data to the alternate site on a regular basis. You must also transmit CICS system logs and forward recovery logs, after they have been archived.

Similarly, you must transmit logs for IMS and DB2 subsystems. Your recovery action is to perform a forward recovery of the data at the alternate site. This allows recovery up to the point of the latest closed log for each subsystem.

You must also copy to the alternate site other vital data that is necessary to run your system. For example, you must copy your load libraries and JCL. You can do this on a regular basis, or when the libraries and JCL change.

The benefits of tier 4 are:

vRecovery is fast, with the required hardware and software already in place at the secondary site.

vRecovery is more complete than in the tier 1 to 3 solutions. You can recover all data to the end of the log for each of your data subsystems.

vRecovery risk is low, because you can easily test your procedures.

The drawbacks are:

vRecovery is more difficult to manage, as you have to ensure that all the logs and copies are transmitted to the other system.

vThis solution introduces synchronization problems. Logs for different data subsystems are transmitted at different times. When you complete your recovery at the secondary site, you could find that your VSAM data is complete up to 30 minutes before the disaster, whereas your DB2 data is complete up to 15 minutes before the disaster. If your data must be synchronized, you may have to develop further procedures to resynchronize data in different subsystems.

230CICS TS for z/OS 4.1: Recovery and Restart Guide