Patent Publication Number: US-2013242426-A1

Title: Shingled magnetic recording disk drive with minimization of the effect of far track erasure on adjacent data bands

Description:
RELATED APPLICATION 
     This application is related to Application No. ______ filed ______, 2012 concurrently with this application and titled “SHINGLED MAGNETIC RECORDING DISK DRIVE WITH INTER-BAND DISK CACHE AND MINIMIZATION OF THE EFFECT OF FAR TRACK ERASURE ON ADJACENT DATA BANDS” (Attorney Docket No. HSJ92012002US1). 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to magnetic recording hard disk drives (HDDs), and more particularly to a shingled magnetic recording (SMR) HDD that minimizes the effect of far track erasure (FTE) on data tracks in the boundary regions of data bands. 
     2. Description of the Related Art 
     Magnetic recording disk drives that use “shingle writing”, also called “shingled recording” or “shingled magnetic recording” (SMR), have been proposed, for example as described in U.S. Pat. No. 6,185,063 B1 and U.S. Pat. No. 6,967,810 B2. In SMR, the write head, which is wider than the read head in the cross-track direction, writes magnetic transitions by making a plurality of consecutive circular paths that partially overlap. The non-overlapped portions of adjacent paths form the shingled data tracks, which are thus narrower than the width of the write head. The data is read back by the narrower read head. The narrower shingled data tracks thus allow for increased data density. The shingled data tracks are arranged on the disk as annular bands separated by annular inter-band gaps or guard bands. 
     The writing of data to an entire band may occur when new data from the host is stored in memory and then written to a band for the first time. It may also occur when a portion of the data in a band is modified, i.e., a “read-modify-write” operation in which all the corresponding data in a band is read and stored in memory, then a portion is modified with the host-provided new write data, and finally all the corresponding data is written data back to the band. The writing of data to an entire band or bands may also occur when a band or bands are “cleaned” or “de-fragmented” to reclaim free space, i.e., the data in one or more bands is read and stored in memory and then re-written to the same band or a new band. 
     A problem in both conventional HDDs and SMR HDDs is wide-area track erasure (WATER) or far track encroachment or erasure (FTE). The write field from the write head is wider than a data track so when the write head is writing to a track, the outer portions of the write field (called the fringe field) overlap onto tracks other than the track being written. Data degradation due to fringe fields is not limited to the tracks immediately adjacent the track being written, but can extend over a range of tracks relatively far from the track being written. This FTE is particularly noticeable with write heads that have side shields. FTE may not affect tracks symmetrically on both sides of the track being written. Tracks on one side may encounter more pronounced FTE effects due to the write head shield design or due to read-write head skew. FTE is described by Liu et al., “Characterization of Skip or Far Track Erasure in a Side Shield Design”, IEEE TRANSACTIONS ON MAGNETICS, VOL. 45, NO. 10, OCTOBER 2009, pp. 3660-3663. U.S. application Ser. No. 12/831,391 filed Jul. 19, 2010, and assigned to the same assignee as this application, describes a conventional HDD where the effect of FTE is minimized by counting the number of writes, incrementing counters based on the known effect of FTE on each track within a range of the track being written, and then rewriting the data when a count reaches a predetermined threshold. 
     In a SMR disk drive, FTE can occur on the tracks in the boundary region of a band, i.e., those tracks near an inter-band gap, when data is written to tracks in the boundary region of an adjacent band. What is needed is a SMR HDD that counts the number of writes to the data tracks in the boundary regions of bands and then rewrites the data in adjacent bands to minimize the effect of FTE. 
     SUMMARY OF THE INVENTION 
     The invention relates to a SMR HDD that essentially eliminates the effect of FTE in the boundary regions of annular data bands caused by writing in the boundary regions of adjacent data bands. The extent of the FTE effect is determined for each track within a range of tracks of the track being written. In one implementation, based on the relative FTE effect for all the tracks in the range, a count increment (CI) is determined for each track. The CI values and their associated track numbers within the range may be stored as a table in memory. A counter is maintained for each track in each boundary region. For every writing to a track in a boundary region, a count for each track in an adjacent boundary region that is within a range of the track being written is increased by the associated CI value. When the count value for a track reaches a predetermined threshold the data is read from that band and rewritten to the same band. In another implementation of the invention, a single cumulative count is maintained for each boundary region of each band and the cumulative count is increased by a cumulative count increment (CCI) for each writing to a track in an adjacent boundary region. When the cumulative count value for a boundary region of a band reaches a predetermined threshold the data is read from that band and rewritten to the band. Because a HDD typically includes multiple disk surfaces, each with an associated read/write head, and because not all heads will have the same exact write profiles and thus not generate the same FTE effect, a CI table or CCI table can be developed for each head and its associated disk surface. 
