Read/write head buffer
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to application Ser. No. 304,788 now abandoned entitled Track Image Read/Write Head Buffer and application Ser. No. 304,664 now abandoned entitled Address Mark Triggered Read/Write Head Buffer, both filed on the same date as this application. 
     FIELD OF THE INVENTION 
     This invention relates to data storage systems and, in particular, to a data buffer that is used in a rotating media data storage system to improve the data transfer performance of the data storage system. 
     BACKGROUND OF THE INVENTION 
     It is a problem in the field of data storage systems to minimize the data retrieval time when reading a data record from a data storage system. In disk drive memory systems for example, a processor is typically connected by a channel interface unit to a plurality of data channels. Each data channel is connected to one or more Direct Access Storage Device (DASD) units which function to store large quantities of data. Each DASD unit includes a control unit for interfacing with the data channel that typically carries eight bit parallel data in a byte serial decoded input/output record format. The control unit performs a data management function to maintain records of the location of all the data stored in the DASD unit. The DASD unit includes rotating data storage media consisting of a plurality of disks, each of which has associated therewith one or more moveable read/write heads. The data is stored on the rotating media in a track format that consists of a plurality of concentric rings of data. A control module is also included at the head of a string of disk drive units and connected to the control unit to convert between the eight bit parallel data format of the data channel and the bit serial data format of the rotating media. In addition, the control module controls data formatting and error correction code generation. The control module is connected by way of a bus to a plurality of read/write heads attached to an actuator, each of which serves to read and write data on an associated disk of the rotating media. 
     In a data record read operation, the processor transmits a seek request, through the channel interface unit and an available data channel, to the DASD control unit associated with the rotating media on which the requested data record is stored. The processor, upon completion of the transmission of the seek request, returns to processing other tasks. The DASD control unit responds to the seek request by determining the physical location of the requested data record on the plurality of disks. The control unit transmits information to the associated control module identifying the physical location and size of the data record in order to retrieve the requested data record from one of the disks of the rotating media. Since the data record is stored in a track format on a rotating media, the actuator associated with the identified disk of the rotating media on which the requested data record is stored must wait until the rotating media rotates a sufficient distance to present the beginning of the requested data record underneath the read/write head associated with the actuator. 
     A predetermined time before the beginning of the requested data record reaches the read/write head associated with the actuator, the control module requests the control unit to obtain a data communication path to the processor in order to transmit the retrieved data record from the rotating media directly to the processor over this data communication path. If a data communication path to the processor is not available, the control module must wait for one entire revolution of the rotating media before the media is again in the position where the requested data record is a sufficient distance away from the read/write head to establish a data communication path to the processor. This process is repeated until a data communication path to the processor is available and the data, as read by the read/write head, can be transmitted directly via the control module and control unit and an available data channel to the processor. It is obvious that there can be numerous delays in this data storage system while data communication paths are established. In a transaction based system, where there are a multitude of random data seeks, such delays can unnecessarily tie up actuators in the disk drive unit and significantly increase the system response time. 
     SUMMARY OF THE INVENTION 
     The above described problems are solved and a technical advance achieved in the field by the read/write head buffer that improves data transfer performance in a rotating media data storage system by rendering the data read operation at the read/write head independent of the control module, control unit and the availability of the data communication path from the control unit to the processor. This is accomplished by providing a buffer memory for each read/write head in the rotating media data storage system so that a seek request from the processor can be handled as soon as the beginning of the requested data record is positioned below the read/write head of the associated actuator. The data record is read from the rotating media by the read/write head and stored in the read/write head buffer independent of the availability of a data communication path to the processor. Thus, the read data record operation need not be synchronized with the availability of a data communication path to the processor. 
