Patent Publication Number: US-7589925-B1

Title: Method of accessing variable density data tracks in a disk drive

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 10/340,855 filed Jan. 10, 2003, which is a continuation of U.S. application Ser. No. 10/053,220 filed Jan. 17, 2002 now abandoned, both of which are incorporated by reference herein in their entireties. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to information storage on a storage media such as a disk in a disk drive. 
     BACKGROUND OF THE INVENTION 
     Data storage devices such as disk drives are used in many data processing systems. Typically a disk drive includes a magnetic data disk having disk surfaces with concentric data tracks, and a transducer head paired with each disk surface for reading data from and writing data to the data tracks. 
     Disk drive storage capacity increases by increasing the data density (or areal density) of the data stored on the disk surfaces. Data density is the linear bit density on the tracks multiplied by the track density across the disk surface. Data density is measured in bits per square inch (BPSI), linear bit density is measured in bits per inch (BPI) and track density is measured in tracks per inch (TPI). As data density increases, the head performance distribution also increases which diminishes disk drive storage capacity and yield. 
     Conventional disk drives fail to account for the different capabilities of the head and disk surface pairs. Conventionally, each disk surface is formatted to store the same amount of data as every other disk surface. However, each head and disk surface pair has unique data recording capability, such as sensitivity and accuracy, which depends on the fly height of the head over the disk surface, the magnetic properties of the head and the quality/distribution of the magnetic media for the disk surface. Thus, in conventional disk drives a head and disk surface pair that has a low error rate is formatted to the same BPI and TPI as a head and disk surface pair that has a high error rate. 
     Conventional disk drive manufacturing applies a single error rate and a single storage capacity for the head and disk surface pairs, and scraps disk drives that include a low performing head and disk surface pair that fails to meet the qualifying requirements. This lowers storage capacity due to inefficient use of high performing head and disk surface pairs that can store more data, and lowers yield due to disk drives being scrapped if they include a low performing head and disk surface pair even if they also include a high performing head and disk surface pair. 
     Conventional disk drives vary BPI to optimize the linear bit density capabilities of the heads. However, with increasing TPI it is difficult to control the head width relative to the shrinking track pitch. As a result, head yield and disk drive yield suffer. 
     There is, therefore, a need for storing data in a disk drive which improves storage capacity and yield and accounts for head performance variation. 
     SUMMARY OF THE INVENTION 
     The present invention uses vertical zoning to improve the performance, storage capacity and yield of data storage devices such as disk drives by optimizing the TPI and optionally BPI across multiple regions for each head/media pair. 
     In an embodiment, a data storage device has multiple storage media surfaces and corresponding heads, each storage media surface includes multiple regions, and each head is for recording on and playback of information from a corresponding storage media surface. A method of defining a storage format for the storage media surfaces includes reading data from each region on each storage media surface with the corresponding head, measuring a record/playback performance of each head for each region on each corresponding storage media surface based on the data read from the regions, and selecting a track density for each region on each storage media surface based on the measured record/playback performance of the corresponding head for the region. 
     In another embodiment, the regions on each storage media surface are concentric regions having an inner and an outer boundary at different radial locations on the storage media surface, each storage media surface includes the same number of regions and the boundaries of radially similarly situated regions on different storage media surfaces are at essentially the same radial locations. 
     In another embodiment, the record/playback performance of each head for each region is measured at multiple locations, at multiple read/write frequencies and/or at multiple track densities. For instance, the data is recorded in the region at a track density, the recorded data is read from the region, the measured error rate of the recorded data read from the region is compared to an acceptable error rate, and if the measured error rate is greater than the acceptable error rate then the previous steps are repeated for a decremented track density until the measured error rate is less than or equal to the acceptable error rate to provide a maximum recordable track density for the region. 
     In another embodiment, the record/playback performance of each head is a squeezed and unsqueezed off-track capability of the head. 
     In another embodiment, the track density is selected from a set of predetermined track densities or derived from an algorithm based on the location of the region on the storage media surface. 
     In another embodiment, servo tracks are written to each region and then the track density is selected for data tracks in each region. 
     In another embodiment, the data track density in a first region on a first storage media surface is greater than the data track density in a first region on a second storage media surface, the data track density in a second region on the first storage media surface is less than the data track density in a second region on the second storage media surface, the first regions are radially similarly situated regions and the second regions are radially similarly situated regions. 
     In another embodiment, the data tracks in a set of radially similarly situated regions are accessed by, for each of the regions, sequentially accessing each data track in the region before accessing data tracks in a subsequent region. 
     In another embodiment, the data storage device is a disk drive and each storage media surface is a disk surface. 
     Advantageously, the present invention increases the performance, storage capacity and yield of data storage devices such as disk drives having storage media surfaces such as disk surfaces. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects and advantages of the present invention will be better understood with reference to the following description, appended claims and accompanying figures where: 
         FIG. 1  shows a disk drive which includes multiple heads and disk surfaces; 
         FIG. 2  shows drive electronics in the disk drive; 
         FIG. 3  shows data tracks and servo tracks on a disk surface; 
         FIG. 4  shows a flow chart for defining a storage format for the disk surfaces; 
         FIG. 5  shows a flow chart for determining the storage capacity of the disk drive; 
         FIG. 6  shows a conventional storage format with fixed data track density and fixed servo track density in a zone on different disk surfaces and the corresponding heads; 
         FIG. 7  shows a conventional zone format on different disk surfaces and the corresponding heads; 
         FIG. 8  shows zones on a disk surface that each include virtual cylinders; 
         FIG. 9  shows a storage format with variable data track density and variable servo track density on different disk surfaces and the corresponding heads; 
         FIG. 10  shows a storage format with variable data track density and fixed servo track density within virtual cylinders and the corresponding heads; 
         FIG. 11  shows a storage format with variable data track density and fixed servo track density within virtual cylinders and the corresponding heads; 
