Raid system including first and second read/write heads for each disk drive

Claim 2

A storage system having one or more disks as data storage mediums, each including a plurality of data records comprising: a first head which reads data stored in a data record; a second head positionable over the data record from which said first head has read data and being spaced from said first head a distance sufficient to produce a time delay of a predetermined amount when said data record moves from a position under said first head to a position under said second head, said second head performing a write operation into the data record when said data record moves to a position under said second head; an actuator having said first head and said second head securely mounted thereon and performing a seek operation; and an updated data generator means for causing said first head to read said data stored in the data record, for generating new data from the data thus read out, and for causing said second head to write the new data into said data record when said data record moves sequentially from a position under said first head to a position under said second head.

Claim 3

A storage system according to claim 1, wherein said first head and said second head are secured to said actuator so that a gap of more than one record length exists between said first and second heads.

Claim 4

A storage system according to claim 1, wherein said time delay equals a time period required for a length of the data record to pass under said first head plus an additional time period equal to a time required for an error correction circuit verify data read by said first head.

Claim 5

A storage system according to claim 2, wherein said gap is subtantially equal to said one record length plus a length corresponding to a time required for an error correction circuit to verify validity of data read by said first head.

Claim 6

In a storage system connected to a processing device for storing data used by the processing device, said storage system comprising: a first disk drive including: a disk having a plurality of records containing data, a first head which reads data held in a record on the disk, and a second head that positions itself over said record read by said first head after a time delay with respect to the positioning of the first head and writes data into said record; a second disk drive including: a disk having a plurality of records wherein at least one record contains redundant data corresponding to the data in a same parity group of disk drives other than said second disk drive, a third head to read redundant data held in a record on the disk, and a fourth head that positions itself over said same record read by said third head after a time delay with respect to the positioning of the third head; a data transfer apparatus which transfers data to and from the processing device; and a controller which, when storing the data received from the data transfer apparatus into a record in the first disk drive, uses the first head of the first disk drive and reads old data stored in a record into which the received data is to be written and also old redundant data stored in a record in the second disk drive and corresponding to the old data, generates new redundant data from the received data, the old data and the old redundant data controls the first disk drive to write the received data into the record in which the old data was stored when the second head first positions itself on the record after the old data was read out, and at the same time controls the second disk drive to write the new redundant data into the record in which the old redundant data was stored when the fourth head first positions itself on the record after the old redundant data was read out.

Claim 7

In a storage system connected to a processing device for storing data used by the processing device, said storage system comprising: a first disk drive including: a disk having a plurality of records containing data, a first head which reads data held in a record on the disk, and a second head that positions itself over said record read by said first head after a time delay with respect to the positioning of the first head and writes data into said record; a second disk drive including: a disk having a plurality of records wherein at least one record contains redundant data corresponding to the data in a same parity group of disk drives other than said second disk drive, a third head to read redundant data held in a record on the disk, and a fourth head that positions itself over said same record read by said third head after a time delay with respect to the positioning of the third head; a data transfer apparatus which transfers data to and from the processing device; and a controller which, when storing the data received from the data transfer apparatus into a record in the first disk drive, uses the first head of the first disk drive and reads old data stored in a record into which the received data is to be written and also old redundant data stored in a record in the second disk drive and corresponding to the old data, generates new redundant data from the received data, the old data and the old redundant data controls the first disk drive to write the received data into the record in which the old data was stored when the second head first positions itself on the record after the old data was read out, and at the same time controls the second disk drive to write the new redundant data into the record in which the old redundant data was stored when the fourth head first positions itself on the record after the old redundant data was read out.

Claim 8

A storage system according to claim 5, wherein said first disk drive has a single actuator that has said first and second heads securely mounted thereon and performs a seek operation, and said second disk drive has a single actuator that has said third and fourth heads securely mounted thereon and performs a seek operation.

Claim 9

A storage system according to claim 5, wherein said first disk drive has a first actuator, which has said first head securely mounted thereon and performs a seek operation, and a second actuator, which has said second head securely mounted thereon and performs a seek operation.

Claim 10

A storage system according to claim 5, wherein when writing the received data, said controller controls said first and second actuators so that said first and second heads are positioned over the same track on the disk that contains the record into which the received data is to be written.