     For a fuller understanding of the nature and advantages of the present invention, reference should be made to the following detailed description taken together with the accompanying figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a top view of a SMR disk drive for use with the method according to the invention. 
         FIG. 2  is a schematic showing a typical band on a SMR disk and illustrates the multiple overlapping tracks that define the shingled data tracks. 
         FIG. 3  is a graph of an example of measured bit error rate (BER) degradation values for a range of tracks written by a perpendicular recording head and illustrates the effect of far track erasure (FTE). 
         FIG. 4  is a table of track number, BER value, and calculated count increment for tracks within a range of tracks for the perpendicular write head that produced the BER data of  FIG. 3 . 
         FIG. 5A  is a schematic representation of a SMR disk showing three annular bands with inter-band gaps and band boundary regions and illustrating the count increment (CI) table aligned with a track being written in one of the band boundary regions. 
         FIG. 5B  is a schematic like  FIG. 5A  but illustrating the CI table aligned with a track being written that is one track shifted from the written track in  FIG. 5A . 
         FIG. 6  is a cumulative count increment (CCI) table for counting the effect of FTE on a band boundary region using a single counter. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a top view of a disk drive  100  with shingled recording according to the invention. The disk drive has a housing or base  101  that supports an actuator  130  and a spindle motor (not shown) for rotating the magnetic recording disk  10  about its center  13  in the direction indicated by arrow  15 . The actuator  130  may be a voice coil motor (VCM) rotary actuator that has a rigid arm  134  and rotates about pivot  132 . A head-suspension assembly includes a suspension  121  that has one end attached to the end of actuator arm  134 , a flexure  123  attached to the other end of suspension  121 , and a head carrier, such as an air-bearing slider  122 , attached to the flexure  123 . The suspension  121  permits the slider  122  to be maintained very close to the surface of disk  10  and the flexure  123  enables the slider  122  to “pitch” and “roll” on an air-bearing generated by the rotating disk  10 . The slider  122  supports the read/write or recording head  109  located on the end face  112  of slider  122 . The recording head  109  is typically a combination of an inductive write head with a magnetoresistive read head (also called a read/write head). Only one disk surface with associated slider and recording head is shown in  FIG. 1 , but there are typically multiple disks stacked on a hub that is rotated by a spindle motor, with a separate slider and recording head associated with each surface of each disk. 
     In this invention the disk drive uses shingled magnetic recording (SMR), also called shingled writing. Thus  FIG. 1  also illustrates portions of the circular shingled data tracks grouped as annular regions or bands on the recording layer of disk  10 . Only portions of five bands  180 ,  182 ,  184 ,  186  and  188  are depicted, but there would typically be a large number of bands. Adjacent bands are separated by inter-band annular gaps, such as typical gaps  181 ,  183 ,  185  and  187 . For example, for a 2.5 inch disk drive with shingled recording, the shingled data tracks may have a cross-track width (TW) of about 50 nm with each band containing several hundred tracks and with each gap separation between the bands being about 100 nm (or about 2 TW). In shingled recording the write head, which is wider than the read head in the cross-track direction, writes magnetic transitions by making a plurality of consecutive circular paths or tracks that partially overlap. The non-overlapped portions of adjacent paths or tracks form the shingled data tracks, which are thus narrower than the width of the write head. The data is read back by the narrower read head. When data is to be re-written in a shingled data track, all of the shingled data tracks that have been written after the track to be re-written are also re-written. 
     As is well known in the art, the data in each shingled data track in each of the bands is also divided into a number of contiguous physical data sectors (not shown). Each data sector is preceded by a synchronization (sync) field, which is detectable by the read head for enabling synchronization of reading and writing the data bits in the data sectors. Also, each shingled data track in each of the bands includes a plurality of circumferentially or angularly-spaced servo sectors (not shown) that contain positioning information detectable by the read head for moving the read/write head  109  to the shingled data tracks and maintaining the read/write head  109  on the tracks. The servo sectors in each shingled data track are typically aligned circumferentially with the servo sectors in the other shingled data tracks so that they extend across the shingled data tracks in a generally radial direction. 
     The disk drive  100  also includes a hard disk controller (HDC)  212  that can include and/or be implemented by a microcontroller or microprocessor. The controller  212  runs a computer program that is stored in memory  214  and that embodies the logic and algorithms described further below. The memory  214  may be separate from controller  212  or as embedded memory on the controller chip. The computer program may also be implemented in microcode or other type of memory accessible to the controller  212 . The controller  212  is connected to a host interface  216  that communicates with the host computer  218 . The host interface  216  may be any conventional computer-HDD interface, such as Serial ATA (Advanced Technology Attachment) or SCSI (Small Computer System Interface). 