         FIG. 12  shows access of data tracks on different disk surfaces in a conventional logical cylinder; 
         FIG. 13  shows access of data tracks on different disk surfaces in a virtual cylinder; 
         FIG. 14  shows access of data tracks on different disk surfaces in a virtual cylinder; and 
         FIG. 15  shows access of data tracks on different disk surfaces in a virtual cylinder. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Data storage devices used to store data for computer systems include, for example, disk drives, floppy drives, tape drives, optical and magneto-optical drives and compact drives. Although the present invention is illustrated by way of a disk drive, the present invention can be used in other data storage devices and other storage media, including non-magnetic storage media, as is apparent to those of ordinary skill in the art and without deviating from the scope of the present invention. 
       FIGS. 1-3  show a hard disk drive  10  diagrammatically depicted for storing user data and/or operating instructions for a host computer  12 . The disk drive  10  includes an electro-mechanical head-disk assembly (HDA)  14  that includes one or more rotating data storage disks  16  mounted in a stacked, spaced-apart relationship upon a spindle  18  rotated by a spindle motor  20  at a predetermined angular velocity. 
     Each disk  16  includes a disk surface  22 , and usually two disk surfaces  22  on opposing sides. Each disk surface  22  has associated magnetic media for recording data. The spindle motor  20  rotates the spindle  18  to move the disks  16  past the magnetic transducer heads  24  suspended by the suspension arms  26  over each disk surface  22 . Generally, each head  24  is attached to a suspension arm  26  by a head gimbal assembly (not shown) that enables the head  24  to swivel to conform to a disk surface  22 . The suspension arms  26  extend radially from a rotary voice coil motor  28 . The voice coil motor  28  rotates the suspension arms  26  and thereby positions the heads  24  over the appropriate areas of the disk surfaces  22  in order to read from or write to the disk surfaces  22 . Because the disks  16  rotate at relatively high speed, the heads  24  ride over the disk surfaces  22  on a cushion of air (air bearing). 
     Each head  24  includes a read element (not shown) for reading data from a disk surface  22  and a write element (not shown) for writing data to a disk surface  22 . Most preferably, the read element is a magneto-resistive or giant magneto-resistive sensor and the write element is inductive and has a write width which is wider than a read width of the read element. 
     Each disk surface  22  is divided into concentric circular data tracks  30  that each have individually addressable data sectors  32  in which user data is stored in the form of magnetic bits. The data sectors  32  are separated by servo tracks  34  that include narrow embedded servo sectors  36  arranged in radially extending servo spokes. The servo sectors  36  include a series of phase-coherent digital fields followed by a series of constant frequency servo bursts. The servo bursts are radially offset and circumferentially sequential, and are provided in sufficient numbers that fractional amplitude read signals generated by the head  24  from at least two servo bursts passing under the head  24  enable the controller  40  to determine and maintain proper position of the head  24  relative to a data track  30 . A servo burst pattern for use with a head that includes a magneto-resistive read element and an inductive write element is described by commonly assigned U.S. Pat. No. 5,587,850 entitled “Data Track Pattern Including Embedded Servo Sectors for Magneto-Resistive Read/inductive Write Head Structure for a Disk Drive” which is incorporated herein by reference. 
     Each disk surface  22  is also divided into concentric circular regions  38  that each include multiple data tracks  30  and multiple servo tracks  34 . The regions  38  each have an inner and an outer boundary at different locations on the disk surface  22 , and thus are radially offset from one another. For instance, the inner region  38 A includes the innermost data tracks  30  on the disk surface  22 , and the outer region  38 B includes the outermost data tracks  30  on the disk surface  22 . Furthermore, the disk surfaces  22  each contain the same number of the regions  38 , and radially similarly situated regions  38  on different disk surfaces  22  have boundaries at essentially the same radial locations and form virtual cylinders. Thus, the virtual cylinders each consist of radially similarly situated regions  38  on different disk surfaces  22 . 
       FIG. 3  shows five servo tracks  34  depicted as servo tracks Sa, Sb, Sc, Sd and Se in relation to three data tracks  30  depicted as data tracks Tk 1 , Tk 2  and Tk 3 . The servo track density is about 150% of the maximum data track density. The servo track density is determined by the minimum read width of a population of heads  24 . After writing the servo tracks  34  at the servo track pitch on the disk surfaces  22 , the data tracks  30  can be written at any radial position between the servo tracks  34 , as opposed to just the null position where read signals from two servo bursts have equal amplitudes. 
     The controller  40  controls the heads  24  to read from and write to the disk surfaces  22 . The controller  40  preferably is an application specific integrated circuit chip (ASIC) which is connected to other ASICs such as a read/write channel  42 , a motor driver  44  and a cache buffer  46  by a printed circuit board  48 . The controller  40  includes an interface  50  which is connected to the host computer  12  via a known bus  52  such as an ATA or SCSI bus. 
     The controller  40  executes embedded or system software including programming code that monitors and operates the disk drive  10 . During a read or write operation, the host computer  12  determines the address where the data is located in the disk drive  10 . The address specifies the head  24 , the data track  30  and the data sector  32 . The data is transferred to the controller  40  which maps the address to the physical location in the disk drive  10 , and in response to reading the servo information in the servo sectors  36 , operates the voice coil motor  28  to position the head  24  over the corresponding data track  30 . As the disk surface  22  rotates, the head  24  reads the servo information embedded in each servo sector  36  and also reads an address of each data sector  32  in the data track  30 . 
     During a read operation, when the identified data sector  32  appears under the head  24 , the data sector  32  containing the desired data is read. In reading data from the disk surface  22 , the head  24  senses a variation in electrical current flowing through the read element when it passes over an area of flux reversals on the disk surface  22 . The flux reversals are transformed into recovered data by the read/write channel  42  in accordance with a channel algorithm such as partial response, maximum likelihood (PRML). The recovered data is then read into the cache buffer  46  where it is transferred to the host computer  12 . The read/write channel  42  most preferably includes a quality monitor which measures the quality of recovered data and provides an indication of the data error rate. One channel implementation which employs channel error metrics is described in commonly assigned U.S. Pat. No. 5,521,945 entitled “Reduced Complexity EPR4 Post-Processor for Sampled Data Detection” which is incorporated herein by reference. The present invention uses the data error rate to select track density as well as linear bit density and/or error correction codes. 
