Caching data in a storage system having multiple caches including non-volatile storage cache in a sequential access storage device

A computer program product for managing data in a sequential access storage device receiving read requests and write requests from a system with respect to tracks stored in a sequential access storage medium in the sequential access storage device, the computer program product comprising a non-transitory computer readable storage medium having computer readable program code embodied therein that executes to perform operations, the operations comprising:

• caching received modified tracks in a non-volatile storage device integrated in the sequential access storage device, wherein the non-volatile storage device is a faster access device than the sequential access storage medium;
• maintaining a spatial index indicating the modified tracks in the non-volatile storage device in an ordering based on their physical location in the sequential access storage medium;
• and
• destaging modified tracks from the non-volatile storage device by comparing a current position of a write head to physical locations of the modified tracks on the sequential access storage medium indicated in the spatial index to select a modified track to destage from the non-volatile storage device to the storage device.

A system in communication with a storage device, comprising:

• a first cache;
• a second cache;
• a processor;
• a computer readable storage medium having code executed by the processor to perform operations, the operations comprising:
• maintaining in the first cache modified and unmodified tracks to Input/Output (I/O) requests;
• demoting modified and unmodified tracks from the first cache;
• promoting the unmodified tracks demoted from the first cache to the second cache;
• destaging modified tracks demoted from the first cache to the storage device, bypassing the second cache;
• and
• discarding unmodified tracks demoted from the second cache.

A storage device, comprising:

• a sequential access storage medium;
• a non-volatile storage device;
• a computer readable storage medium having computer readable program code embodied therein that executes to perform operations, the operations comprising:
• caching received modified tracks in the non-volatile storage device integrated in the sequential access storage device, wherein the non-volatile storage device is a faster access device than the sequential access storage medium;
• maintaining a spatial index indicating the modified tracks in the non-volatile storage device in an ordering based on their physical location in the sequential access storage medium;
• and
• destaging modified tracks from the non-volatile storage device by comparing a current position of a write head to physical locations of the modified tracks on the sequential access storage medium indicated in the spatial index to select a modified track to destage from the non-volatile storage device to the storage device.