What is a write consistency group?

is a configuration option that is available for any or that is participating in a . These logical drives have been configured to use the and to preserve write order.

When Global copy is selected, any write requests to the primary logical drive are completed by the . The controller does not wait for an I/O completion indication from the secondary, remote that the data was successfully copied to the secondary logical drive.

As result of selecting this , write requests are not guaranteed to be completed in the same order on the secondary logical drive as they are on the primary logical drive. If the order of write requests is not retained, data on the secondary logical drive might become inconsistent with the data on the primary logical drive. This condition could jeopardize any attempt to recover data if a disaster occurs on the primary storage subsystem.

Sometimes, multiple remote logical drive mirrors that exist in a single storage subsystem have been configured to use Global copy and to preserve consistent write order. In this case, the remote logical drive mirrors are considered to be an interdependent group, known as a write consistency group. The data in the secondary, remote storage subsystem cannot be considered fully synchronized until all remote logical drive mirrors in the write consistency group are synchronized.

If one remote logical drive mirror in a write consistency group becomes Unsynchronized, all of the remote logical drive mirrors in the write consistency group become Unsynchronized. In addition, any write activity to the remote, secondary storage subsystem is prevented to protect the consistency of the remote data set.


Note:

Mirrored pairs that have been configured with the option stay in until you run a resume command. Mirrored pairs that are configured with the option automatically resynchronize and restart write-consistent I/O operation.

Example

On a campus site, the Enhanced Remote Mirroring premium feature has been configured between two storage subsystems. At the primary site, the primary storage subsystem has three remote logical drive mirrors (RM-A, RM-B, and RM-C). Each mirrored pair is configured to copy data to the secondary storage subsystem, at a remote secondary site.

All three remote logical drive mirrors have been configured to use the Global copy and to preserve write order. If the mirrored pair RM-A becomes Unsynchronized due to a link interruption, the controller automatically changes RM-B and RM-C into Unsynchronized status until communication can be resumed.