Claim 11

A storage system according to claim 5, wherein the second disk drive has a third actuator having said third head securely mounted thereon and a fourth actuator having said fourth head securely mounted thereon.

Claim 12

A storage system according to claim 9, wherein when writing the received data, said controller controls said third and fourth actuators so that said third and fourth heads are positioned over the same track on the disk that contains the record into which the received data is to be written.

Claim 13

A storage system according to claim 5, wherein said controller comprises: differential data generation apparatus which generates, from the received data and the old data, differential data used to generate new redundant data; and redundant data generation apparatus which generates new redundant data from the differential data produced by the differential data generation means and the old redundant data.

Claim 14

A storage system according to claim 11, wherein the differential data generation apparatus has apparatus which obtains an exclusive-OR of the received data and the old data, and the redundant data generation apparatus has apparatus which obtains an exclusive-OR of the differential data and the old redundant data.

Claim 15

A storage system according to claim 11, wherein said redundant data generation apparatus is provided in said second disk drive.

Claim 16

A storage system according to claim 5, wherein at least a part of said controller is included in said first and the second disk drives.

Claim 17

A storage system according to claim 5, wherein said controller includes redundant data generation apparatus which generates the new redundant data from the received data, the old data and the old redundant data.

Claim 18

A storage system according to claim 15, wherein said redundant data generation apparatus is provided in the second disk drive.

Claim 19

A storage system according to claim 5 to claim 14, wherein said first disk drive and said second disk drive each has a plurality of disk drives.

Claim 20

A storage system according to claim 5, wherein an interval of more than one record length is provided between said first head and said second head and between said third head and said fourth head.

Claim 21

A storage system according to claim 18, wherein said first disk drive has a fifth head used to position said second head over the record read by said first head, and said second disk drive has a sixth head used to position said fourth head over the record read by said third head.

Claim 22

A storage system according to claim 19, wherein said fifth and sixth heads are dedicated for reading.

Claim 23

A storage system connected to a processing device for storing data used by the processing device, said storage system comprising: a first disk drive including: a disk having a plurality of records containing data, a first head which reads data held in a record on the disk, and a second head that positions itself over said record read by said first record after a time delay with respect to the positioning of said first head and writes data into said record; a second disk drive including: a disk having a plurality of records wherein at least one record contains redundant data corresponding to the data held in a same parity group of disk drives other than said second disk drive, a third head which reads redundant data held in a record on the disk; and a fourth head that positions itself over said record read by said third head with a time delay with respect to said third head; a data transfer apparatus which transfers data to and from the processing device; a first controller which, through the first head of the first disk drive, reads old data stored in a record into which the received data is to be written when the data apparatus receives from the processing device the data to be written into the first disk drive, and which writes the received data into the record in which the old data was stored when the second head is first positioned on the record after the old data has been read out; a second controller which reads old redundant data stored in a record in the second disk drive and corresponding to the old data when the data transfer apparatus receives from the processing device the data to be written into the first disk drive, and which writes new redundant data into the record when the fourth head is first positioned on the record in which the old redundant data was stored; and apparatus which generates new redundant data from the received data, the old data read out by the first controller, and the old redundant data read out by the second controller.

Claim 24

A storage system connected to a processing device for storing data used by the processing device, said storage system comprising: a first disk drive including: a disk having a plurality of records containing data, a first head which reads data held in a record on the disk, and a second head that positions itself over said record read by said first record after a time delay with respect to the positioning of said first head and writes data into said record; a second disk drive including: a disk having a plurality of records wherein at least one record contains redundant data corresponding to the data held in a same parity group of disk drives other than said second disk drive, a third head which reads redundant data held in a record on the disk; and a fourth head that positions itself over said record read by said third head with a time delay with respect to said third head; a data transfer apparatus which transfers data to and from the processing device; a first controller which, through the first head of the first disk drive, reads old data stored in a record into which the received data is to be written when the data apparatus receives from the processing device the data to be written into the first disk drive, and which writes the received data into the record in which the old data was stored when the second head is first positioned on the record after the old data has been read out; a second controller which reads old redundant data stored in a record in the second disk drive and corresponding to the old data when the data transfer apparatus receives from the processing device the data to be written into the first disk drive, and which writes new redundant data into the record when the fourth head is first positioned on the record in which the old redundant data was stored; and apparatus which generates new redundant data from the received data, the old data read out by the first controller, and the old redundant data read out by the second controller.

