Metro Mirror

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Metro Mirror functions offer a synchronous long-distance copy option that constantly updates a secondary copy of a volume to match changes made to a source volume.

With Metro Mirror copying, the source and target volumes can be on the same storage system or on separate storage systems. You can locate the storage system at another site some distance away. Synchronous mirroring means that each update to the source storage system must also be updated in the target storage system before another update can process. When Metro Mirror receives a host update to the source volume, it completes the corresponding update to the target volume. This guarantees data consistency by ensuring that a write operation that completes is received by the host application after the update has been committed to the target storage system and acknowledged by both the source and target storage systems. (Typically, the target volumes are on a different storage system.) This results in near perfect data consistency but can result in lag time between transactions.

With Metro Mirror, consistency is guaranteed across all volumes on which an application does write operations as long as all volume pairs are in full duplex state. When error conditions affect some of the volume pairs (or different volume pairs at different time), this consistency might be lost. For example, if one of the target volumes cannot be updated because a path fails, the corresponding source volume normally goes into a suspended state, but still allows updates. However, these updates are no longer transferred to the target volume. Only the bitmap of changed tracks is created and maintained. So the consistency across volumes is lost, although the order of write operations is still guaranteed for the other target volumes.

Metro Mirror copying supports a maximum distance of 300 km (186 mi). Delays in response times for Metro Mirror are proportional to the distance between the volumes. However, 100% of the source data is available at the recovery site when the copy operation ends.

The following procedure describes how data is written for a Remote Mirror and Copy operation. A copy to the target storage system is synchronous with the source volume’s I/O operation.
  1. An application requests a write I/O to the source storage system. The write I/O is written into cache and nonvolatile storage (NVS).
  2. Metro Mirror sends the write I/O to the target storage system cache and NVS.
  3. The storage system at the recovery site signals that the write operation has completed when the updated data is in its cache and NVS.
  4. When the storage system at the production site receives notification from the target storage system that the write operation has completed, it returns the I/O completed status to your application.