     The electronics associated with disk dive  100  also include servo electronics  240 . In the operation of disk drive  100 , the read/write channel  220  receives signals from the read head and passes servo information from the servo sectors to servo electronics  240  and data signals from the data sectors to controller  212 . Servo electronics  240  typically includes a servo control processor that uses the servo information from the servo sectors to run a control algorithm that produces a control signal. The control signal is converted to a current that drives actuator  130  to position the read/write head  109 . In the operation of disk drive  100 , interface  216  receives a request from the host computer  218  for reading from or writing to the data sectors. Controller  212  receives a list of requested data sectors from interface  215  and converts them into a set of numbers that uniquely identify the disk surface, track and data sector. The numbers are passed to servo electronics  240  to enable positioning read/write head  109  to the appropriate data sector. 
     The controller  212  acts as a data controller to transfer blocks of write data from the host computer  218  through the read/write channel  220  for writing to the disk  10  by the write head, and to transfer blocks of read data from the disk  10  back to the host computer  218 . Disk drives typically include, in addition to the rotating disk storage, solid state memory (referred to as “cache”) that temporarily holds data before it is transferred between the host computer and the disk storage. The conventional cache is dynamic random access memory (DRAM), a volatile form of memory that can undergo a significant number of write/erase cycles and that has a high data transfer rate. Disk drives may also include nonvolatile memory. One type of nonvolatile memory is “flash” memory, which stores information in an array of floating gate transistors, called “cells” which can be electrically erased and reprogrammed in blocks. Thus in disk drive  100 , the controller  212  also communicates with volatile memory  250  (shown as DRAM) and optional nonvolatile memory  252  (shown as FLASH) via data bus  254 . 
       FIG. 2  is a schematic of a shingled region or band, like band  186 , for use in describing the method of SMR. A typical band will have a large number, i.e., several hundred or thousand, shingled data tracks (SDTs); however only  7  are shown in band  186  for ease of illustration. Band  186  has inter-band gaps (IBGs)  185 ,  187  that separate it from radially adjacent bands. The write head makes successive paths or tracks (TRs) to form the SDTs which, in the example of  FIG. 2 , are written in the direction from disk outside diameter (OD) to disk inside diameter (ID). The write pole tip of the write head has a cross-track width (WTW) that is wider than the sensing edge of the read head cross-track width (RTW). When writing data, the write head generates paths of magnetic transitions, represented by the vertical lines, as the recording layer moves in the direction of arrow  15 . For example, the actuator positions the write head to write data along track 1 (TR1), then moves the write head to write data along track 2 (TR2). The writing of data along TR2 overwrites a portion of the previously written TR1 and thus “squeezes” the data of TR1 to thereby form the first shingled data track (SDT1). In the example of  FIG. 2 , the shingled data tracks are written in the direction from the disk OD to ID. However, a disk drive can be formatted such that writing of the shingled data tracks in one or more bands can be from ID to OD, with different bands being written in different directions. 
     In general, in SMR, whenever any portion of the data in an annular band is to be re-written or updated, all of the shingled data tracks in that annular band that were written after the shingled data track being updated are also re-written. The writing of data to an entire band may occur when new data from the host is stored in memory and then written to a band for the first time. It may also occur when a portion of the data in a band is modified, i.e., a “read-modify-write” operation in which all the data in a band is read and stored in memory, then a portion is modified with the host-provided new write data, and finally all the data is written data back to the band. The writing of data to an entire band or bands may also occur when a band or bands are “cleaned” or “de-fragmented” to reclaim free space, i.e., the data in one or more bands is read and stored in memory and then re-written to the same band or a new band. 
     A problem in both conventional HDDs and SMR HDDs is wide-area track erasure (WATER) or far track encroachment or erasure (FTE). The write field from the write head is wider than a data track so when the write head is writing to a track, the outer portions of the write field (called the fringe field) overlap onto tracks other than the track being written. The fringe fields can extend over a range of tracks relatively far from the track being written. FTE generally translates into an increase in bit error rate (BER), resulting in degradation of the performance of the disk drive. In some severe cases, poor BER will lead to a significant increase of unrecoverable data errors. FTE is particularly noticeable with write heads that have side shields. FTE may not affect tracks symmetrically on both sides of the track being written. Tracks on one side may encounter more pronounced FTE effects due to the write head shield design or due to read-write head skew. In a SMR disk drive, FTE can occur on the tracks in the boundary region of a band, i.e., those tracks near an inter-band gap, when data is written to tracks in the boundary region of an adjacent band. 