Claim 25

A storage system according to claim 21, wherein said first controller is provided in said first disk drive.

Claim 26

A storage system according to claim 21, wherein said second controller is provided in said second disk drive.

Claim 27

A storage system according to claim 21, wherein said apparatus which generates the redundant data is provided in said second disk drive.

Claim 28

In a storage system connected to a processing device for storing data used by the processing device, said storage system comprising: a first disk drive including: a disk having a plurality of records containing data, a first head which reads data held in a record on the disk, and a second head that positions itself over said record read by said first head after a time delay with respect to the positioning of the first head and writes data into said record; a second disk drive including: a disk having a plurality of records wherein at least one record contains redundant data corresponding to the data held in a same parity group of disk drives other than said second disk drive, a third head to read redundant data held in a record on the disk, and a fourth head that positions itself over said record read by said third head after a time delay with respect to said positioning of said third head; data transfer means to transfer data to and from said processing device; a first controller which, through said first head of the first disk drive, reads old data stored in a record into which the received data is to be written when the data transfer apparatus receives from the processing device the data to be written into said first disk drive, and writes the received data into the record in which the old data was stored when said second head is first positioned on the record after the old data has been read out; a differential data generation apparatus which generates differential data from the old data read out by the first control means and the received data; a second controller which reads old redundant data stored in a record in the second disk drive and corresponding to the old data when the data transfer apparatus receives from the processing device the data to be written into the first disk drive, generates new redundant data from the differential data generated by the differential data generation apparatus and the old redundant data, and which writes the new redundant data into the record when the fourth head is first positioned on the record in which the old redundant data was stored.

Claim 29

In a storage system connected to a processing device for storing data used by the processing device, said storage system comprising: a first disk drive including: a disk having a plurality of records containing data, a first head which reads data held in a record on the disk, and a second head that positions itself over said record read by said first head after a time delay with respect to the positioning of the first head and writes data into said record; a second disk drive including: a disk having a plurality of records wherein at least one record contains redundant data corresponding to the data held in a same parity group of disk drives other than said second disk drive, a third head to read redundant data held in a record on the disk, and a fourth head that positions itself over said record read by said third head after a time delay with respect to said positioning of said third head; data transfer means to transfer data to and from said processing device; a first controller which, through said first head of the first disk drive, reads old data stored in a record into which the received data is to be written when the data transfer apparatus receives from the processing device the data to be written into said first disk drive, and writes the received data into the record in which the old data was stored when said second head is first positioned on the record after the old data has been read out; a differential data generation apparatus which generates differential data from the old data read out by the first control means and the received data; a second controller which reads old redundant data stored in a record in the second disk drive and corresponding to the old data when the data transfer apparatus receives from the processing device the data to be written into the first disk drive, generates new redundant data from the differential data generated by the differential data generation apparatus and the old redundant data, and which writes the new redundant data into the record when the fourth head is first positioned on the record in which the old redundant data was stored.

Claim 30

A storage system according to claim 25, wherein said first controller is provided in said first disk drive.

Claim 31

A storage system according to claims 25 or 24, wherein said second controller is provided in said second disk drive.

Claim 32

In a storage system connected to a processing device to store data used by the processing device, the storage system comprising: a first disk drive including: a disk having a plurality of records containing data, a first head to read data held in a record on the disk, and a second head that positions itself on the same record with a time delay with respect to the first head and writes data into the record on the disk; a second disk drive including: a disk having a plurality of records containing redundant data corresponding to the data held in each record of the first disk drive, a third head to read redundant data held in a record on the disk, and a fourth head that positions itself on the same record with a time delay with respect to the third head; a data transfer means to transfer data to and from the processing device; and a control means which, when storing the data received from the data transfer means into a record in the first disk drive, uses the first head of the first disk drive and reads old data stored in a record into which the received data is to be written and also old redundant data stored in a record in the second disk drive and corresponding to the old data; which generates new redundant data from the received data, the old data and the old redundant data; which controls the first disk drive to write the received data into the record in which the old data was stored when the second head first positions itself on the record after the old data was read out; and which at the same time controls the second disk drive to write the new redundant data into the record in which the old redundant data was stored when the fourth head first positions itself on the record after the old redundant data was read out.

