Method, system, and program for mirroring data between sites

摘要

Provided are a method, system, and program for mirroring data. A relationship is established between a plurality of source data units in a primary storage and a plurality of corresponding target data units in a secondary storage. An operation is initiated to perform an initial copy of all source data units in the relationship to the corresponding target data units. An update is received to a source data unit and a determination is made of whether a first write mode is indicated. The update is copied to the source data unit in the first write mode to the corresponding target data unit if the first copy write is indicated. Otherwise, the update is copied to the source data unit corresponding target data unit in a second write mode if the first write mode is not indicated. If the first write mode is not indicated, then a determination is made of whether a number of writes for all data units subject to the relationship falls below a threshold and the first write mode is indicated if the number of writes for all data units subject to the relationship falls below the threshold.