Retry off-track positioning table

An improved heroic data recovery technique in which the inventive mechanism initially calibrates data storage devices' read/write mechanism misalignment characteristics and then reduces such calibration to an off-track positioning table. The table reflects a histogram indicating statistically where the misalignment is most likely to be found based on measurement of the characteristics of the device. The device's off-track positioning mechanism uses this table to execute a series of off-track read attempts in statistical order of likelihood of recovering unreadable data. The read head moves first to the off-track position in the table where statistically the misalignment is most likely to be found, and then to the next most likely, and so on, until the data is either found and recovered, or the series embodied in the table is exhausted. If the table becomes exhausted, an alternative heroic technique must be applied.

TECHNICAL FIELD OF THE INVENTION
 This invention relates generally to heroic techniques in the recovery of
 unreadable data stored previously on mass data storage devices, and more
 specifically to executing a series of off-track reading attempts in
 accordance with prior measurement of off-track error characteristics of
 specific data storage devices.
 BACKGROUND OF THE INVENTION
 In the art of mass data storage, it is to be expected that occasionally
 stored data becomes unreadable on the medium on which it is stored. A
 common reason for the data becoming unreadable is that the write mechanism
 loses correspondence with the read mechanism. When this happens, although
 the data as stored may have full integrity, the read mechanism is simply
 unable to retrieve the data as stored, since the write mechanism has
 deposited the data on the medium in a different location from the location
 where the read mechanism expects to find it.
 Standard algorithmic data recovery techniques such as Error Correction Code
 ("ECC") are typically unable to recover data lost in this way. Since the
 read and write mechanisms are out of correspondence, there is generally
 too much data missing for ECC to recover mathematically. In such cases,
 heroic data recovery techniques must be resorted to. Off-track positioning
 is the primary heroic technique used to recover data suspected to be lost
 through loss of correspondence between read and write mechanisms. If
 off-track positioning fails, the success of other heroic techniques such
 as modifying read channel characteristics (read bias, filter boost, etc.)
 or re-tensioning the tape (in the case of tape data storage) becomes very
 unpredictable.
 Standard off-track positioning techniques typically employ a predetermined
 sequence of read track adjustment steps where, starting at the track
 centerline (where the data is expected), the read head is displaced in
 successively larger off-track distances symmetrically either side of the
 centerline until the data is discovered. This methodology assumes that the
 small misalignments between read and write mechanisms are more likely to
 occur than large misalignments in every case.
 This "centerline increment" approach is flawed for at least two reasons.
 First, the forces that actually work to produce read/write mechanism
 misalignment include contributions from conditions where a large
 misalignment may be just as likely as a small misalignment. For example, a
 primary cause of misalignment is a "wander" between write head and storage
 medium. This "wander" is just as likely to be large as small, depending on
 the effects that are causing it. The "wander" may, however, be measured
 and subsequently predicted with some accuracy according to particular
 characteristics of the data storage device.
 Second, the "centerline increment" approach tends to magnify the effect of
 hysteresis inherent in the reciprocal movement of the read head. Most read
 head mechanisms employ meshing worm gears to enable small but repeatably
 accurate displacements across the storage medium. Slight imperfections (or
 normal wear and tear) in the gear profiles cause "gear lash", where, over
 time, "slop" in the gears diminishes the actual displacement of the read
 head for a given rotation of the drive worm gear. This hysteresis effect
 is generally optimized by biasing the meshing worm gears, normally with a
 spring. Nonetheless, this hysteresis effect always tends to have the most
 impact on small movements of the read head.
 The hysteresis effect is thus magnified in the "centerline increment"
 approach to off-track positioning, since the approach starts with small
 displacements either side of the centerline, and progresses to larger
 increments (where hysteresis may have a less pronounced effect) only when
 small displacements have failed to retrieve the data. Indeed, under the
 "centerline increment" approach in a device where hysteresis is
 pronounced, the off-track positioning mechanism may not actually move the
 read head off the track centerline for several consecutive incremental
 off-track read attempts. In such cases, the time to recover data via
 off-track positioning is clearly extended.