     The data record read by the read/write head is stored in the read/write head buffer in two, seven run length limited (RLL) self clocking code format with the error correction code bits appended to the data record. The read/write head buffer does not process the image read from the rotating media but instead stores this image for subsequent deformatting and processing by the control module. The read/write head buffer includes a phase locked loop to maintain signal clocking independent of the control module. By providing the read/write head buffer on a one per actuator basis, the response time of the rotating media data storage system for a memory access is significantly reduced since the requested data record is retrieved from the rotating media as soon as the data record is properly positioned and in no case will this time be greater than one revolution of the rotating media. 
     The error correction codes written on the rotating media to protect the integrity of the requested data record are maintained since they are stored in the read/write head buffer along with the data record. Thus, once a data communication path is established to the processor, the control module receives the image that was stored on the rotating media from the read/write head buffer and can deformat this data from the bit serial, run length limited self clocking coding with error correction characters format of this data and convert this information to eight bit parallel data that is in an input/output record format for use by the control unit. The control unit stores the decoded error checked data record and transmits the requested data record to the processor over the available data channel if a full track buffer is present in the cache. If the cache is not equipped with a full track buffer, only the count and key records are buffered. In this fashion, the read/write head buffer enables the processor to effectively &#34;start&#34; and &#34;stop&#34; the rotating media to obtain data stored thereon. While the rotation of the rotating media is not interrupted, the use of the read/write head buffer enables the processor to have access to a data record independent of the operation of the rotating media. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 illustrates the overall architecture of the read/write head buffer as placed in a data processing environment; 
     FIG. 2 illustrates additional circuit details of the read/write head buffer. 
    
    
     DETAILED DESCRIPTION 
     In computer systems, a processor is typically connected by a channel interface unit to a plurality of data channels. Each data channel is connected to one or more Direct Access Storage Device (DASD) units which function to store large quantities of data. These DASD units typically use rotating data storage media comprised of either optical disks or magnetic disks to store data records thereon. 
     A typical DASD unit is the IBM 3380-type disk drive unit that uses magnetically readable/writable disks as the data storage media. Each of these 3380-type DASD units includes a control unit for interfacing with a data channel to the processor that typically carries eight bit parallel data in a byte serial decoded input/output record format. The control unit performs a data management function to maintain records of the physical location of all the data stored on the DASD unit. The DASD unit includes rotating data storage media typically consisting of a plurality of magnetically readable/writable disks, each of which has associated therewith one or more moveable read/write heads. The data is stored on the rotating media in a track format that consists of a plurality of concentric rings of data. A control module is also included at the head of a string disk drive units and connected to the control unit to convert between the eight bit parallel data format of the data channel and the bit serial data format of the rotating media. In addition, the control module controls data formatting and error correction code generation. The control module is connected by way of a bus to a plurality of read/write heads attached to an actuator, each of which serves to read and write data on an associated disk of the rotating media. 
     Data Record Read Operation 
     In a data record read operation, the processor transmits a seek request, through the channel interface unit and an available data channel, to the DASD control unit associated with the rotating media on which the requested data record is stored. The processor, upon completion of the transmission of the seek request, returns to processing other tasks. The DASD control unit responds to the seek request by determining the physical location of the requested data record on the plurality of disks. The control unit transmits information to the associated control module identifying the physical location and size of the data record in order to retrieve the requested data record from one of the disks of the rotating media. Since the data record is stored in a track format on a rotating media, the actuator associated with the identified disk of the rotating media on which the requested data record is stored must wait until the rotating media rotates a sufficient distance to present the beginning of the requested data record underneath the read/write head associated with the actuator. 
     A predetermined time before the beginning of the requested data record reaches the read/write head associated with the actuator, the control module requests the control unit to obtain a data communication path to the processor in order to transmit the retrieved data record from the rotating media directly to the processor over this data communication path. If a data communication path to the processor is not available, the control module must wait for one entire revolution of the rotating media before the media is again in the position where the requested data record is a sufficient distance away from the read/write head to establish a data communication path to the processor. This process is repeated until a data communication path to the processor is available and the data, as read by the read/write head, can be transmitted directly via the control module and control unit and an available data channel to the processor. It is obvious that there can be numerous delays in this data storage system while data communication paths are established. In a transaction based system where there are a multitude of random data seeks, such delays can unnecessarily tie up actuators in the disk drive unit and significantly increase the system response time. 
