Importance

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Abstract

A snapshot copy of a production data set is maintained while a host may continue write access to the production data set. The data storage system responds to a host request to write to a storage location of the production data set by checking whether or not the storage-location has been modified since the time when the snapshot copy was created, and upon finding that the storage location of the production data set has not been modified, copying data from the storage location of the production data set to an allocated storage location of the snapshot copy, and after copying data from the storage location of the production data set to the allocated storage location of the snapshot copy, performing the write operation upon the storage location of the production data set. In the preferred implementation, the data storage system allocates to the snapshot copy a bit map to indicate storage locations in the production data set that have been modified, and a list of pointers to allocated storage locations for the snapshot copy. The snapshot copy facility is useful so that a host write operation upon a storage location being backed up need not be delayed until original data in the storage location is written to secondary storage. The snapshot copy facility is also useful for other applications such as transaction processing and debugging.

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