Claim 33

In a storage system connected to a processing device to store data used by the processing device, the storage system comprising: a first disk drive including: a disk having a plurality of records containing data, a first head to read data held in a record on the disk, and a second head that positions itself on the same record with a time delay with respect to the first head and writes data into the record on the disk; a second disk drive including: a disk having a plurality of records containing redundant data corresponding to the data held in each record of the first disk drive, a third head to read redundant data held in a record on the disk, and a fourth head that positions itself on the same record with a time delay with respect to the third head; a data transfer means to transfer data to and from the processing device; and a control means which, when storing the data received from the data transfer means into a record in the first disk drive, uses the first head of the first disk drive and reads old data stored in a record into which the received data is to be written and also old redundant data stored in a record in the second disk drive and corresponding to the old data; which generates new redundant data from the received data, the old data and the old redundant data; which controls the first disk drive to write the received data into the record in which the old data was stored when the second head first positions itself on the record after the old data was read out; and which at the same time controls the second disk drive to write the new redundant data into the record in which the old redundant data was stored when the fourth head first positions itself on the record after the old redundant data was read out.

Claim 34

In a storage system connected to a processing device to store data used by the processing device, the storage system comprising: a first disk drive including: a disk having a plurality of records containing data, a first head to read data held in a record on the disk, and a second head that positions itself on the same record with a time delay with respect to the first head and writes data into the record on the disk; a second disk drive including: a disk having a plurality of records containing redundant data corresponding to the data held in each record of the first disk drive, a third head to read redundant data held in a record on the disk, and a fourth head that positions itself on the same record with a time delay with respect to the third head; a data transfer means to transfer data to and from the processing device; a first control means which, through the first head of the first disk drive, reads old data stored in a record into which the received data is to be written when the data transfer means receives from the processing device the data to be written into the first disk drive; and which writes the received data into the record in which the old data was stored when the second head is first positioned on the record after the old data has been read out; a second control means which reads old redundant data stored in a record in the second disk drive and corresponding to the old data when the data transfer means receives from the processing device the data to be written into the first disk drive; and Which writes new redundant data into the record when the fourth head is first positioned on the record in which the old redundant data was stored; and a means to generate new redundant data from the received data, the old data read out by the first control means, and the old redundant data read out by the second control means.

Claim 35

In a storage system connected to a processing device to store data used by the processing device, the storage system comprising: a first disk drive including: a disk having a plurality of records containing data, a first head to read data held in a record on the disk, and a second head that positions itself on the same record with a time delay with respect to the first head and writes data into the record on the disk; a second disk drive including: a disk having a plurality of records containing redundant data corresponding to the data held in each record of the first disk drive, a third head to read redundant data held in a record on the disk, and a fourth head that positions itself on the same record with a time delay with respect to the third head; a data transfer means to transfer data to and from the processing device; a first control means which, through the first head of the first disk drive, reads old data stored in a record into which the received data is to be written when the data transfer means receives from the processing device the data to be written into the first disk drive; and which writes the received data into the record in which the old data was stored when the second head is first positioned on the record after the old data has been read out; a second control means which reads old redundant data stored in a record in the second disk drive and corresponding to the old data when the data transfer means receives from the processing device the data to be written into the first disk drive; and Which writes new redundant data into the record when the fourth head is first positioned on the record in which the old redundant data was stored; and a means to generate new redundant data from the received data, the old data read out by the first control means, and the old redundant data read out by the second control means.

Claim 36

In a storage system connected to a processing device to store data used by the processing device, the storage system comprising: a first disk drive including: a disk having a plurality of records containing data, a first head to read data held in a record on the disk; and a second head that positions itself on the same record with a time delay with respect to the first head and writes data into the record on the disk, a second disk drive including: a disk having a plurality of records containing redundant data corresponding to the data held in each record of the first disk drive, a third head to read redundant data held in a record on the disk, and a fourth head that positions itself on the same record with a time delay with respect to the third head; a data transfer means to transfer data to and from the processing device; a first control means which, through the first head of the first disk drive, reads old data stored in a record into which the received data is to be written when the data transfer means receives from the processing device the data to be written into the first disk drive, and which writes the received data into the record in which the old data was stored when the second head is first positioned on the record after the old data has been read out; a differential data generation means to generate differential data from the old data read out by the first control means and the received data; a second control means which reads old redundant data stored in a record in the second disk drive and corresponding to the old data when the data transfer means receives from the processing device the data to be written into the first disk drive; which generates new redundant data from the differential data generated by the differential data generation means and the old redundant data, and which writes the new redundant data into the record when the fourth head is first positioned on the record in which the old redundant data was stored.