     During a write operation, the host computer  12  remembers the address for each file on the disk surface  22  and which data sectors  32  are available for new data. The controller  40  operates the voice coil motor  28  in response to the servo information read from the servo sectors  36  to position the head  24 , settles the head  24  into a writing position, and waits for the appropriate data sector  32  to rotate under the head  24  to write the data. To write data on the disk surface  22 , an electrical current is passed through a write coil in the write element of the head  24  to create a magnetic field across a magnetic gap in a pair of write poles that magnetizes the disk surface  22  under the head  24 . When the data track  30  is full, the controller  40  moves the head  24  to the next available data track  30  with sufficient contiguous space for writing data. If still more track capacity is required, another head  24  writes data to a data sector  32  of another data track  30  on another disk surface  22 . 
     In every disk drive, there is a distribution associated with the performance of the heads and corresponding disk surfaces. The present invention takes advantage of this distribution to determine track density assignments. 
       FIG. 4  shows a flow chart  100  for defining a storage format for the disk surfaces  22 . Preferably, the servo tracks  34  are written to the disk surfaces  22 , and then the track density is selected for the data tracks  30 . The servo track density is greater than the maximum data track density to assure accurate positioning of the heads  24  regardless of the selected data track density. In addition, the track density for the data tracks  30  is individually selected for each region  38  on each disk surface  22  by measurement of recorded data in each region  38  on each disk surface  22  to optimize the storage capacity of each region  38  on each disk surface  22 . 
     Initially, predetermined track densities (TPI) are stored in a table (step  102 ). Generally, these track densities are incremental or decremental values of one another, for example, a maximum track density can be the highest value in a series of five track densities. Alternatively, the track densities can be derived from an algorithm based on the location of the region  38  on the disk surface  22 . The track densities can vary by changing the track width of the data tracks  30  or the space between adjacent data tracks  30 . Preferably, the track densities vary by changing the space between adjacent data tracks  30  because the track width of the data tracks  30  is determined by the write width of the write element of the head  24 . Optionally, predetermined linear bit densities (BPI) are also stored in the table. In addition, an acceptable error rate, which represents the greatest error rate than can be tolerated, is established (step  104 ). 
     The predetermined track densities and linear bit densities and the acceptable error rate are provided to a testing and formatting program during a self-scan operation during manufacturing, and the process starts (step  106 ). An initial track density, linear bit density and region  38  are determined (step  108 ). Preferably, the initial track density is the maximum value for the head  24  and the region  38  so that track density for the region  38  can be selected rapidly with few iterations, assuming the region  38  has a track density that is closer to the maximum value than the minimum value. The maximum track density can be calculated or estimated from statistically compiled measured track densities for a population of pairs of heads  24  and disk surfaces  22 . The maximum track density can also be based on the location of the region  38  on the disk surface  22 . For example, the maximum track density can increase from the inner region  38 A to the outer region  38 B due to the skew angle of the head  24  relative to the data tracks  30 . 
     The head  24  writes data to the region  38  at the track density and linear bit density (step  110 ), and then the head  24  reads the recorded data from the region  38  (step  112 ) and an error rate of the recorded data is measured (step  114 ). The measured error rate is compared to the acceptable error rate (step  116 ), and if the measured error rate is greater than the acceptable error rate then the track density and/or the linear bit density is decreased (step  118 ) and steps  110  to  116  are repeated for the region  38 . Steps  110  to  118  repeat as continued iterations until the measured error rate is less than or equal to the acceptable error rate (step  116 ), at which time the current track density and linear bit density are selected as the maximum recordable track density and linear bit density for the region  38 . If another region  38  remains to be tested (step  122 ) then the process returns to step  108 , otherwise the process ends (step  124 ). 
     In this manner, the track density for each region  38  is selected based on the performance of the corresponding head  24 . For example, if the head  24  has strong measured performance for the region  38  then a high track density (narrow track pitch) is selected for the region  38 , whereas if the head  24  has weak measured performance for the region  38  then a low track density (wide track pitch) is selected for the region  38 . As a result, variable TPI is provided on a region-by-region basis for each disk surface  22  based on the measured performance of the heads  24 . Likewise, variable BPI can be provided on a region-by-region basis. Advantageously, the variable TPI (and optional variable BPI) optimize the storage format of the disk drive  10 , thereby increasing the performance, storage capacity and yield of the disk drive  10  over conventional disk drives. 
       FIG. 5  shows a flow chart  200  for determining the storage capacity of the disk drive  10 . Initially, a target storage capacity for the disk drive  10  is established (step  202 ) and the process starts (step  204 ). The track density and optionally the linear bit density are selected for each region  38  as in the flow chart  100  (step  206 ). Thereafter, the storage capacity of each disk surface  22  is calculated as TPI×BPI×(1+ECC)/FE, where TPI is the track density, BPI is the linear bit density, ECC is the fractional level of error correction code which is typically about 0.1, and FE is the format efficiency which is typically about 0.57 (step  208 ). The storage capacities of the disk surfaces  22  are summed to provide the storage capacity of the disk drive  10 , and the storage capacity of the disk drive  10  is compared with the target storage capacity (step  210 ). If the storage capacity of the disk drive  10  equals or exceeds the target storage capacity then the disk drive  10  qualifies (step  212 ), otherwise the disk drive  10  fails (step  214 ). 
     The flow chart  200  can be modified to perform steps  206 ,  208  and  210  for a single disk surface  22  and repeat steps  206 ,  208  and  210  until the disk drive  10  qualifies. That is, if the storage capacity of the disk drive  10  equals or exceeds the target storage capacity based on less than all the disk surfaces  22  then it is not necessary to calculate the storage capacity of the remaining disk surfaces  22  since the disk drive  10  qualifies. However, if the storage capacity of the disk drive  10  is less than the target storage capacity based on less than all the disk surfaces  22  then it is necessary to calculate the storage capacity of the next disk surface  22  until the disk drive  10  qualifies or all the disk surfaces  22  have been evaluated. 
     After the disk drive  10  is qualified, the controller  40  is programmed to provide the selected track density and linear bit density for formatting the data tracks  30  in each region  38 . 
       FIG. 6  shows a conventional storage format with fixed data track density and fixed servo track density in a zone on different disk surfaces and the corresponding heads. The data track density and the servo track density are the same for each disk surface and each head, regardless of the capabilities of the heads. In this example, the disk drive includes N heads depicted as heads  0 ,  1  . . . N−1, and the data tracks and servo tracks are formatted in zone  0  as follows: 
     