     The read/write head buffer improves data transfer performance in a rotating media data storage system by rendering the data read operation at the read/write head independent of the control module, control unit and the availability of the data communication path from the control unit to the processor. This is accomplished by providing a buffer memory for each read/write head in the rotating media data storage system so that a seek request from the processor can be handled as soon as the beginning of the requested data record is positioned below the read/write head of the associated actuator. The data record is read from the rotating media by the read/write head and stored in the read/write head buffer independent of the availability of a data communication path to the processor. Thus, the read data record operation need not be synchronized with the availability of a data communication path to the processor. 
     The data record read by the read/write head is stored in the read/write head buffer in run length limited self clocking code format (such as 2, 7 coding) with the error correction code bits appended to the data record. The read/write head buffer does not process the image read from the rotating media but instead temporarily stores this image for subsequent deformatting and processing by the control module. The read/write head buffer includes a phase locked loop to maintain signal clocking independent of the control module. By providing the read/write head buffer on a one per actuator basis, the response time of the rotating media data storage system for a memory access is significantly reduced since the requested data record is retrieved from the rotating media as soon as the data record is properly positioned and in no case will this time be greater than one revolution of the rotating media. 
     The error correction codes written on the rotating media to protect the integrity of the requested data record are maintained since they are stored in the read/write head buffer along with the data record. Thus, once a data communication path is established to the processor, the control module receives the image that was stored on the rotating media from the read/write head buffer and can deformat this data from the bit serial, run length limited self clocking (two, seven RLL) coding with error correction characters format of this data and convert this information to eight bit parallel data that is in an input/output record format for use by the control unit. The control unit stores the decoded error checked data record and transmits the requested data record to the processor over the available data channel. In this fashion, the read/write head buffer enables the processor to effectively &#34;start&#34; and &#34;stop&#34; the rotating media to obtain data stored thereon. While the rotation of the rotating media is not interrupted, the use of the read/write head buffer enables the processor to have access to a data record independent of the operation of the rotating media. 
     System Architecture 
     FIG. 1 illustrates the overall architecture of a data processing system that is equipped with an associated data storage system that includes a plurality of memory devices, one of which is illustrated in FIG. 1 in the form of a conventional 3380-type of disk drive unit 120. Processor 100 is connected via bus 101 to a channel interface unit 102 that serves to interconnect processor 100 with a plurality of data channels 103-1 to 103-n, each of which is an eight bit parallel data bus. Connected to one of these data channels 103-1 is the disk drive unit 120 mentioned above. 
     Disk drive unit 120 includes a control unit 104 that functions to provide overall management of the disk drive unit 120. This management function includes storing information that identifies the exact physical location of all data stored on the rotating media 111-0 to 111-15 of disk drive unit 120. Processor 100 identifies a data record by volume identification and address. While this information provides a general physical location of the data, the exact storage location on the rotating media 111-0 to 111-15 requires additional specificity. Control unit 104 provides this additional specific information by converting the volume and address information received from the processor into head, actuator, track and sector identification information to precisely define the physical location of the requested data record on disk drive unit 120. 
     The control unit 104 transfers data between data channel 103-1 and one or more control modules (ex. 106) in an eight bit parallel data format wherein each eight bits of data comprise one of a series of bits of the data record in a decoded input/output record format. The data in this format is converted by control module 106 into a bit serial format wherein error correction code characters are appended thereto for error detection and control purposes. The resultant data is then encoded into two, seven code. The control module 106 responds to the control signals transmitted by control unit 104 identifying the head, actuator, track and sector for storing a data record by selecting one of the actuators 110-0 to 110-15 that are used by disk drive unit 120 to read/write data on the rotating media 111-0 to 111-15. While sixteen actuators are illustrated herein, it is expected that future devices may be equipped with 32 or more actuators. The actuators include the read/write head and signal control circuitry for reading and writing the data on the rotating media 111-0 to 111-15. 