Claim 37

In a storage system connected to a processing device to store data used by the processing device, the storage system comprising: a first disk drive including: a disk having a plurality of records containing data, a first head to read data held in a record on the disk; and a second head that positions itself on the same record with a time delay with respect to the first head and writes data into the record on the disk, a second disk drive including: a disk having a plurality of records containing redundant data corresponding to the data held in each record of the first disk drive, a third head to read redundant data held in a record on the disk, and a fourth head that positions itself on the same record with a time delay with respect to the third head; a data transfer means to transfer data to and from the processing device; a first control means which, through the first head of the first disk drive, reads old data stored in a record into which the received data is to be written when the data transfer means receives from the processing device the data to be written into the first disk drive, and which writes the received data into the record in which the old data was stored when the second head is first positioned on the record after the old data has been read out; a differential data generation means to generate differential data from the old data read out by the first control means and the received data; a second control means which reads old redundant data stored in a record in the second disk drive and corresponding to the old data when the data transfer means receives from the processing device the data to be written into the first disk drive; which generates new redundant data from the differential data generated by the differential data generation means and the old redundant data, and which writes the new redundant data into the record when the fourth head is first positioned on the record in which the old redundant data was stored.

Claim 38

In a storage system connected to a processing device for storing data used by the processing device, said storage system comprising: a first disk drive including: a disk having a data record containing data, a first head which reads data held in a record on the disk, and a second head that positions itself over said record read by said first head after a time delay with respect to the positioning of said first head and writes data into said record; a second disk drive including: a disk having a parity record containing redundant data corresponding to the data held in said data record of first disk drive, a third head which reads redundant data held in a record on the disk, and a fourth head that positions itself over said record read by said third head after a time delay with respect to the positioning of the third head; a data transfer apparatus which transfers data to and from the processing device; and a controller which, when storing the data received from the data transfer apparatus into a record in the first disk drive, uses the first head of the first disk drive and reads old data stored in a record into which the received data is to be written and also old redundant data stored in a record in the second disk drive and corresponding to the old data, generates new redundant data from the received data, the old data and the old redundant data controls the first disk drive to write the received data into the record in which the old data was stored when the second head first positions itself on the record after the old data was read out, and at the same time controls the second disk drive to write the new redundant data into the record in which the old redundant data was stored when the fourth head first positions itself on the record after the old redundant data was read out.

Claim 39

In a storage system connected to a processing device for storing data used by the processing device, said storage system comprising: a first disk drive including: a disk having a data record containing data, a first head which reads data held in a record on the disk, and a second head that positions itself over said record read by said first head after a time delay with respect to the positioning of said first head and writes data into said record; a second disk drive including: a disk having a parity record containing redundant data corresponding to the data held in said data record of first disk drive, a third head which reads redundant data held in a record on the disk, and a fourth head that positions itself over said record read by said third head after a time delay with respect to the positioning of the third head; a data transfer apparatus which transfers data to and from the processing device; and a controller which, when storing the data received from the data transfer apparatus into a record in the first disk drive, uses the first head of the first disk drive and reads old data stored in a record into which the received data is to be written and also old redundant data stored in a record in the second disk drive and corresponding to the old data, generates new redundant data from the received data, the old data and the old redundant data controls the first disk drive to write the received data into the record in which the old data was stored when the second head first positions itself on the record after the old data was read out, and at the same time controls the second disk drive to write the new redundant data into the record in which the old redundant data was stored when the fourth head first positions itself on the record after the old redundant data was read out.

