Global Copy

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Global Copy functions offer a nonsynchronous long-distance copy option whereby write operations to a storage unit at your production site are considered complete before they are transmitted to a storage unit at your recovery site.

Global Copy is a nonsynchronous mirroring function and is an alternative mirroring approach to Metro Mirror operations. Host updates to the source volume are not delayed by waiting for the update to be confirmed by a storage unit at your recovery site. The source volume sends a periodic, incremental copy of updated tracks to the target volume instead of a constant stream of updates. There is no guarantee that dependent write operations are transferred in the same sequence that they have been applied to the source volume. This nonsynchronous operation results in a “fuzzy copy” at the recovery site; however, through operational procedures, you can create a point-in-time consistent copy at your recovery site that is suitable for data migration, backup, and disaster recovery purposes.

To ensure that a consistent copy of the data is created, you can periodically switch from Global Copy to Metro Mirror mode. Then, either stop application I/O or freeze write applications to source volumes and wait for all pending updates to be copied to the recovery site. At this point, you can create a FlashCopy operation at your recovery site to obtain a consistent data.

The Global Copy function can operate at very long distances—well beyond the 300 km distance that is supported for Metro Mirror—and with minimal impact to applications, with the distance limited only by the network and the channel extended technology.

During a disaster, data can be restored only to the last known consistent increment that was created. This means that data that is written to the production site but is waiting to be transferred to the recovery site is lost whenever the two storage units can no longer communicate. Be aware that the use of Global Copy functions do not guarantee against data loss. Global Mirror functions, on the other hand, provide recoverable copies of production data at a long distance recovery site by continually forming consistent sets of data with no significant performance impact. This allows a quick restart at your recovery site in case of a disaster at the production site.

The following describes the Global Copy write sequence:
  1. During a Global Copy operation, the storage unit at your production site captures information about updates to the source and periodically sends those updates to the target volume at the recovery site.
  2. After the initial copy of tracks, the storage unit series periodically starts a synchronization cycle where all updated tracks, in ascending order from the lowest numbered track, are copied from the source volume to the target volume. The storage unit updates the target tracks with the current information for each track, regardless of the number of updates between the time that each track was last copied, the current time, and the order in which the updates occurred.
  3. When this process completes, the cycle is repeated. There is little response time degradation on application write operations in extended distance mode.
  4. Write updates to the source volume receive an immediate completion because the synchronization cycle is independent of the updates to the source volume.