Patent Publication Number: US-6903892-B1

Title: Minimization of tape repositions using multiple read elements per track

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to accessing data stored on tape. 
   2. Background Art 
   Tape is increasingly used as a data storage media due to its high volumetric data density and its low cost per bits stored. Typically, data is written onto tape with a tape head having a plurality of write elements for simultaneously writing data tracks. The tape head typically includes a plurality of read elements for simultaneously reading these data tracks. One tape head configuration includes two modules, each module having a plurality of write elements and associated read elements. The modules are arranged such that, for a given tape direction over the access head, the read elements precede the write elements on one module and the write elements precede the read elements on the other module. This configuration supports read-after-write in either tape direction. 
   When a tape access system is unable to read data written on the tape, the tape is typically rewound and an attempt is made to read the data again. This attempt is successful in most cases. However, this recovery effort takes a significant amount of time. The tape drive is required to stop the tape, run the tape backwards a small distance, stop the tape again, and then start the tape in the original direction of motion. This repositioning interrupts operation and causes performance degradation of the tape access system. 
   What is needed is to decrease the number of repositions during read back mode. Preferably, any decrease in repositions will be accomplished without significantly increasing the tape access system complexity. 
   SUMMARY OF THE INVENTION 
   The present invention decreases the number of repositions during read back by making use of multiple read elements associated with each data track. 
   To this end, a tape access system is provided. A tape head has two read elements associated with each data track to be simultaneously read. Each read element generates a read signal in response to information recorded on the data track. Each of a plurality of sets of read electronics receives the read signal from either one or the other of the two read elements associated with each data track and converts the received read signal into read data. For each data track, control logic selects one of the two read elements as a source for output data. In response to a determination that a read error has occurred, the control logic selects the other of the two read elements as the source for output data. 
   In an embodiment of the present invention, the two read elements comprise a leading read element preceding a trailing element in the tape direction. The control logic selects the trailing read element prior to determining that a read error has occurred and selects the leading read element after determining that a read error has occurred. A buffer holds read data generated by the leading read clement. The control logic reads data from the buffer after determining that a read error has occurred. 
   In another embodiment of the present invention, the control logic selects the leading read element prior to determining that a read error has occurred and selects the trailing read element after determining that a read error has occurred. 
   In yet another embodiment of the present invention, each read element in the two read elements associated with each data track is offset relative to the other read element in an offset direction normal to the tape direction. A write element associated with one read element may be offset in the offset direction so as to be aligned in the offset direction with the other read element. A write element associated with the other read element may be offset in the offset direction so as to be aligned in the offset direction with the one read element. 
   A method of reading data written onto a plurality of data tracks is also provided. Each data track is accessed with a leading read element and a trailing read element. The leading read element accesses a particular location on the data track before the trailing read element accesses that particular location. Each read element generates a read signal. For each data track, output data is generated based on the read signals from the leading read element. If an error is determined, the output data is generated based on the read signals from the trailing read element. 
   A tape head for accessing a plurality of data tracks written along a length of a tape is also provided. The tape head includes a plurality of leading read elements, each leading read element accessing one data track, and a trailing read element corresponding to each leading read element. Each read element has a width less than the accessed data track width. Each trailing read element is offset across the width of the data track relative to the corresponding leading read element. 
   In an embodiment of the present invention, the tape head includes a plurality of write elements. Each write element is constructed as part of a leading read element and is offset across the width of the data track relative to the leading read clement by an amount substantially the same as the amount that the corresponding trailing read element is offset across the width of the data track relative to the leading read element. Similarly, a write element may be constructed as part of a trailing read element with the write element offset across the width of the data track relative to the trailing read element by an amount substantially the same as the amount that the corresponding leading read element is offset across the width of the data track relative to the trailing read element. 
