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Disaster Recovery Levels

When IBM Dual Copy (i.e., RAID-1 disk mirroring) was introduced in the late 1980s, the necessity of solving the dependent-write issue was the main roadblock to implementing disk mirroring across multiple storage subsystems. Creating Consistency Groups through timestamping and data freeze techniques has overcome these issues.

When considering the RTOs and RPOs in any disaster-recovery solution involving data replication, its critical to understand the need for cross-volume data integrity and data consistency. Essential elements for creating cross-volume data integrity and data consistency include the ability to:

  • Create PiT copies of the data as necessary
  • Provide a site "Data Freeze" causing all data at the remote site to be consistent with a PiT
  • Use a Consistent timestamp across all write updates to order all writes at the remote site
  • Create data set/file Consistency Groups

Cross-volume data integrity and data consistency enable database RESTARTs if the second copy of the data is actually used. Solutions that employ cross-volume mirroring and remote-disk mirroring must address the issue of data consistency to support cross-volume and cross-storage subsystem data integrity.

Most customers, when designing a multi-site solution, must minimize the time it takes to restart applications once the data at the secondary site has been recovered. Today's technology allows you to avoid database recovery options that typically take hours to days.

Tiers of Multi-Site Service Availability

In the late 1980s, the SHARE Technical Steering Committee, working with IBM, developed a whitepaper that described levels of service for disaster recovery using Tiers 0 through 6. Since then, a number of businesses using IBM zSeries* have moved toward an IBM TotalStorage* solution called the Geographically Dispersed Parallel Sysplex* (GDPS), which allows an installation to manage end-to-end application and data availability across multiple geographically separate sites. This resulted in an additional seventh tier representing the industry's highest level of availability driven by technology improvements.Figure 1 illustrates Tiers 1 through 7.

Tier 0: No disaster recovery - Most customers in the industry today understand the need for disaster recovery, as well as the need for backup of critical data. However, Tier 0 is still common, especially in the low-end marketplace.

Robert Kern works in Disk Storage Architecture for the IBM. Robert can be reached at

Victor Peltz works in Business Line Management for the IBM Systems and Technology Group in San Jose, Calif. Victor can be reached at

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