       
         
           
               
               
               
               
             
               
                   
               
             
            
               
                 Head 0: 
                 15 Data Tracks 
                   
                 15 Data Tracks per 45 Servo Tracks 
               
               
                 Head 1: 
                 15 Data Tracks 
                   
                 15 Data Tracks per 45 Servo Tracks 
               
               
                 . . . 
                   
                 . . . 
               
               
                 Head N − 1: 
                 15 Data Tracks 
                   
                 15 Data Tracks per 45 Servo Tracks 
               
               
                   
               
            
           
         
       
     
       FIG. 7  shows a conventional zone format on different disk surfaces and the corresponding heads. Each disk surface is divided into several concentric zones, and each zone includes multiple data tracks and multiple servo tracks. In this example, the disk drive includes N disk surfaces depicted as disk surfaces  0 ,  1  . . . N−1, N heads depicted as heads  0 ,  1  . . . N−1 and zone  0 . 
       FIG. 8  shows zones  54  on disk surface  22  that each include virtual cylinders  56 . Each disk surface  22  is divided into concentric circular zones  54  that each include multiple regions  38  and multiple virtual cylinders  56 . The boundaries of the zones  54  are shown as dark circles, and the boundaries of the regions  38  and the virtual cylinders  56  within the zones  54  are shown as light circles. The zones  54  each have an inner and an outer boundary at different locations on the disk surface  22 , and thus are radially offset from one another. Furthermore, the disk surfaces  22  each contain the same number of the zones  54 , and radially similarly situated zones  54  on different disk surfaces  22  have boundaries at essentially the same radial locations. Also, in each zone  54 , the regions  38  and the virtual cylinders  56  are co-extensive since the virtual cylinders  56  each consist of radially similarly situated regions  38  on different disk surfaces  22 . 
     The zones  54  are depicted as zones  0  to M−1, and within each zone  54 , the regions  38  are depicted as regions  0  to P−1 and the virtual cylinders  56  are depicted as virtual cylinders  0  to P−1. Thus, the disk surface  22  includes M zones  54 , and each zone  54  includes P regions  38  and P virtual cylinders  56 . For example, zone  0  (the outermost zone) includes regions  0  to P−1 and virtual cylinders  0  to P−1, and zone M−1 (the innermost zone) includes regions  0  to P−1 and virtual cylinders  0  to P−1. 
     Within each virtual cylinder  56 , the data track density can change from disk surface  22  to disk surface  22 , the servo track density can change from disk surface  22  to disk surface  22 , and the ratio of the data track density to the servo track density can change from disk surface  22  to disk surface  22 . Likewise, within each virtual cylinder  56 , the data track density can change and the servo track density can be the same from disk surface  22  to disk surface  22 , and the data track density can be the same and the servo track density can change from disk surface  22  to disk surface  22 . Likewise, within each virtual cylinder  56 , the data track density can change between some disk surfaces  22  and be the same between other disk surfaces  22 , and the servo track density can change between some disk surfaces  22  and be the same between other disk surfaces  22 . Likewise, within each virtual cylinder  56 , the data track density of a disk surface  22  can be the same as another disk surface  22  and different than another disk surface  22 , and the servo track density of a disk surface  22  can be the same as another disk surface  22  and different than another disk surface  22 . 
     Within each virtual cylinder  56 , the linear bit density can also vary as described above for the track density. Furthermore, the virtual cylinders  56  need not have track densities that are related to one another. That is, since the heads  24  are measured for record/playback performance in each region  38  on each disk surface  22 , the virtual cylinders  56  can have storage formats that are independent from one another. However, the virtual cylinders  56  can have similar or identical storage formats. For example, the servo tracks  34  can be written at a fixed servo track pitch across the disk surfaces  22 , and then a head  24  may exhibit strong measured performance across the corresponding disk surface  22  whereas another head  24  may exhibit weak measured performance across the corresponding disk surface  22 . 
       FIG. 9  shows a storage format with variable data track density and variable servo track density on different disk surfaces  22  and the corresponding heads  24 . Furthermore, the ratio of the data tracks  30  to the servo tracks  34  varies between different disk surfaces  22  and different heads  24 . In this example, the data track density for head  0  is less than the data track density for head  1 , the servo track density for head  0  is less than the servo track density for head  1 , and the ratio of the data track density to the servo track density for head  0  (3:2) is less than the ratio of the data track density to the servo track density for head  1  (2:1). 
       FIG. 10  shows a storage format with variable data track density and fixed servo track density within virtual cylinders  56  and the corresponding heads  24 . The data track density varies for the disk surfaces  22  within the virtual cylinders  56 , however the servo track density is the same for the disk surfaces  22  within the virtual cylinders  56 . In this example, the disk drive  10  includes N heads depicted as heads  0 ,  1  . . . N−1 and zone  0  that includes 3 virtual cylinders  56  depicted as virtual cylinders  0 ,  1  and  2 , the virtual cylinders  56  each include 6 data tracks  30  depicted as data tracks  0 ,  1 ,  2 ,  3 ,  4  and  5 , and the data tracks  30  and the servo tracks  34  are formatted in zone  0  as follows: 
     
       
         
           
               
               
               
               
             
               
                   
               
             
            
               
                 Head 0: 
                 15 Data Tracks 
                   
                 15 Data Tracks per 45 Servo Tracks 
               
               
                 Head 1: 
                 18 Data Tracks 
                   
                 18 Data Tracks per 45 Servo Tracks 
               
               
                 . . . 
                   
                 . . . 
               
               
                 Head N − 1: 
                 12 Data Tracks 
                   
                 12 Data Tracks per 45 Servo Tracks 
               
               
                   
               
            
           
         
       
     
