Mirroring information on modified data from a primary storage controller to a secondary storage controller for the secondary storage controller to use to calculate parity data

A system comprising a primary storage controller storing data in a primary storage to mirror data to a secondary storage controller storing data in a secondary storage, comprising:

• a processor;
• and
• a computer readable storage medium having computer readable program code embodied therein that when executed by the processor performs operations, the operations comprising:
• calculating new primary parity data from modified data for a primary group of tracks in the primary storage and difference data from the modified data and a pre-modified version of the modified data;
• sending the difference data and one of the modified data and the new primary parity data to the secondary storage controller to cause the secondary storage controller to write new secondary parity data and the modified data to a secondary group of tracks at the secondary storage;
• and
• writing the modified data and the new primary parity data to the primary group of tracks in the primary storage.

A method for mirroring data from a primary storage controller storing data in a primary storage and a secondary storage controller storing data in a secondary storage, comprising:

• calculating new primary parity data from modified data for a primary group of tracks in the primary storage and difference data from the modified data and a pre-modified version of the modified data;
• sending the difference data and one of the modified data and the new primary parity data to the secondary storage controller to cause the secondary storage controller to write new secondary parity data and the modified data to a secondary group of tracks at the secondary storage;
• and
• writing the modified data and the new primary parity data to the primary group of tracks in the primary storage.