 There is therefore a need in the art to conduct data recovery via off-track
 positioning in accordance with predictable misalignment characteristics of
 the storage device rather than by some arbitrary approach such as
 "centerline increments". It would be further advantageous if improved
 off-track positioning techniques minimized the effect of hysteresis in the
 read head mechanism.
 SUMMARY OF THE INVENTION
 These and other objects, features and technical advantages are achieved by
 a system and method which initially calibrates data storage devices'
 read/write mechanism misalignment characteristics and then reduces such
 calibration to an off-track positioning table. The table is based on a
 statistical distribution indicating where the misalignment is most likely
 to be found, from the most likely off-track location down to the least
 likely, based on measurement of the characteristics of the device. The
 off-track positioning mechanism uses this table to execute a series of
 off-track reading attempts in statistical order of likelihood in
 recovering unreadable data.
 In a preferred embodiment, the read head moves off-track first to one side
 of the centerline, to the off-track position in the table where
 statistically the misalignment is most likely to be found. The read head
 then tries to recover data from there. If no data is found, the read head
 moves to the corresponding off-track position on the other side of the
 centerline. If no data is found on the other side, then the read head
 switches track position in the table in statistically the second-most
 likely place. If this read yields no data, then the read tries at the
 corresponding off-track position on the other side of the centerline. The
 process continues until either the data is found and recovered, or the
 series of offsets embodied in the table is exhausted. If the table is
 exhausted, then off-track data recovery was unsuccessful, and an
 alternative heroic technique must be applied.
 Note that the preferred embodiment uses symmetric off-track positioning, as
 described immediately above, in order to take advantage of proximity
 characteristics expected to be exhibited by "sequential" storage devices
 such as tape drives. It will be appreciated, however, that the invention
 is broader in concept, and is not limited to symmetric off-track
 positioning either side of the track centerline. Operators of ordinary
 skill in the art will know when proximity is likely to be a factor, as may
 vary from one type of storage device to another. Such operators will then
 apply the invention in conjunction with proximity considerations when
 appropriate.
 When proximity is not a consideration, the invention may be implemented in
 its broader form, in which a table is developed representing the
 statistical likelihood of finding off-track data without regard to the
 track centerline. In such an alternative embodiment, the statistical
 priorities established by the inventive table may call for a series of
 off-track read attempts that moves from one position to the next anywhere
 within the sampled off-track position spectrum.
 It is therefore a technical advantage of the present invention to use
 off-track positioning to recover data in accordance with the read/write
 mechanism misalignment characteristics of the data storage device, rather
 than an arbitrary method such as "centerline increments" as described in
 the previous section. As a result of correlating off-track data recovery
 to predictable misalignment characteristics, rapid data recovery becomes
 more attainable.
 It is a further technical advantage of the present invention to minimize
 the effect of hysteresis in the read head mechanism when implementing
 off-track positioning data recovery techniques. Because the distances
 traveled by the read head in following the sequence of off-track read
 attempts according to the inventive positioning table tend to be
 irregular, the effect of hysteresis tends to have a less pronounced impact
 on accurate movement of the read head mechanism.
 The foregoing has outlined rather broadly the features and technical
 advantages of the present invention in order that the detailed description
 of the invention that follows may be better understood. Additional
 features and advantages of the invention will be described hereinafter
 which form the subject of the claims of the invention. It should be
 appreciated by those skilled in the art that the conception and the
 specific embodiment disclosed may be readily utilized as a basis for
 modifying or designing other structures for carrying out the same purposes
 of the present invention. It should also be realized by those skilled in
 the art that such equivalent constructions do not depart from the spirit
 and scope of the invention as set forth in the appended claims.

DETAILED DESCRIPTION OF THE INVENTION
 The invention will be described in detail with reference to a preferred
 embodiment in which an inventive off-track positioning table is
 implemented on a tape-driven mass data storage device. It will be
 appreciated, however, that the invention is broad in concept, and is not
 limited to tape drive embodiments. The inventive off-track positioning
 table may be implemented equally well in any data storage application in
 which read/write mechanisms and data storage media move relative to each
 other. For example, the medium may be rotational in configuration (such as
 a cylinder or a platter), or it may be sequential (such as a tape). So
 long as data written off-track on the medium is susceptible to being
 recovered by making selected corresponding off-track read attempts, the
 inventive off-track positioning table may be utilized to improve the data
 recovery efficiency of such off-track read attempts.