       FIG. 11  shows a storage format with variable data track density and fixed servo track density within virtual cylinders  56  and the corresponding heads  24 . The data track density varies for the disk surfaces  22  within the virtual cylinders  56 , however the servo track density is the same for the disk surfaces  22  within the virtual cylinders  56 . Furthermore, the ratio of the data tracks  30  to the servo tracks  34  varies within the virtual cylinders  56  between different disk surfaces  22  and different heads  24 . In this example, the data track density for head  0  is greater than the data track density for head  1 , the servo track density for heads  0  and  1  is the same, and the ratio of the data track density to the servo track density for head  0  (3:2) is greater than the ratio of the data track density to the servo track density for head  1  (7:4). Furthermore, the data tracks  30  are not vertically aligned within the virtual cylinders  56 . 
       FIG. 12  shows access of data tracks on different disk surfaces in a conventional logical cylinder. Each disk surface is divided into concentric circular zones that each include data tracks and servo tracks. On a disk surface, the zones each have an inner and an outer boundary at different locations and thus are radially offset from one another. Furthermore, the disk surfaces each contain the same number of zones, and radially similarly situated zones on different disk surfaces have boundaries at essentially the same radial locations and form logical cylinders. Thus, the logical cylinders each consist of radially similarly situated zones on different disk surfaces. Within each logical cylinder, the data track density is the same and the data tracks on different disk surfaces are radially aligned with one another, although different logical cylinders may have different data track densities. 
     Data is sequentially accessed (read from or written to) in a logical cylinder by positioning the heads over a set of vertically aligned data tracks, then a head accessing a data track on a disk surface, then the next head accessing a data track on the next disk surface, and so on until the heads have accessed the vertically aligned data tracks on the disk surfaces, then positioning the heads over the next set of vertically aligned data tracks that are adjacent to the previous set and sequentially accessing the next set of vertically aligned data tracks head by head, and so on. In other words, the heads are positioned over vertically aligned data tracks during a seek operation and then head switches are performed between adjacent heads (without a seek operation) so that the heads sequentially access the vertically aligned data tracks on the disk surfaces, then the heads are positioned over adjacent vertically aligned data tracks during a seek operation, and so on. 
     In this example, the disk drive includes heads  0 ,  1 ,  2  and  3 , corresponding disk surfaces  0 ,  1 ,  2  and  3  and logical cylinder  0  that includes vertically aligned data tracks on the disk surfaces. The heads are positioned over a first set of vertically aligned data tracks (the first row of data tracks) during a seek operation, then head  0  accesses the data track on disk surface  0 , then head  1  accesses the data track on disk surface  1 , then head  2  accesses the data track on disk surface  2 , and then head  3  accesses the data track on disk surface  3 . The heads are then positioned over a second set of vertically aligned data tracks (the second row of data tracks) during a seek operation, then head  0  accesses the data track on disk surface  0 , then head  1  accesses the data track on disk surface  1 , then head  2  accesses the data track on disk surface  2 , and then head  3  accesses the data track on disk surface  3 . The seek operation followed by sequential access of each disk surface using head switches are repeated until the access operation is complete. 
       FIGS. 13-15  show access of data tracks  30  on different disk surfaces  22  in a virtual cylinder  56 . In the virtual cylinder  56 , the data track density on different disk surfaces  22  may be different, and therefore the data tracks  30  on different disk surfaces  22  may not be vertically aligned. As a result, the access operation for a conventional logical cylinder is undesirable because a head switch may also require a seek operation, thereby degrading the performance of the disk drive  10 . 
       FIG. 13  shows data sequentially accessed in (read from or written to) the virtual cylinder  56  by positioning a head  24  over a data track  30  on a disk surface  22 , then the head  24  accessing the data track  30 , then positioning the head  24  over an adjacent data track  30  on the disk surface  22 , then the head  24  accessing the data track  30 , and so on until the head  24  sequentially accesses the data tracks  30  on the disk surface  22  in the virtual cylinder  56  (the data tracks  30  in the region  38 ), then positioning the next head  24  over a data track  30  on the next disk surface  22 , then the next head  24  sequentially accessing the data tracks  30  on the next disk surface  22 , and so on. In other words, a head  24  sequentially accesses the data tracks  30  on the corresponding disk surface  22  in the virtual cylinder  56  by sweeping across the virtual cylinder  56  in a radial direction, then a head switch and a seek operation are performed, then the next head  24  sequentially accesses the data tracks  30  on the corresponding disk surface  22  in the virtual cylinder  56  by sweeping across the virtual cylinder  56  in the same radial direction, and so on. 