   The above features, and other features and advantages of the present invention are readily apparent from the following detailed descriptions thereof when taken in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram illustrating tape access electronics that may be used with the present invention; 
       FIG. 2  is a schematic diagram of a tape access system according to an embodiment of the present invention; 
       FIG. 3  is a schematic diagram of a tape access system with buffered read elements according to an embodiment of the present invention; 
       FIG. 4  is a schematic diagram of a tape access system with a full set of primary read access elements and less than a full set of recovery read access elements according to an embodiment of the present invention; 
       FIG. 5  is a schematic diagram illustrating a tape access head having aligned elements that may be used with the present invention, 
       FIG. 6  is a schematic diagram illustrating a tape access head having offset read elements that may be used with the present invention; 
       FIG. 7  is a schematic diagram illustrating a tape access head with offset read and write elements that may be used with the present invention; and 
       FIG. 8  is a schematic diagram illustrating read access with a tape head having offset read and write elements. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1 , a block diagram illustrating tape access electronics that may be used with the present invention is shown. A tape access system, shown generally by  20 , processes data  22  from processor  24 . Data to be written onto tape  26  is received by data formatter  28  which properly formats data  22 . Formatting may include inserting special fields to identify each record as being unique, provide the record length, describe the compression status of the record, determine how data is to be spread across one or more tracks, and the like. Data formatter  28  may also perform block level error correction and detection. Formatted data is received by encoder  30  for further format conversion such as, for example, run length limited encoding. Encoder  30  produces write signal  32 . Write equalizer  34  modifies write signal  32  in a manner that enables more robust detection during subsequent read operations. Write driver  36  converts write equalized signals  38  into write current  40  having alternating polarities. Write element  42  converts write currents  40  into a write field which writes data patterns onto tape  26 . 
   Read element  44  generates read signals  46  in response to fields written on tape  26 . Preamp  48  amplifies read output signals  46 . If read element  44  is a magnetoresistive (MR) element, preamp  48  may also include a bias supply to provide the required bias for MR read element  44 . Read equalizer  50  shapes amplified read signals  46 . Shaping may include one or more of amplitude equalization as a function of frequency, pulse shaping to reduce the width of isolated pulses, low pass filtering to improve read signal-to-noise ratio, and the like. Automatic gain control (AGC)  52  normalizes the output of read equalizer  50 . Typical read elements  44  exhibit variations in output amplitude. Also, system variations such as write current  40  amplitude, spacing between elements  42 ,  44  and tape  26 , and the like contribute to amplitude variation. AGC  52  reduces these variations. Detector and clock recovery  54  accepts the magnitude normalized read signal and determines the location and time of recorded data transitions. Clock recovery determines the reference frequency and phase required to determine transition locations. Decoder  56  undoes the encoding performed by encoder  30 . Data formatter  28  accepts the unencoded data stream and formats the data for reception by processor  24 . The read electronics illustrated here, shown generally by  58 , are typical of read electronics used in tape access systems  20 . However, as will be recognized by one of ordinary skill in the art, the present invention applies to a wide variety of read electronics. 
   Referring now to  FIG. 2 , a schematic diagram of a tape access system according to an embodiment of the present invention is shown. Tape system  20  includes a tape head, shown generally by  70 , with leading module  72  and trailing module  74 . Leading module  72  precedes trailing module  74  when accessing tape moving past tape head  70  in tape direction  76 . Each module  72 ,  74 , includes a plurality of write elements  42  and read elements  44 . Typically, modules  72 ,  74  are arranged such that, for a given tape direction  76 , read elements  44  precede write elements  42  in one module  72  and read elements  44  follow write elements  42  in the other module  74 . Tape head  70  as illustrated includes read elements  44  and write elements  42  for accessing four data tracks simultaneously. As will be recognized by one of ordinary skill in the art, the number of data tracks simultaneously accessed may be varied within the scope of the present invention. 
   Read electronics  58  are illustrated for one data track. In an embodiment of the present invention, electronics  58  are duplicated for each data track. Electronics  58  for only one data track is shown for clarity. At least a portion of each set of electronics  58  is duplicated so that both leading read element  44   L  and trailing read element  44   T  are simultaneously supported. In the embodiment shown, read signals  46  from both read elements  44   L ,  44   T  are amplified and fed into multiplexer  78 . Select line  80  determines which input to multiplexer  78  is fed into electronics  82  which handle equalization, gain control, detection, decoding, formatting, and the like to generate output data  84 . Control logic  86  monitors output data  84  for errors that would otherwise require a rewind recovery. Control logic  86  instead switches multiplexer  78  to obtain output data  84  from the other read element. 
   During operation in one embodiment of the present invention, control logic  86  causes output data  84  to be generated by leading read element  44   L . When a read error occurs, control logic  86  switches multiplexer  78  to use trailing read element  44   T  as the source for output data  84 . Trailing read element  44   T  has the chance to re-read the problematic portion of tape  26  on the fly. Trailing read element  44   T  could continue to be the source of output data until the end of a file or until a reposition is required. At this point, control logic  86  switches back to leading read element  44   L . Tape head  70  is constructed such that leading read element  44   L  is separated from trailing read element  44   T  by a sufficient distance to allow error detection of output data  84  and subsequent switching of multiplexer  78 . This spacing depends on the format of data recorded, linear recording density, tape speed, geometry of tape head  70 , and the like. 
