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Disaster Recovery Levels

Tiers 1 and 2: Physical transport - The majority of today's customers use a traditional method (known unofficially within the industry as the pickup truck access method (PTAM)) of creating tapes nightly and transporting them to a remote site overnight. Tier 1 users send the tapes to a warehouse or "cold" site for storage. Tier 2 users send the tapes to a "hot" site where the tapes can be quickly restored in the event of a disaster.

Various schemes have been developed to improve the process of offloading data nightly from production sites and production volumes to tapes. Some of these solutions provide full integration with various databases (Oracle, DB2* UDB*, etc.), as well as applications (e.g., SAP). Here are some of the names created to describe these off-line backup solutions:

  • Server-less backup
  • LAN-less backup
  • Split mirroring

Hardware vendors have created products to fit into this marketplace. The IBM Enterprise Storage Server* (ESS) FlashCopy* function provides this capability and, when coupled with ESS disk mirroring solutions, can create a PiT copy of data within the same ESS logical storage subsystem without impacting production applications.

Tier 3: Electronic vault transport - There are two ways to accomplish electronic tape vaulting:

  • Write the tape from the primary site directly into a tape storage subsystem located at the remote secondary site
  • Use the IBM Virtual Tape Subsystem (VTS) and its Peer-to-Peer Tape (PtP) copy capability (GDPS can use this capability)

Both replace the need to physically transport tapes, with the tradeoff of the added expense for telecommunication lines.

With PtP tape, a host writes data to a tape in the primary site. The VTS PtP hardware then transmits the data to a second VTS located at the secondary site. Various options are provided as to when the actual tape copying takes place, for example during tape rewind/unload, asynchronously after rewind/unload, etc.

Tier 4: Two active sites with application software mirroring - Various database, file system or application-based replication techniques also have been developed to replicate current data to a second site, but these techniques are limited to data contained in the particular database or file system for which they were designed. An example of this in the open systems world is software mirroring at the file-system level. If all of your data resides within the file system, these techniques can be a fast and efficient method of replicating data locally or remotely. Software-based file system mirroring can also be fully integrated with various host-base server clustering schemes like AIX* High Availability Geographic Cluster (HAGEO). Host failover causes the software mirroring to failover as well.

These techniques are effective for a specific server platform or file system. But, from an operational point of view, in an enterprise with multiple server types and host OSs, it can be difficult to understand which technique is used with each server and, when things change, maintenance of the solution can be a challenge.

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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