     In this example, the disk drive includes heads  0 ,  1 ,  2  and  3 , corresponding disk surfaces  0 ,  1 ,  2  and  3  and virtual cylinder  0  that includes data tracks  30  on the disk surfaces. Head  0  is positioned over the first data track  30  (the top data track  30 ) on disk surface  0  during a seek operation, then head  0  accesses the first data track  30  on disk surface  0 , then head  0  is positioned over the second data track  30  (adjacent to the top data track  30 ) on disk surface  0  during a seek operation, then head  0  accesses the second data track  30  on disk surface  0 , and so on until head  0  is positioned over the last data track  30  (the bottom data track  30 ) on disk surface  0  within virtual cylinder  0  during a seek operation, and then head  0  accesses the last data track  30  on disk surface  0  within virtual cylinder  0 . A head switch is then performed and head  1  is positioned over the first data track  30  (the top data track  30 ) on the disk surface  1  during a seek operation, then head  1  accesses the first data track  30  on disk surface  1 , then head  1  is positioned over the second data track  30  (adjacent to the top data track  30 ) on disk surface  1  during a seek operation, then head  1  accesses the second data track  30  on disk surface  1 , and so on until head  1  is positioned over the last data track  30  (the bottom data track  30 ) on disk surface  1  within virtual cylinder  0  during a seek operation, and then head  1  accesses the last data track  30  on disk surface  1  within virtual cylinder  0 . A head switch is then performed to head  2 , then the sequential seek and access operations are repeated for disk surface  2 , then a head switch is performed to head  3 , and then the sequential seek and access operations are repeated for disk surface  3  until the access operation is complete. 
     Furthermore, since heads  0 ,  1 ,  2  and  3  sequentially access the data tracks  30  on disk surfaces  0 ,  1 ,  2  and  3 , respectively, in the same radial direction, the access operation follows a repeating sweeping pattern. 
       FIG. 14  shows data sequentially accessed in (read from or written to) the virtual cylinder  56  by positioning a head  24  over a data track  30  on a disk surface  22 , then the head  24  accessing the data track  30 , then positioning the head  24  over an adjacent data track  30  on the disk surface  22 , then the head  24  accessing the data track  30 , and so on until the head  24  sequentially accesses a first subset of the data tracks  30  on the disk surface  22  in the virtual cylinder  56  (a first subset of the data tracks  30  in the region  38 ), then switching to the next head  24 , then the next head  24  sequentially accessing a first subset of the data tracks  30  on the next disk surface  22 , then switching to the initial head  24  and positioning the head  24  over the next data track  30  on the initial disk surface  22 , then the head  24  sequentially accessing a second subset of the data tracks  30  on the disk surface  22 , then switching to the next head  24 , then the next head  24  sequentially accessing a second subset of the data tracks  30  on the next disk surface  22 , and so on. In other words, a head  24  sequentially accesses a first subset of the data tracks  30  on the corresponding disk surface  22  in the virtual cylinder  56  in a first radial direction, then a head switch (without a seek operation) is performed and the next head  24  sequentially accesses a first subset of the data tracks  30  on the corresponding disk surface  22  in the virtual cylinder  56  in a second radial direction opposite the first radial direction, then a head switch and a seek operation are performed and the next head  24  sequentially accesses a second subset of the data tracks  30  on the corresponding disk surface  22  in the virtual cylinder  56  in the first radial direction, then a head switch (without a seek operation) is performed and the next head  24  sequentially accesses a second subset of the data tracks  30  on the corresponding disk surface  22  in the virtual cylinder  56  in the second radial direction, and so on. 
     In this example, the disk drive includes heads  0  and  1 , corresponding disk surfaces  0  and  1  and virtual cylinder  0  that includes data tracks  0  to  27  on disk surface  0  and data tracks  0  to  13  on disk surface  1 . Head  0  is positioned over data track  0  on disk surface  0  during a seek operation, then head  0  accesses data track  0  on disk surface  0 , then head  0  is positioned over data track  1  on disk surface  0  during a seek operation, then head  0  accesses data track  1  on disk surface  0 , and so on until head  0  is positioned over data track  12  on disk surface  0  during a seek operation, and then head  0  accesses data track  12  on disk surface  0 . A head switch (without a seek operation) is then performed and head  1  accesses data track  6  on disk surface  1 , then head  1  is positioned over data track  5  on disk surface  1  during a seek operation, then head  1  accesses data track  5  on disk surface  1 , and so on until head  1  is positioned over data track  0  on disk surface  1  during a seek operation, and then head  1  accesses data track  0  on disk surface  1 . A head switch is then performed and head  0  is positioned over data track  13  on the disk surface  0  during a seek operation, then head  0  accesses data track  13  on disk surface  0 , and the sequential seek and access operations for subsets of the data tracks on disk surfaces  0  and  1  are repeated until the access operation is complete. 
     Advantageously, since data track  12  on disk surface  0  is vertically aligned with data track  6  on disk surface  1 , and data track  25  on disk surface  0  is vertically aligned with data track  13  on disk surface  0 , the head switches from head  0  to head  1  do not involve a seek operation. Positioning head  0  over data track  12  on disk surface  0  also positions head  1  over data track  6  on disk surface  1 , and positioning head  0  over data track  25  on disk surface  0  also positions head  1  over data track  13  on disk surface  1 . Furthermore, since heads  0  and  1  sequentially access radially aligned subsets of the data tracks on disk surfaces  0  and  1 , respectively, in opposite radial directions, the access operation follows a zig-zag pattern. 
       FIG. 15  shows data sequentially accessed in (read from or written to) the virtual cylinder  56  by positioning a head  24  over a data track  30  on a disk surface  22 , then the head  24  accessing the data track  30 , then positioning the head  24  over an adjacent data track  30  on the disk surface  22 , then the head  24  accessing the data track  30 , and so on until the head  24  sequentially accesses the data tracks  30  on the disk surface  22  in the virtual cylinder  56  (the data tracks  30  in the region  38 ), then switching to the next head  24 , then the next head  24  sequentially accessing the data tracks  30  on the next disk surface  22 , then switching to another head  24 , and so on. In other words, a head  24  sequentially accesses the data tracks  30  on the corresponding disk surface  22  in the virtual cylinder  56  in a first radial direction, then a head switch (without a seek operation) is performed and the next head  24  sequentially accesses the data tracks  30  on the corresponding disk surface  22  in the virtual cylinder  56  in a second radial direction opposite the first radial direction, then a head switch (without a seek operation) is performed and another head  24  sequentially accesses the data tracks  30  on the corresponding disk surface  22  in the virtual cylinder  56  in the first radial direction, and so on. 
