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Disaster Recovery Levels

Tier 5: Two-site, two-phase commit - This technique is specific to the database and its configuration used in conjunction with the application environment. Various databases provide specific data replication of database logs, coupled with programs to apply the log changes at the secondary site. Databases that use this approach include IMS* with IMS RSR and IBMs Replidata Solution for DB2. Typically, one only gets data consistency within the specific database, and transactions across multiple databases arent supported.

Tier 6: Disk and tape storage subsystem mirroring - This technique includes two types of mirroring.

  • Disk mirroring - Disk mirroring is popular because it can be implemented in the storage subsystem and, as a result, is independent of the host applications, databases and file systems that utilize the storage subsystem. There are four popular storage subsystem mirroring architectures (each with various options) available in the industry today:

    • IBM PtP Remote Copy
    • IBM Extended Remote Copy (XRC)
    • EMC Symmetrix Remote Data Facility (SRDF)
    • Hitachi HARC

    You can use disk mirroring in conjunction with other enterprise disaster-recovery packages and solutions to address all data within an enterprise.

  • Tape mirroring - You can mirror tape data via the IBM VTS PtP hardware capability or with various software packages. For example, for zSeries hosts, DFSMShsm has a feature known as Aggregate Backup and Recovery Support (ABARS). Use ABARS to create remote tape volumes for data sets that arent part of XRC or Peer-to-Peer Remote Copy (PPRC) pairs. Typically this data is non-critical but still needs to be recovered in the event of a disaster that prevents moving back to the primary site for an extended period of time.

    All tape-replication schemes have the potential for some data loss. In the event of a disaster, the tapes actually being written to the remote site at the time of the disaster, as well as any open tape data sets, won't have all of their data copied successfully to the remote site. To address this issue, most customers place critical production data on disk and use one or more of the various available disk-mirroring solutions. A similar software product for open systems is the Tivoli* Storage Manager feature, "Disaster Recovery Manager."

Tier 7: IBM GDPS - Use GDPS to implement the highest level in the multi-site availability hierarchy. GDPS can enable an installation to provide the means to support the highest level of application availability. GDPS combines software and hardware as the means for managing a complete switch of all resources from one site to another automatically, providing continuous operations as well as disaster recovery support for both planned and unplanned outages. A whitepaper on GDPS is here.

Today's e-business world places renewed emphasis on RTO and RPO. These factors, among others, are metrics used to evaluate an applications robustness. Figure 2 provides some guidelines relative to the service levels of RTO and RPO made possible with various technologies.

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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