     In this invention variable incremented counting is performed for the shingled data tracks in the band boundary regions that are subjected to the FTE effect from writing to boundary regions in adjacent bands. The magnitude or extent of the FTE effect is determined for each track in a boundary region that is within a range of tracks of the track being written in the boundary region of an adjacent band, and based on the relative FTE effect for all the tracks in the range a count increment (CI) is determined. A count may be maintained for each track in a boundary region or a cumulative count maintained for all the tracks in a boundary region. In one implementation a counter is maintained for each of N tracks in each boundary region, where N is the track range of the effect of FTE from the write head. When data is written to one of the N tracks in a boundary region, the counters for the N tracks in the adjacent boundary region are increased by the predetermined increments based on the number of tracks from the track being written. When the count for any one of the N tracks of a boundary region reaches a predetermined threshold, the data in that band is rewritten. The data is rewritten before the FTE effects can build up, so the reliability of the data is improved. In another implementation, a single counter is maintained for each boundary region of N tracks. When data is written to one of the N tracks in a boundary region, the counter for the adjacent boundary region is increased by a predetermined cumulative increment based on the number of N tracks that are within the range of the track being written. When the cumulative count for a boundary region reaches a predetermined threshold, the data in that band is rewritten. 
     In one approach for determining the relative FTE effects on the tracks within a range of tracks of the track being written, the error rate is used to determine the count increments. A predetermined data pattern is written to all the tracks within a range of −N to +N tracks from a track (designated track 0). An initial “bit” error rate (BER) is then measured for each track in the range of 2N tracks. In one well-known approach for measuring BER, the HDD&#39;s error correction circuitry is deactivated, for example by setting to zero the value in the error correction register for the maximum number of errors to correct, and then the data pattern is read back and the number of bytes in error is counted. Since there must be at least one bit in error for each byte in error, this is the initial BER for each track in the range. Then track 0 is written a very large number of times (for example 100,000 writes). The BER is then again measured for all 2N tracks in the range. The degradation in BER is the difference between the measured BER after the writes to track 0 and the initial BER.  FIG. 3  is a graph of measured BER degradation values for a range of 32 shingle data tracks written by a perpendicular write head. The y-axis of  FIG. 3  is the difference in the logarithm of the measured BER after writes and the logarithm of the initial BER (Δlog (BER)). This graph shows the expected relatively large effect of the fringe fields at immediately adjacent tracks −1 and +1. The FTE effect is clearly shown by the high BER values for tracks −9 to −15, which are significantly higher than the BER values for tracks closer to track 0 (tracks −2 to −8).  FIG. 3  also shows the unsymmetrical characteristic of FTE, with very low BER values for tracks between +2 and +16. From the measured BER degradation values, which represent the relative weightings of FTE for all the tracks within the range, a set of count increments can be calculated for all the tracks within the range.  FIG. 4  is a table of shingled data track number (TR#), BER degradation value (logarithmic), and calculated count increment (CI) for 32 shingled data tracks within a range of −N to +N tracks (where N=16 in this example) for the perpendicular write head that produced the BER data of  FIG. 3 . In this example a Δlog (BER) of 0.75 is an arbitrary reference value (REF) and assigned a count increment of 1 (as shown by track −1). The count increments are then calculated for each track based on the BER degradation for that track. Because the BER values are logarithmic, a count increment (CI) is calculated for each track number (TR#) according to the following: 
         CI   TR# =10 [Δlog(BER     TR#     )−REF]   
     In this invention, for every writing to a data track in one of the N boundary region tracks, at least one count is maintained for the adjacent boundary region. The method of the invention will be explained with  FIGS. 5A-5B . In one implementation a count is maintained for each track in a boundary region that is within N tracks of the track being written in the adjacent boundary region and each count is increased by its value of CI according to a table of CI values. In  FIG. 5A , three annular bands  184 ,  186 ,  188  are depicted, with one-track wide inter-band gaps (IBGs)  185 ,  187 . Each band has 2 boundary regions, BR 1  at the ID side and BR 2  at the OD side. In this example, the effect of FTE is from −8 tracks to +8 tracks, so N=8, a relatively small number for ease of illustration. In the example of  FIG. 5A , track 3 in boundary region BR 1  of band  186  is being written, as represented by the cross-hatching. Thus the center of the CI table is depicted to the right of this track being written. As shown, the range of N tracks from the track being written (track 3 in BR 1  of band  186 ) extends only into tracks 1 through 5 in the adjacent boundary region, i.e. BR 2  of band  184 . Thus, for boundary region BR 2  in band  184 , the counters for tracks 1-5 would be incremented by 5, 12, 21, 1 and 0, respectively, based on the corresponding CI values in the CI table.  