Point-in-time snap copy withdrawal management

A storage controller, comprising:

• snap copy relationship establishment logic configured to establish a first point-in-time snap copy relationship between a snap copy source and a snap copy target as of a first point-in-time over a first range of tracks of the snap copy source wherein the first range of tracks includes a first subrange of tracks and a second subrange of tracks;
• snap copy relationship withdrawal logic configured to receive a withdraw request issued as of a second point-in-time to withdraw the first subrange of tracks from the point-in-time snap copy relationship, the snap copy relationship withdrawal logic having:
• snap copy relationship splitting logic configured to create a second point-in-time snap copy relationship between the snap copy source and the snap copy target as of the first point-in-time over the second subrange of tracks of the snap copy source and reduce the range of the first point-in-time snap copy relationship to the first subrange of tracks of the withdraw request to provide a modified first point-in-time snap copy relationship as of the first point-in-time between the snap copy source and the snap copy target over the first subrange of tracks of the withdraw request;
• and
• withdrawal delay logic configured to determine whether to delay withdrawal of the modified first point-in-time snap copy relationship in response to the withdrawal request, to temporarily preserve data of the first subrange of tracks of the modified first point-in-time snap copy relationship.

A computer program product for use with a host and a primary storage system at a first location, the primary storage system having a primary storage controller and at least one storage unit controlled by the primary storage controller and configured to store a plurality of data units in tracks, wherein the computer program product comprises a computer readable storage medium having program instructions embodied therewith, the program instructions executable by a processor a storage system to cause processor operations, the processor operations comprising:

• establishing a first point-in-time snap copy relationship between a snap copy source of a storage unit and a snap copy target of a storage unit as of a first point-in-time over a first range of tracks of the snap copy source wherein the first range of tracks includes a first subrange of tracks and a second subrange of tracks;
• receiving a withdraw request issued as of a second point-in-time to withdraw the first subrange of tracks from the point-in-time snap copy relationship;
• creating a second point-in-time snap copy relationship between the snap copy source and the snap copy target as of the first point-in-time over the second subrange of tracks of the snap copy source;
• reducing the range of the first point-in-time snap copy relationship to the first subrange of tracks of the withdraw request to provide a modified first point-in-time snap copy relationship as of the first point-in-time between the snap copy source and the snap copy target over the first subrange of tracks of the withdraw request;
• and
• determining whether to delay withdrawal of the modified first point-in-time snap copy relationship in response to the withdrawal request, to temporarily preserve data of the first subrange of tracks of the modified first point-in-time snap copy relationship.