     The above-described conventional disk drive unit 120 of FIG. 1 is also equipped with a plurality of read/write head buffers 108-0 to 108-15 on a one per read/write head basis. The read/write head buffer 108-0, for example, is interposed between bus 107 and read/write head 110-0. The exact physical implementation of this read/write head buffer can also be a single memory connected to bus 107 and partitioned into segments, one segment for each read/write head 108 in disk drive unit 120. Read/write head buffer 108-0 serves to temporarily store the track image captured by the read/write head 110-0. 
     Read/Write Head Buffer Circuit 
     FIG. 2 illustrates additional detail of read/write head buffer 108-0. Data is read in analog form by read/write circuit 201 of read/write head 110-0 and converted into a digital signal that is typically self clocked. The digital data signal is transmitted by read/write circuit 201 on lead 202 to read/write head buffer 108-0 where it is applied to phase locked loop 203 and address mark detect circuit 214. The address mark detect circuit 214 monitors the digital data signal to identify the presence of an address mark that precedes the count field in each data record stored on the media 111-0. Once address mark detect circuit 214 verifies the presence of the address mark, phase locked loop 203 is activated to separate the data from the clock in the digital data signal and applies the data via lead 215 and the clock via lead 204 to shift register 205. The clock and data signals on leads 204 and 215 enable shift register 205 to receive the serial bits of data and store these data bits in its memory. Each 16 bit byte of 2, 7 encoded data that is stored in shift register 205 is transmitted over bus 206 in parallel form to buffer 207. The addressing of buffer 207 is accomplished by the use of the disk drive servo 208 which identifies the sector count of the rotating media in well-known fashion and outputs this information on lead 210 to control logic 211. The sector count is converted by control logic 211 to a pointer value that is output on lead 212 to address buffer 207. The clock signal from phase locked loop 203 is used to maintain clock timing synchronization to load data into buffer 207. Thus, the digital data signal that is read from the rotating media is maintained in the proper clocking synchronization by the use of phase locked loop 203 so that the data stored via shift register 205 in buffer 207 accurately represents the data stored on the track of the rotating media 111-0. 
     The accuracy of this data stored in buffer 207 is insured because this data along with its associated error correction code bits is stored in a two, seven code format. The entire track image is thereby temporarily stored in buffer 207 for transmission to control module 106 via shift register 205 and bus 107. Read/write head buffer 108-0 therefore appears transparent to control module 106 in that the output of read/write head buffer 108-0 is identical to the signal output by read/write head 110-0 except for its lack of synchronization with the rotating media 111-0. Control module 106 resynchronizes the data obtained from read/write head buffer 108-0, therefore any data storage errors occasioned by read/write head buffer 108-0 due to defects in the rotating media 111-0 or address marks are easily corrected by control module 106 by use of its own internal phase locked loop and error correction circuitry. Maintenance of the error correction codes through read/write head buffer 108-0 minimizes the possibility of an error occurring in the data transfer process. 
     Direct Transfer Mode 
     In a certain percentage of read data record operations, the data record need not be buffered since a data communication path from the control unit 104 to processor 100 is available when the data record is read from the rotating media 111-0 to 111-15. In this case, the retrieved data record is read on a bitwise basis from the rotating media 111-0 by read/write circuit 201 and stored on a bytewise basis in shift register 205. The retrieved data record is then transmitted in parallel via bus 107 to control module 106. Buffer 207 is therefore switchably connectable to bus 107 since it is used only when the data communication path from the control unit 104 to processor 100 is unavailable at the time of reading the data record from the rotating media 111-0. 
     While a specific embodiment of the present invention has been disclosed, it is expected that those skilled in the art can and will devise alternate embodiments that fall within the scope of the appended claims.