FIG. 5B  is identical to  FIG. 5A , except that now the actuator has moved the write head towards the ID by one track and thus track 2 in boundary region BR 1  of band  186  is being written. Thus the center of the CI table is now depicted to the right of the new track being written (track 2 in BR 1  of band  186 ). As shown, the range of N tracks from the tracks being written now extends into tracks 1 through 6 in BR 2  of band  184 . Thus, for boundary region BR 2  in band  184 , the counters for tracks 1-6 would again be incremented, but this time by 0, 5, 12, 21, 1, 0, respectively, based on the corresponding CI values in the CI table.  FIGS. 5A-5B  are for an example where tracks in a BR 1  (a boundary region on the ID side of a band) are being written, which causes FTE in a BR 2  (a boundary region on the OD side of a band) in the adjacent band. This results in the use of CI values for the −N range (−1 to −8 SDTs) in the CI table. However, if tracks in a BR 2  (a boundary region on the OD side of a band) are being written, for example tracks in BR 2  of band  184 , this would cause FTE in a BR 1  (a boundary region on the ID side of a band) in the adjacent band, for example BR 1  of adjacent band  186 . This would result in the use of CI values for the +N range (+1 to +8 SDT#s) in the CI table. 
     During operation of the HDD, the controller (HDC  12  in  FIG. 1 ), or another controller or microprocessor in the HDD, identifies the track number where data is being written, recalls from the table the CI values for each track within the range and increases the counters for each track within the range by the recalled CI values. The table and the counters are stored in memory associated with controller  12 , for example memory  14 , which may be embedded in controller  12 , volatile memory  50  or nonvolatile memory  52 . When the count value for a track in the boundary region of a band reaches a predetermined threshold (T) the data is read from that band and rewritten to the band. The value for T can be chosen based on several factors, including the known track density of the HDD, the intended purpose of the HDD, the desired reliability, and the BER of the HDD measured during manufacturing. Thus, depending on these factors, T may be chosen to be a relatively high value, for example higher than 10,000, or a relatively low value, for example less than several hundred. After the data has been rewritten to a band, the counter or counters are reset to 0. 
     In another implementation of the invention, a single cumulative count is maintained for each boundary region of each band and the cumulative count is incremented by a cumulative count increment (CCI) for each writing to a track in an adjacent boundary region. For example, in  FIG. 5A , the FTE effect on tracks 1-5 of BR 2  in band  184  due to the writing track 3 in BR 1  of band  186  can be represented by a CCI corresponding to the sum of the CI values for these tracks. Thus for track 3 of a BR 1  (a boundary region on the ID side of a band), CCI=5+12+21+1+0=39. Similarly, as shown in  FIG. 5B , for track 2 of a BR 1 , CCI=3+5+12+21+1+0=42. A complete CCI table for the example of  FIGS. 5A-5B  is shown in  FIG. 6 . Thus the track number for the track being written in a boundary region is determined and the corresponding CCI value is recalled from the table and added to the cumulative count for the adjacent boundary. The CCI values are related to the number of tracks between the track being written and the adjacent boundary region and represent the cumulative effect of FTE on all the tracks within the range of the track being written. When the cumulative count value for a boundary region of a band reaches a predetermined threshold the data is read from that band and rewritten to the band. In this implementation only a single counter is required for a boundary region, i.e., only two counters for each band. 
     Because a HDD typically includes multiple disk surfaces, each with an associated read/write head, and because not all heads will have the same exact write profiles and thus not generate the same FTE effects, a table like that in  FIG. 4  can be developed for each head and its associated disk surface. Also, because of head skew, the write profile and thus the FTE effect for a particular head may vary depending on the radial position of the head. Thus multiple tables like the table in  FIG. 4  may be maintained for each head, depending on the radial position of the head. 
     The operation of the HDD as described above may be implemented as a set of computer program instructions stored in memory and executable by a processor, such as the HDC, or a separate controller or microprocessor in the HDD. The controller performs logical and arithmetic operations based on the program instructions stored in memory, and is thus capable of performing the functions described above and represented in the figures. 
     While the present invention has been particularly shown and described with reference to the preferred embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the spirit and scope of the invention. Accordingly, the disclosed invention is to be considered merely as illustrative and limited in scope only as specified in the appended claims.