Claim 40

A storage system connected to a processing device for storing data used by the processing device, said storage system comprising: a first disk drive including: a disk having a data record containing data, a first head which reads data held in a record on the disk, and a second head that positions itself over said record read by said first head after a time delay with respect to the positioning of said first head and writes data into said record; a second disk drive including: a disk having a parity record containing redundant data corresponding to the data held in said data record, a third head which reads redundant data held in a record on the disk; and a fourth head that positions itself over said record read by said third head with a time delay with respect to said third head; a data transfer apparatus which transfers data to and from the processing device; a first controller which, through the first head of the first disk drive, reads old data stored in a record into which the received data is to be written when the data apparatus receives from the processing device the data to be written into the first disk drive, and writes the received data into the record in which the old data was stored when the second head is first positioned on the record after the old data has been read out; a second controller which reads old redundant data stored in a record in the second disk drive and corresponding to the old data when the data transfer apparatus receives from the processing device the data to be written into the first disk drive, and writes new redundant data into the record when the fourth head is first positioned on the record in which the old redundant data was stored; and apparatus which generates new redundant data from the received data, the old data read out by the first controller, and the old redundant data read out by the second controller.

Claim 41

A storage system connected to a processing device for storing data used by the processing device, said storage system comprising: a first disk drive including: a disk having a data record containing data, a first head which reads data held in a record on the disk, and a second head that positions itself over said record read by said first head after a time delay with respect to the positioning of said first head and writes data into said record; a second disk drive including: a disk having a parity record containing redundant data corresponding to the data held in said data record, a third head which reads redundant data held in a record on the disk; and a fourth head that positions itself over said record read by said third head with a time delay with respect to said third head; a data transfer apparatus which transfers data to and from the processing device; a first controller which, through the first head of the first disk drive, reads old data stored in a record into which the received data is to be written when the data apparatus receives from the processing device the data to be written into the first disk drive, and writes the received data into the record in which the old data was stored when the second head is first positioned on the record after the old data has been read out; a second controller which reads old redundant data stored in a record in the second disk drive and corresponding to the old data when the data transfer apparatus receives from the processing device the data to be written into the first disk drive, and writes new redundant data into the record when the fourth head is first positioned on the record in which the old redundant data was stored; and apparatus which generates new redundant data from the received data, the old data read out by the first controller, and the old redundant data read out by the second controller.

Claim 42

In a storage system connected to a processing device for storing data used by the processing device, said storage system comprising: a first disk drive including: a disk having a data record containing data, a first head which reads data held in a record on the disk, and a second head that positions itself over said record read by said first head after a time delay with respect to the positioning of said first head and writes data into said record; a second disk drive including: a disk having a parity record containing redundant data corresponding to the data held in a said parity data record, a third head to read redundant data held in a record on the disk, and a fourth head that positions itself over said record read by said third head after a time delay with respect to said positioning of said third head; data transfer means to transfer data to and from said processing device; a first controller which, through said first head of the first disk drive, reads old data stored in a record into which the received data is to be written when the data transfer apparatus receives from the processing device the data to be written into said first disk drive, and writes the received data into the record in which the old data was stored when said second head is first positioned on the record after the old data has been read out; a differential data generation apparatus which generates differential data from the old data read out by the first control means and the received data; a second controller which reads old redundant data stored in a record in the second disk drive and corresponding to the old data when the data transfer apparatus receives from the processing device the data to be written into the first disk drive, generates new redundant data from the differential data generated by the differential data generation apparatus and the old redundant data, and which writes the new redundant data into the record when the fourth head is first positioned on the record in which the old redundant data was stored.

Claim 43

In a storage system connected to a processing device for storing data used by the processing device, said storage system comprising: a first disk drive including: a disk having a data record containing data, a first head which reads data held in a record on the disk, and a second head that positions itself over said record read by said first head after a time delay with respect to the positioning of said first head and writes data into said record; a second disk drive including: a disk having a parity record containing redundant data corresponding to the data held in a said parity data record, a third head to read redundant data held in a record on the disk, and a fourth head that positions itself over said record read by said third head after a time delay with respect to said positioning of said third head; data transfer means to transfer data to and from said processing device; a first controller which, through said first head of the first disk drive, reads old data stored in a record into which the received data is to be written when the data transfer apparatus receives from the processing device the data to be written into said first disk drive, and writes the received data into the record in which the old data was stored when said second head is first positioned on the record after the old data has been read out; a differential data generation apparatus which generates differential data from the old data read out by the first control means and the received data; a second controller which reads old redundant data stored in a record in the second disk drive and corresponding to the old data when the data transfer apparatus receives from the processing device the data to be written into the first disk drive, generates new redundant data from the differential data generated by the differential data generation apparatus and the old redundant data, and which writes the new redundant data into the record when the fourth head is first positioned on the record in which the old redundant data was stored.