U.S. patent application number 13/423177 was filed with the patent office on 2013-09-19 for shingled magnetic recording disk drive with minimization of the effect of far track erasure on adjacent data bands.
The applicant listed for this patent is Zvonimir Z. Bandic, Cyril Guyot, Tomohiro Harayama, Robert Eugeniu Mateescu, Shad Henry Thorstenson, Timothy Kohchih Tsai. Invention is credited to Zvonimir Z. Bandic, Cyril Guyot, Tomohiro Harayama, Robert Eugeniu Mateescu, Shad Henry Thorstenson, Timothy Kohchih Tsai.
Application Number | 20130242426 13/423177 |
Document ID | / |
Family ID | 49122388 |
Filed Date | 2013-09-19 |
United States Patent
Application |
20130242426 |
Kind Code |
A1 |
Bandic; Zvonimir Z. ; et
al. |
September 19, 2013 |
SHINGLED MAGNETIC RECORDING DISK DRIVE WITH MINIMIZATION OF THE
EFFECT OF FAR TRACK ERASURE ON ADJACENT DATA BANDS
Abstract
A shingled magnetic recording (SMR) hard disk drive (HDD)
essentially eliminates the effect of far track erasure (FTE) in the
boundary regions of annular data bands caused by writing in the
boundary regions of adjacent annular data bands. The extent of the
FTE effect is determined for each track within a range of tracks of
the track being written. Based on the relative FTE effect for all
the tracks in the range, a count increment (CI) table or a
cumulative count increment (CCI) table is maintained for all the
tracks in the range. For every writing to a track in a boundary
region, a count for each track in an adjacent boundary region, or a
cumulative count for the adjacent boundary region, is increased.
When the count reaches a predetermined threshold the data is read
from that band and rewritten to the same band.
Inventors: |
Bandic; Zvonimir Z.; (San
Jose, CA) ; Guyot; Cyril; (San Jose, CA) ;
Harayama; Tomohiro; (Sunnyvale, CA) ; Mateescu;
Robert Eugeniu; (San Jose, CA) ; Thorstenson; Shad
Henry; (Rochester, MN) ; Tsai; Timothy Kohchih;
(Alviso, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Bandic; Zvonimir Z.
Guyot; Cyril
Harayama; Tomohiro
Mateescu; Robert Eugeniu
Thorstenson; Shad Henry
Tsai; Timothy Kohchih |
San Jose
San Jose
Sunnyvale
San Jose
Rochester
Alviso |
CA
CA
CA
CA
MN
CA |
US
US
US
US
US
US |
|
|
Family ID: |
49122388 |
Appl. No.: |
13/423177 |
Filed: |
March 17, 2012 |
Current U.S.
Class: |
360/31 |
Current CPC
Class: |
G11B 20/1258 20130101;
G11B 2020/10898 20130101; G11B 27/36 20130101; G11B 5/012 20130101;
G11B 2020/1298 20130101; G11B 20/1816 20130101 |
Class at
Publication: |
360/31 |
International
Class: |
G11B 27/36 20060101
G11B027/36 |
Claims
1. A shingled magnetic recording disk drive comprising: a magnetic
recording disk having a recording surface with a plurality of
concentric shingled data tracks arranged in annular bands separated
by annular inter-band gaps, each band having a boundary region of
tracks adjacent a gap, whereby each gap is located between adjacent
boundary regions; a write head associated with said disk surface
for generating a magnetic write field to write data to the data
tracks; a read head for reading data written in the data tracks; a
controller for controlling the writing of data by the write head to
the data tracks; and memory coupled to the controller and
containing a program of instructions readable by the controller for
minimizing the effect of encroachment of the write field on data
tracks in a boundary region when a data track is being written in
an adjacent boundary region, the program of instructions
undertaking the method acts comprising: (a) maintaining in memory
at least one count for each boundary region; (b) for each writing
of data to a track in a boundary region, increasing said at least
one count for an adjacent boundary region by a predetermined
increment, said increment being determined from the number of
tracks between the track being written and said adjacent boundary
region; and (c) when a count reaches a predetermined threshold,
reading the data from the band containing said threshold-count
boundary region and rewriting the data read from said band.
2. The disk drive of claim 1 wherein there are N tracks in each
boundary region and wherein maintaining in memory at least one
count for each boundary region comprises maintaining a count for
each of said N tracks.
3. The disk drive of claim 2 wherein step (b) comprises: for each
writing of data to a track in a boundary region, increasing said at
least one count for each of said N tracks in an adjacent boundary
region by a predetermined increment, said increment being
determined from the number of tracks between the track being
written and the track whose count is being incremented.
4. The disk drive of claim 3 wherein the effect of encroachment of
the write field on data tracks is within a range between -N tracks
and +N tracks and further comprising a table in memory of 2N tracks
having range numbers between -N and +N and corresponding 2N count
increment values, and wherein the method act of increasing said at
least one count for each of said N tracks in an adjacent boundary
region by a predetermined increment includes determining the range
number for each track within said range and recalling from the
table in memory the corresponding count increment value.
5. The disk drive of claim 4 wherein each of the count increment
values is related to a measured error rate for the corresponding
track.
