IBM SC34-7012-01 - Manual (Page 242)

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CICS system logs and forward recovery logs, after they have been archived.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:
v Recovery is fast, with the required hardware and software already in place at the
secondary site.
v Recovery 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.
v Recovery risk is low, because you can easily test your procedures.
The drawbacks are:
v Recovery is more difficult to manage, as you have to ensure that all the logs and
copies are transmitted to the other system.
v This 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.
Approach
Workload may be shared
Site one backs up site two
and the reverse
Critical applications and
data are online
Switch network
Recover other applications
Recovery
Continuous transmission of data
Dual online for critical data
Network switching capability
Recovery in minutes to hours
Site Two Site One
VTAM
Channel extender
3745 3745
3990 3990
ESCON
Figure 22. Disaster recovery tier 4: active secondary site
230 CICS TS for z/OS 4.1: Recovery and Restart Guide