     In this example, the disk drive includes heads  0 ,  1 ,  2  and  3 , corresponding disk surfaces  0 ,  1 ,  2  and  3  and virtual cylinder  0  that includes data tracks  30  on the disk surfaces. Head  0  is positioned over the first data track  30  on disk surface  0  during a seek operation, then head  0  accesses the first data track  30  on disk surface  0 , then head  0  is positioned over the second data track  30  on disk surface  0  during a seek operation, then head  0  accesses the second data track  30  on disk surface  0 , and so on until head  0  is positioned over the last data track  30  on disk surface  0  within virtual cylinder  0  during a seek operation, and then head  0  accesses the last data track  30  on disk surface  0  within virtual cylinder  0 . A head switch (without a seek operation) is then performed and head  1  accesses the last data track  30  on disk surface  1 , then head  1  is positioned over the second-to-last data track  30  on disk surface  1  during a seek operation, then head  1  accesses the second-to-last data track  30  on disk surface  1 , and so on until head  1  is positioned over the first data track  30  on disk surface  1  within virtual cylinder  0  during a seek operation, and then head  1  accesses the first data track  30  on disk surface  1  within virtual cylinder  0 . A head switch (without a seek operation) is then performed and head  2  sequentially accesses the data tracks  30  (from the first data track  30  to the last data track  30 ) on disk surface  2  within virtual cylinder  0 , and then a head switch (without a seek operation) is performed and head  3  sequentially accesses the data tracks  30  (from the last data track  30  to the first data track  30 ) on disk surface  3  within virtual cylinder  0  until the access operation is complete. 
     Advantageously, since the last data tracks  30  on disk surfaces  0  and  1  are vertically aligned, the first data tracks  30  on disk surfaces  1  and  2  are vertically aligned, and the last data tracks  30  on disk surfaces  2  and  3  are vertically aligned, the head switches from head  0  to head  1 , head  1  to head  2 , and head  2  to head  3  do not involve a seek operation. Positioning head  0  over the last data track  30  on disk surface  0  also positions head  1  over the last data track  30  on disk surface  1 , positioning head  1  over the first data track  30  on disk surface  1  also positions head  2  over the first data track  30  on disk surface  2 , and positioning head  2  over the last data track  30  on disk surface  2  also positions head  3  over the last data track  30  on disk surface  3 . Furthermore, since heads  0  and  2  and heads  1  and  3  sequentially access radially aligned data tracks  30  on disk surfaces  0  and  2  and disk surfaces  1  and  3 , respectively, in opposite radial directions, the access operation follows a zig-zag pattern. 
     The track density and the linear bit density can be optimized for each region  38  independently of the other regions  38 . Alternatively, the track density and the linear bit density for the regions  38  can be selected to provide a predetermined or optimized storage capacity and/or performance for the disk drive  10 . For example, the track density and the linear bit density can be selected from predetermined values based on the performance of the head  24  for the region  38 . Likewise, the track density and the linear bit density can be selected for each region  38  based on whether the head performance for the region  38  is strong, medium or weak. In addition, the storage capacity of the disk drive  10  can be maximized for the measured squeezed and unsqueezed off-track capability of the heads  24 . Likewise, the track density can be selected to maximize the squeezed and unsqueezed off-track capability of the heads  24 . 
     The track density can be selected for the region  38  using a wide variety of record/playback performance measurements between the head  24  and the region  38 . For example, the performance measurement can be a 747 measurement. The term “747” comes from the resultant data profile having a similar appearance to the elevational outline of a Boeing 747 airplane. During 747 measurement, the head  24  is moved off-track until the error rate exceeds a threshold, and the distance to failure is the off-track capability. This process is repeated with adjacent tracks at smaller spacing until the off-track capability drops to zero. The resulting off-track capability versus track pitch is then analyzed to determine the optimum track pitch, typically chosen as the track pitch with the maximum off-track capability. The 747 measurement is described by Jensen et al. in “Demonstration of 500 Megabits per Square Inch with Digital Magnetic Recording,” IEEE Transactions on Magnetics, Vol. 26, No. 5, September 1990, pp. 2169 et seq. See also “Measure a Disk-Drive&#39;s Read Channel Signals,” Test &amp; Measurement World, August 1999, published by Cahners Business Information, Newton, Mass. The record/playback performance measurement can also be a simple in-drive erase width measurement. 
     For example, a 747 profile defines head performance by squeezed off-track capability (SOTC) and unsqueezed off-track capability (UOTC) at a fixed error rate. The 747 profile for each head is moved (by changing BPI and/or TPI) to the minimum SOTC at a pre-defined track squeeze so that the disk drive has the maximum storage capacity. The 747 profiles for the heads are then moved collectively (by changing BPI and/or TPI) to maintain the SOTC so that the disk drive has the minimum storage capacity and the heads have similar SOTC and UOTC. 
     The SOTC and UOTC can be determined from write width (WW), read width (RW), erase width (EW), track pitch (TP), squeeze (SQZ), on-track bit error rate (BER) and a function (f) as follows:
 