   Referring now to  FIG. 3 , a schematic diagram of a tape access system with buffered read elements according to an embodiment of the present invention is shown. In this embodiment, each read element  44  has its own substantially complete set of read electronics  58 . During operation, control logic  86  commands multiplexer  92  to select trailing read element  44   T  as the source for output data  84 . Meanwhile, data generated by leading read clement  44   L  is held in buffer  90 . If output data  84  from trailing read element  44   T  is determined to be erroneous, control logic  86  can splice in good data from buffer  90  read by leading read element  44   L . When trailing read elements  44   T  continue correctly reading the data stream, buffer  90  can be emptied and refilled with additional data from leading read element  44   L . Alternatively, buffer  90  can be implemented as a first in-first out buffer. 
   Referring now to  FIG. 4 , a schematic diagram of a tape access system with full set of primary read access elements and less than a full set of recovery read access elements according to an embodiment of the present invention is shown. The amount of electronics  82  required may be reduced by multiplexing readers  44  employed in error recovery. Typically, a defect will affect only a subset of data tracks that are simultaneously read by tape head  70 . Thus, information from only one or two channels is needed to avoid a rewind recovery. In the embodiment shown, each read element  44  in leading module  72  has a substantially complete set of electronics  82 . In contrast, read elements  44  in trailing module  74  have fewer sets of read electronics  82  than read elements  44 . Thus, if N is the number of data channels to be simultaneously read and y is the number of sets of electronics  82  supporting trailing module  74 , y can be expressed as 0&lt;y&lt;N. 
   Select line  80  for multiplexer  100  selects between read elements  44  in trailing module  74  to be serviced by electronics  82 . Control logic  102  which may be, for example, data formatter  28 , generates multiplexing control signal  80 . Since rewind avoidance is being performed by a subset of trailing readers  44 , control logic  102  can note which blocks are in error and select which channel from trailing module  74  to use in a recovery attempt. Control logic  102  generates output data  84  using read signals  46  from leading module  72  corrected by information from trailing module  74  as needed. 
   Referring now to  FIG. 5 , a schematic diagram illustrating a tape access head having aligned elements that may be used with the present invention is shown. Leading write element  42   L  includes long pole  110  and short pole  112 . The width of data track  114  written by write element  42   L  is determined by the width of short pole  112 . Similarly, trailing write element  42   T  is formed by long pole  116  and short pole  118 . Leading read element  44   L  includes MR sensor  120  shielded by long pole  110  and read shield  122 . Similarly, trailing read element  44   T  includes MR sensor  124  shielded by long pole  116  and read shield  126 . In a typical read head, each of elements  110 ,  112 ,  116 ,  118 ,  120 ,  122 ,  124 ,  126  is centered in a direction normal to tape direction  76  about centerline  128 . One difficulty with this arrangement is that any defect on data track  114  will be sensed by MR sensor  120  and MR sensor  124  in approximately the same manner. Construction of elements  110 ,  112 ,  116 ,  118 ,  120 ,  122 ,  124 ,  126  is well known in the art of tape access systems. 
   Referring now to  FIG. 6 , a schematic diagram illustrating a tape access head having offset read elements that may be used with the present invention is shown. In  FIG. 6 , data track  114  includes defect  130 . To minimize the effect of defect  130 , MR element  120  is offset across the width of data track  114  relative to read element  124 . Thus, the effect of defect  130  will be different on leading read element  44   L  than on trailing read element  44   T . Effectively offsetting read elements  44   L ,  44   T  increases the chance that one or the other of read elements  44   L ,  44   T  will generate data signal  46  free of error. 
   Referring now to  FIG. 7 , a schematic diagram illustrating a tape access head with offset read and write elements that may be used with the present invention is shown. Leading read element  44   L  and trailing write element  42   T  are centered about centerline  128   a . Trailing read element  44   T  and leading write element  42   L  are centered about centerline  128   b . Centerlines  128   a ,  128   b  are separated in a direction normal to tape direction  76  by separation distance d. 
   Referring now to  FIG. 8 , a schematic diagram illustrating read access with a tape head having offset read and write elements is shown. During read of data track  114 , leading read element  44   L  and trailing read element  44   T  are each offset from data track centerline  140  by one half of the separation distance between centerlines  128   a ,  128   b . This positioning keeps read elements  44   L ,  44   T  well within data track  114 . 
   While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.