Data processing: database and file management or data structures – Database design – Data structure types
Reexamination Certificate
1998-06-12
2001-11-06
Shah, Sanjiv (Department: 2172)
Data processing: database and file management or data structures
Database design
Data structure types
C714S758000, C360S053000
Reexamination Certificate
active
06314433
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates generally to heroic techniques in the recovery of corrupt data detected on mass storage devices, and more specifically to accumulating the corrective effects of selected heroic techniques sequentially performed on an entire frame of data in which corrupt blocks have been detected.
BACKGROUND OF THE INVENTION
In the art of mass data storage, it is to be expected that occasionally stored data becomes corrupt on the medium on which it is stored. The read mechanism then becomes unable to retrieve the data from the storage device. Reasons for data becoming corrupt include irregularities in the physical medium on which the data is stored, loss of correspondence between the write mechanism and the read mechanism, or exposure of the data as stored on the medium to adverse environmental conditions. Whatever the reason for the data becoming corrupt, steps must be taken to try to recover the data or else it is lost.
A standard data recovery technique is store the data with a periodic Error Correction Code (“ECC”). The ECC is derived from a predetermined algorithm, in which actual stored data values in the data group to which the ECC pertains are used as the values of variables in the algorithm. The result of the algorithm is the ECC for the data group to which the ECC pertains, and, as noted, the ECC is physically stored on the storage medium in association with the data group. Then, where a portion of the data group is detected to be corrupt, the value of the ECC can be “reversed” back through the algorithm to recover the lost data values to a limited extent.
The extent to which a standard data recovery technique such as ECC can universally recover corrupt data depends on the sophistication of the underlying algorithm and how badly the data is corrupt. The ability of standard data recovery techniques such as ECC to recover data is therefore limited.
“Heroic” data recovery techniques are resorted to in the recovery of stored data when the level of data corruption is too great for standard techniques such as ECC to work. Hence the term “heroic”, which for purposes of this disclosure means measures above and beyond standard data recovery techniques such as ECC. Examples of these “heroic” techniques include:
re-positioning of the read sensor from the original
ominal location;
modification of read channel characteristics (read bias, filter boost, etc.); and
re-tensioning of the tape (in the case of tape data storage).
Data is generally stored in blocks of a specific, predetermined size. ECC is typically then implemented on a frame basis, a frame being simply a predetermined logical set of data blocks. Error detection and correction during read operations commonly uses a Cyclic Redundancy Check (CRC) at the block level to detect blocks which were incorrectly read from the storage medium. Heroic data recovery operations traditionally are also performed at the block level, so that when CRC detects a corrupt block, heroic operations can be performed on each sequential corrupt block, one block at a time, in the heroic data recovery process.
An inconsistency in traditional data recovery approaches has therefore always been present in that standard techniques such as ECC are done on a frame basis, while heroic data techniques are done on a block basis. Even though only one or two blocks may be detected in error, an entire frame has to be isolated in order to attempt standard data recovery using ECC. If ECC is unsuccessful, and heroic techniques are required, the frame must then be broken back down into blocks to allow heroic recovery on the traditional block-by-block basis. Considerable data processing and physical movement of the read mechanism would be saved if heroic recovery could be done on a frame basis. There is therefore a need in the art to perform heroic data recovery on a frame basis.
SUMMARY OF THE INVENTION
These and other objects, features and technical advantages are achieved by a system and method in which selected heroic data recovery techniques are performed repeatedly on a frame of data blocks in which errors have been detected. The invention becomes operable when standard data recovery techniques such as ECC, working on a frame-by-frame basis, are unable to recover all the corrupt data within the frame because there are just too many corrupt data blocks. The invention then sequentially applies a preselected series of heroic techniques, each heroic technique in the series applied concurrently to all blocks in the frame. Each technique can be expected to recover some, but probably not all of the corrupt blocks. Further, repetition of a particular technique in the series, or each different technique in the series may be expected to recover different corrupt blocks in the frame. The invention combines the corrective effects of the individual applications of heroic techniques in the series, until sufficient individual data blocks overall in the frame are recovered to allow ECC to complete the job.
A preferred embodiment describes the invention with reference to recovering frames of data stored on a tape. The embodiment further describes an exemplary frame size of
64
blocks each storing 1KB of data. ECC is the embodiment's selected standard data recovery technique to be assisted by the inventive application of frame-based heroic data recovery techniques. It will be appreciated, however, that the invention is not limited in these exemplary regards. The invention is independent of any particular storage medium, or any specific standard data recovery technique to be assisted by heroic techniques. The invention is broader in concept in that the advantage may be had by successively applying heroic techniques to a frame of data blocks, the cumulative corrective effect thereof on individual corrupt blocks eventually enabling a frame-based standard data recovery technique such as ECC to take over and completely recover the frame.
It is therefore a technical advantage of the present invention to increase the efficiency with which data is recovered in a combination of standard and heroic techniques. As noted, when the presence of corrupt data is detected at a block level using, for example, checking techniques such as CRC, the first step is to allow standard data recovery techniques such as ECC to attempt to recover the data. The data recovery mechanism therefore goes back and reads in the entire frame in which the corrupt data has been detected. If the data is too corrupt (i.e. there are too many corrupt blocks) for the standard technique to be able to recover the entire frame, then in a traditional approach the data recovery mechanism will then proceed to apply selected heroic data recovery techniques on a block-by-block basis. A block will be read in repeatedly until a heroic technique recovers it. Then the mechanism will move on to the next block.
Under the inventive mechanism, however, once it has been determined at a frame level that standard recovery techniques are insufficient and heroic techniques are needed, a selected series of heroic techniques are applied sequentially, each to an entire frame of blocks at once. Some blocks may be recovered, others may not, but with each successive frame-based application of a heroic technique in which one or more new blocks recovered, the frame becomes increasingly disposed to be recovered by a standard technique such as ECC.
A first improved efficiency of the inventive mechanism over the traditional approach arises out of obviating the need to repeatedly read in individual data blocks to apply heroic techniques until the block is recovered. Instead, entire frames are read in until sufficient blocks are recovered to allow standard techniques such as ECC to recover the frame. The number of starts, stops, reverses and reads required of the data recovery mechanism is thus reduced, saving time required for data recovery as well as wear and tear on the mechanism itself. Further, the full corrective capability of standard techniques is enabled, reducing the amount of heroic data recovery ac
Mills Mark E.
Moore John L.
Hewlett--Packard Company
Shah Sanjiv
LandOfFree
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