 It will be further appreciated that, as described above in the previous
 section, the preferred embodiment makes symmetric off-track reads either
 side of the track centerline so as to take advantage of proximity
 considerations inherent in the misalignment characteristics expected in a
 sequential storage device such as the tape drive of the preferred
 embodiment. As noted above, though, the inventive off-track positioning
 table is not limited to such symmetric off-track reads, which are used in
 the preferred embodiment only because they are appropriate in view of the
 exemplary tape storage device used in a preferred embodiment. An
 alternative embodiment will be described summarily below wherein off-track
 reads are made without regard to the track centerline, and so therefore
 appear completely asymmetric about the track centerline.
 With reference to FIG. 1, the first stage of recovering the data in
 accordance with the inventive off-track positioning table is to calibrate
 the read/write mechanism misalignment characteristics of the data storage
 device on which it is to be implemented (block 101). It will be
 appreciated that these characteristics will vary from one storage device
 type to another, and even among storage devices of the same type. As
 noted, for example, it may be advantageous to implement the invention in
 conjunction with proximity considerations. Those of ordinary skill in this
 art, however, may be expected to already understand and identify the
 individual read/write mechanism misalignment characteristics of the
 various storage devices on which they may wish to implement the inventive
 off-track positioning table, and to select and apply them accordingly. For
 example, in the tape drive storage device of the preferred embodiment, it
 is well understood that tape "wander" may occur perpendicular to the
 direction of travel of the tape as the tape is unwound from one reel and
 wound onto the other reel. It is further well understood that such
 "wander" is generally cyclic and tends to be more pronounced at the end of
 a tape than at the beginning. Additional effects well understood in the
 tape drive devices to contribute to loss of correspondence between read
 and write mechanisms include variation in tape tension (tape tends to be
 tighter near the beginning than at the end), variation of the attitude at
 which the tape addresses the read/write head as the tape passes from one
 reel to another, and thermal expansion effects.
 It will therefore be appreciated that operators of ordinary skill in the
 art will know which read/write mechanism misalignment characteristics to
 measure and calibrate for the particular device. Calibration is enabled by
 monitoring the misalignment behavior of the read/write mechanisms for a
 suitable sample of naturally-occurring data, and charting a statistical
 representation of the distribution of the off-track repositioning required
 to recover the data sample completely. The distribution then allows
 specific off-track read positions to be ranked according to the
 statistical probability that the data is likely to be recovered there.
 With further reference to FIG. 1, the next stage of recovering data in
 accordance with the invention is to develop the inventive table (block
 102). The table comprises the off-track read positions ranked in order of
 likelihood that data will be recovered there, from most likely to least
 likely.
 The table is then implemented in the firmware of the device (block 103) and
 is followed whenever off-track positioning is used by the device to
 attempt to recover unreadable data (block 104). The device makes off-track
 read attempts in a series following the table, starting with the off-track
 read position most likely to find the data, followed by the next most
 likely, and so on down to the least likely.
 Turning now to FIG. 2, histogram 200 represents a hypothetical calibration
 of an exemplary tape storage device used in a preferred embodiment of the
 present invention. Since a tape storage device is being used, the operator
 wishes to take advantage of proximity effects, and so symmetrical
 implementation either side of the track centerline will be used. As
 emphasized at length above, however, other embodiments are possible
 consistent with the scope of the invention in which such symmetrical
 implementations are not selected.
 With further reference to histogram 200 on FIG. 2, the incidence of data
 recovery at each off-track position (cumulative incidences for that
 position either side of the track centerline) is charted to indicate a
 distribution. From this distribution, table 300 on FIG. 3 can be
 developed, ranking off-track positions in order of statistical likelihood
 of recovering data there.
 FIG. 4 shows table 300 on FIG. 3 implemented on the exemplary tape drive
 during off-track data recovery. FIG. 4 represents a data track 400 on a
 tape, 401C being the track centerline, 401U being the upper boundary with
 the data track above, and 401L being the lower boundary with the data
 track below. Data track 400 is divided into 24 off-track positions 402, +1
 through +12 above track centerline 401C, and -1 through -12 below the
 track centerline 401C.