UOTC=(WW−RW)/2+EW+f(BER)  (1)
 
SOTC=TP−SQZ−(WW+RW)/2+f(BER)  (2)
 
     For linear bit density optimization, UOTC is the performance metric. For a given head, WW, RW and EW are constant (C). Therefore, UOTC can be determined from BER or BPI and SOTC can be determined from BPI if TP and SQZ are constant as follows:
 
UOTC=f(BER)+C
 
BER=f(BPI)
 
UOTC=f(BPI)+C  (3)
 
SOTC=TP−SQZ+f(BER)+C
 
SOTC=f(BPI)+C  (4)
 
     For track density optimization, SOTC is the performance metric. For a given linear bit density, SOTC is a function of TP and SQZ. Therefore, the TP can be determined as follows:
 
SOTC=TP−SQZ+C  (5)
 
TP=SOTC+SQZ−C  (6)
 
     The optimization algorithm can be as follows:
         UOTC1: minimum required UOTC+margin   SOTC1: minimum required SOTC at SQZ1   SQZ1: SQZ test point for SOTC1       

     1. Find the minimum acceptable performance point for each head by optimizing BPI: (a) run channel optimization for new BPI (for every different data rate, there is channel optimization) and (b) optimize BPI within the allowed range of formats or data rates such that the difference (UOTC−UOTC1) is minimized while satisfying the requirement of UOTC1&lt;=UOTC. 
     2. Find the minimum acceptable performance point for each head by optimizing TPI: optimize track pitch within an allowed adjacent track pitch (ATP) range such that difference (SOTC−SOTC1) is minimized while satisfying the minimum performance requirement of SOTC1&lt;=SOTC. 
     3. Optimize BPIs for all heads to meet the storage capacity requirement: (a) calculate: delta storage capacity=(current storage capacity−minimum storage capacity), and (b) if delta storage capacity &lt; &gt; nBPI step size then increase/decrease BPI by nx % within the allowed BPI for each head if possible. 
     4. Optimize TPIs for all heads to meet the storage capacity requirement: (a) calculate the new storage capacity, determine the new delta storage capacity, (b) calculate the delta ATP allowed for each head, and if delta storage capacity &lt; &gt; nATP step size then decrease/increase track pitch by delta ATP within the allowed ATP range for each head if possible. 
     The present invention has been described in considerable detail with reference to certain preferred versions thereof, however other versions are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.