Dynamic magnetic information storage or retrieval – Automatic control of a recorder mechanism – Controlling the head
Reexamination Certificate
1998-06-19
2001-02-13
Psitos, Aristotelis M. (Department: 2651)
Dynamic magnetic information storage or retrieval
Automatic control of a recorder mechanism
Controlling the head
C369S053370, C714S015000, C714S710000
Reexamination Certificate
active
06188538
ABSTRACT:
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 p
Mills Mark E.
Moore John L.
Hewlett -Packard Company
Psitos Aristotelis M.
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