6. The disk drive of claim 1 wherein there are N tracks in each
boundary region and wherein maintaining in memory at least one
count for each boundary region comprises maintaining a cumulative
count for each boundary region.
7. The disk drive of claim 6 wherein step (b) comprises: for each
writing of data to a track in a boundary region, increasing said
cumulative count for an adjacent boundary region by a predetermined
cumulative count increment, said cumulative count increment being
determined from which of the data tracks is being written.
8. The disk drive of claim 7 wherein the effect of encroachment of
the write field on data tracks is within a range between -N tracks
and +N tracks and further comprising a table in memory of 2N tracks
having range numbers between -N and +N and corresponding 2N
cumulative count increment values, and wherein the method act of
increasing said cumulative count by a predetermined increment
includes determining the track number for the track being written
and recalling from the table in memory the corresponding cumulative
count increment value.
9. The disk drive of claim 8 wherein each of the cumulative count
increment values is related to the sum of the measured error rates
for all the tracks within the range of the track being written.
10. The disk drive of claim 1 wherein the disk drive has a
plurality of disk surfaces, each having a plurality of concentric
data tracks, and associated write heads, and wherein the program of
instructions comprises undertaking method acts (a) through (c) for
each disk surface.
11. The disk drive of claim 1 wherein the memory is nonvolatile
memory.
12. A magnetic recording disk drive comprising: a rotatable
magnetic recording disk comprising a substrate and a magnetic
recording layer on the substrate; a head carrier having a
recording-layer-facing surface; a write head on the head carrier,
the write head generating a generally circular path of magnetic
transitions in the recording layer as the disk rotates; an actuator
connected to the head carrier for moving the head carrier generally
radially across the disk, the actuator being capable of moving the
head in an increment less than the radial width of a path, whereby
the write head generates partially overlapping generally circular
paths of magnetic transitions, the non-overlapping portions of the
circular paths representing data tracks, the data tracks being
arranged in annular bands separated by annular inter-band gaps,
each band having a boundary region of data tracks adjacent a gap,
whereby each gap is located between adjacent boundary regions; a
read head on the carrier for reading written data from the data
tracks; memory coupled to the controller and containing a program
of instructions readable by the controller for minimizing the
effect of encroachment of the write field on data tracks in a
boundary region when a data track is being written in an adjacent
boundary region, the program of instructions undertaking the method
acts comprising: (a) maintaining in memory a cumulative count for
each boundary region; (b) maintaining in memory a table of track
numbers between -N tracks and +N tracks and corresponding 2N
cumulative count increment (CCI) values; (c) for each writing of
data to a track in a boundary region, determining the track number
and recalling from said table the corresponding CCI; (d) increasing
said cumulative count by said recalled CCI for the boundary region
adjacent the boundary region containing the track being written;
and (e) when said cumulative count reaches a predetermined
threshold, reading the data from the band containing said
threshold-count boundary region and rewriting the data read from
said band.
13. The disk drive of claim 12 wherein each of the CCI values is
related to the sum of the measured error rates for all the tracks
within a range of N tracks of the track being written.
14. The disk drive of claim 12 wherein the disk drive has a
plurality of disk surfaces, each having a plurality of data tracks,
and associated write heads, and wherein the program of instructions
comprises undertaking method acts (a) through (e) for each disk
surface.
15. The disk drive of claim 12 wherein the memory is nonvolatile
memory.
Description
RELATED APPLICATION
[0001] 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
[0002] 1. Field of the Invention
[0003] 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.
[0004] 2. Description of the Related Art
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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
[0009] 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.
[0010] 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
[0011] FIG. 1 is a top view of a SMR disk drive for use with the
method according to the invention.
[0012] 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.
[0013] 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).
[0014] 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.
[0015] 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.
[0016] 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.
[0017] 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
[0018] 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.
[0019] 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.
[0020] 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.
[0021] 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).
[0022] 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.
[0023] 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.
[0024] 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.
[0025] 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.
[0026] 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.
[0027] 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.
[0028] 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'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 (.DELTA.log
(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 .DELTA.log (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.sub.TR#=10.sup.[.DELTA.log(BER.sup.TR#.sup.)-REF]
[0029] 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, BR1 at the ID side and BR2 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 BR1 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 BR1 of band 186) extends only into tracks 1
through 5 in the adjacent boundary region, i.e. BR2 of band 184.
Thus, for boundary region BR2 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 BR1 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 BR1 of band 186). As shown, the range of N tracks from
the tracks being written now extends into tracks 1 through 6 in BR2
of band 184. Thus, for boundary region BR2 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 BR1 (a boundary region on the ID side of a band) are being
written, which causes FTE in a BR2 (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 BR2 (a boundary region on the OD side of a band) are
being written, for example tracks in BR2 of band 184, this would
cause FTE in a BR1 (a boundary region on the ID side of a band) in
the adjacent band, for example BR1 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.
[0030] 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.
[0031] 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
BR2 in band 184 due to the writing track 3 in BR1 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 BR1 (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 BR1, 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.
[0032] 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.
[0033] 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.
[0034] 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.
* * * * *