Picket fence staging in a multi-tier cache

A method, comprising:

• storing in storage, a track having a set of sectors of data;
• caching update data for the track in a fast cache tier of a multi-tier cache in a first track write caching operation which includes caching update data for a first subset of sectors of the track in the fast cache tier and omitting caching data for a second subset of sectors of the track in the fast cache tier to define upon completion of the first track write caching operation, a number of track holes equal to zero or more track holes of the track in the fast cache tier in which a track hole corresponds to one or more contiguous sectors of the second subset of sectors of the track for which data caching is omitted during the first track write caching operation,
• determining the number of track holes defined by the second subset of sectors omitted from the first track write caching operation;
• and
• queuing prestage requests in one of a plurality of prestage request queues as a function of the number of track holes determined to be present in the second subset of sectors absent from the fast cache tier, wherein each prestage request when executed prestages read data from storage to a slow cache tier of the multi-tier cache, for one or more sectors for the second subset of sectors identified by one or more track holes.

A computer system, comprising:

• a host having a processor,
• a data storage system having a processor, a memory having a plurality of prestage request queues, storage and a multi-tier cache having a fast cache tier and a slow cache tier;
• and
• a computer program product comprising a computer readable storage medium having program instructions embodied therewith, the program instructions executable by a processor of the computer system to cause computer system operations, the computer system operations comprising:
• storing in storage, a track having a set of sectors of data;
• caching update data for the track in the fast cache tier of the multi-tier cache in a first track write caching operation which includes caching update data for a first subset of sectors of the track in the fast cache tier and omitting caching data for a second subset of sectors of the track in the fast cache tier to define upon completion of the first track write caching operation, a number of track holes equal to zero or more track holes of the track in the fast cache tier in which a track hole corresponds to one or more contiguous sectors of the second subset of sectors of the track for which data caching is omitted during the first track write caching operation,
• determining the number of track holes defined by the second subset of sectors omitted from the first track write caching operation;
• and
• queuing prestage requests in one of the plurality of prestage request queues as a function of the number of track holes determined to be present in the second subset of sectors absent from the fast cache tier, wherein each prestage request when executed prestages read data from storage to the slow cache tier of the multi-tier cache, for one or more sectors for the second subset of sectors identified by one or more track holes.