 Column 403 on FIG. 4 indicates the sequence in which the device will make
 off-track read attempts in accordance with table 300 on FIG. 3. Since
 off-track position 5 is at the top of table 300, column 403 shows attempts
 1 and 2 at off-track positions +5 and -5, respectively. Positions +7 and
 -7 are next, followed by positions +2 and -2, and so on until the last
 attempts are +10 and -10. The off-track read sequence thus follows the
 misalignment characteristics sampled and calibrated in developing
 histogram 200 on FIG. 2.
 The symmetric implementation on FIG. 4 is self-evident. It will nonetheless
 be appreciated, without the need for further illustration, that the
 present invention applies with equivalent enabling effect on embodiments
 where there is no symmetric implementation (i.e. the incidence of data
 recovery at each off-track position 402 on FIG. 4 is recorded and
 distributed independently, instead of cumulatively with the corresponding
 position on the other side of the track centerline). A histogram is
 developed similar to histogram 200 on FIG. 2, in which the statistical
 likelihood of recovering data at each track is charted. This histogram is
 converted into a corresponding table 300 on FIG. 3, from which an
 off-track read position sequence can be implemented corresponding to
 column 403 on FIG. 4. The only difference is that the off-track read
 sequence in column 403 is likely to be distributed unevenly about track
 centerline 401C.
 The technical advantages as described in the previous section are thus
 achieved. Off-track data recovery is implemented in a sequence correlated
 to the misalignment characteristics of the read/write mechanisms of the
 current device. As a result, data recovery is accelerated. Implementations
 of the invention in practice have consistently demonstrated at least a
 12-fold performance improvement in the time required to recover otherwise
 unreadable data.
 The advantage to minimize the effect of mechanical hysteresis is also
 achieved. In the embodiment illustrated on FIG. 4, the read head moves the
 following distances in following column 403 (even though the
 implementation is chosen to be symmetric): +5, -10, +12, -14, +9, -4, +11,
 -18 and so on. It will be appreciated that these steps are large and
 irregular, thus tending to minimize the effect of mechanical hysteresis to
 degrade data recovery performance.
 Similarly, when symmetrical implementation is not selected, the steps taken
 by the read head will continue to be irregular, merely by the nature of
 the statistical sampling of individual off-track read positions on an
 independent basis. Mechanical hysteresis will also be minimized in these
 implementations.
 An exemplary algorithm employing the present invention may comprise the
 following steps:
 (a) recording a set of data on the data track;
 (b) detecting instances of data recorded in step (a) becoming unreadable by
 the device due to the read head not tracking the data as recorded on the
 data track;
 (c) performing selected off-track reads so as to recover the unreadable
 data detected in step (b), each off-track read performed at an off-track
 position in a spectrum thereof across the data track;
 (d) for each off-track position in the spectrum, counting the number of
 times that data is recovered from that off-track-position during step (c);
 (e) ranking off-track positions into a series corresponding to incidents of
 data recovery counted in step (d), the series in order of the off-track
 position having most incidents down to the off-track position having least
 incidents;
 (f) embodying the series in a table;
 (g) loading the table into firmware on the device; and
 (h) following the table in future off-track read attempts by the device to
 recover unreadable data.
 Other embodiments of the invention are also possible. Since the tables are
 implemented in firmware on each device, they may be periodically updated
 as new statistical findings are made. For example, a new type of storage
 medium may be recommended for use with the device, such as a new CD or
 tape cartridge, having improved read/write mechanism alignment
 characteristics. Consistent with the present invention, new off-tracking
 positioning tables will be advantageously be loaded into the device's
 firmware. Alternatively, further research and development may be done on
 existing tables to enhance data recovery efficiency, requiring updating of
 such existing tables previously implemented in firmware.
 Although the present invention and its advantages have been described in
 detail, it should be understood that various changes, substitutions and
 alterations can be made herein without departing from the spirit and scope
 of the invention as defined by the appended claims.