Maintaining duplex-paired storage devices during gap processing using of a dual copy function

In a data processing system of the type comprising a host computer having a plurality of input/output channels, a direct access storage device controller connected to input/output channels of said host computer, and two strings of direct access storage devices connected to said direct access storage device controller, said direct access storage device controller having a memory and channel switches and storage paths, said channel switches and storage paths establishing paths between said connected input/output channels and said direct access storage devices, each direct access storage device in each of said strings having rotating recording media characterized by having a plurality of tracks on which are recorded a plurality of records, each record containing a count field and a plurality of additional fields, said fields being separated by small gaps, each record being identified by a cell number which designates an exact location of the count field for that record on a track, said direct access storage devices further including a head positioned by an actuator responsive to said direct access storage device controller for reading and writing data of said records in response to requests from said host computer, the improvement in said direct access storage device control whereby duplex-paired storage devices are maintained during gap processing by a dual copy function performed by said direct access storage device controller transparently to said host computer, said direct access storage device further comprising:

• means for selecting two of said direct access storage devices as said duplex-paired storage devices, a first one of said direct access storage devices being designated as a primary storage device and a second one of said direct access storage devices being designated as a secondary storage device, said primary and secondary storage devices being physically separated but substantially identical in configuration and type;
• means for synchronizing said secondary storage device with said primary storage device;
• a plurality of bits of each of said cell numbers of records on said primary storage device being designated to indicate a status of a corresponding one of said count field and said plurality of fields which comprise each of said records as modified or unmodified as a result of a write operation to a record on said primary storage device;
• means for setting the corresponding one of said plurality of bits to indicate a modified status when one of said count field or said plurality of fields which comprise a record is modified by a write operation to said record on said primary storage device;
• means for storing modified records in said memory;
• said head accessing the records on said primary storage device in sequence and means responsive to records accessed by said head for searching for a count field for each of said records;
• when a count field for a record is encountered by said means for searching, said means for searching further saving in said memory the cell number from the count field for the record;
• means for identifying by cell number saved in memory each record on said primary storage device containing a field that has been modified as indicated by one of said plurality of bits having been set, said step of identifying being performed during time periods corresponding to said gaps between fields;
• and
• means for transferring from said memory to said secondary storage device only those fields updated on said primary storage device in said identifying step thereby maintaining on said secondary storage device an exact duplicate of records stored on said first storage device without degrading the performance of said data processing system.