Data storage device saving absolute position at each servo wedge for previous write operations

Disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and servo sectors. The servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo control system to control the actuator arm as it seeks from track to track.

FIG. 1 shows a prior art disk format 2 as comprising a number of servo tracks 4 defined by servo sectors 6 0 - 6 N recorded around the circumference of each servo track. Each servo sector 6 , comprises a preamble 8 for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark 10 for storing a special pattern used to symbol synchronize to a servo data field 12 . The servo data field 12 stores coarse head positioning information, such as a servo track address, used to position the head over a target data track during a seek operation. Each servo sector 6 i further comprises groups of servo bursts 14 (e.g., N and Q servo bursts), which are recorded with a predetermined phase relative to one another and relative to the servo track centerlines. The phase based servo bursts 14 provide fine head position information used for centerline tracking while accessing a data track during write/read operations. A position error signal (PES) is generated by reading the servo bursts 14 , wherein the PES represents a measured position of the head relative to a centerline of a target servo track. A servo controller processes the PES to generate a control signal applied to a head actuator (e.g., a voice coil motor) in order to actuate the head radially over the disk in a direction that reduces the PES.

FIG. 1 shows a prior art disk format comprising a plurality of tracks defined by servo sectors that form servo wedges.

FIG. 2A shows a data storage device in the form of a disk drive according to an embodiment comprising a head actuated over a disk comprising a plurality of tracks defined by servo sectors, wherein each servo sector comprises a plurality of servo bursts and the servo sectors define a plurality of servo wedges.

FIG. 2B is a flow diagram according to an embodiment wherein an absolute position of the head is evaluated to detect an off-track write condition.

FIG. 2C illustrates an off-track write condition detected based on the absolute positions of adjacent track writes according to an embodiment.

FIGS. 3A-3D illustrate an embodiment wherein the absolute position of the head at each servo wedge during write operations is saved in a position buffer representing a circumference of the disk.

FIG. 4 is a flow diagram according to an embodiment wherein the position buffer is updated based on a current write operation.

FIG. 5 is a flow diagram according to an embodiment wherein an off-track write condition may be detected based on the absolute position of the head and an absolute reference position.

FIG. 6 is a flow diagram according to an embodiment wherein when an off-track write condition is detected, at least one data wedge of the previously written, adjacent track is rewritten.

FIG. 7 is a flow diagram according to an embodiment wherein an off-track write condition may be predicted before it occurs.

FIG. 8 shows an embodiment wherein the disk is divided into a number of radial zones, wherein each radial zone is tracked using a position buffer and a data buffer.

FIG. 9A shows a data storage device in the form of a disk drive according to an embodiment comprising a head actuated over a disk.

FIG. 9B is a flow diagram according to an embodiment wherein an off-track write threshold is selected based on a write throughput and a read throughput of the data storage device.

FIG. 10 illustrates an embodiment wherein the off-track write threshold is selected as the minimum of the measured off-track write thresholds that substantially maximizes the write throughput biased by a first weighting and the read throughput biased by a second weighting.

FIG. 2A shows a data storage device in the form of a disk drive according to an embodiment comprising a head 16 actuated over a disk 18 comprising a plurality of tracks 20 defined by servo sectors 22 0 - 22 N , wherein each servo sector comprises a plurality of servo bursts and the servo sectors 22 0 - 22 N define a plurality of servo wedges. The disk drive further comprises control circuitry 24 configured to execute the flow diagram of FIG. 2B , wherein a first write operation is executed to a first track and a first absolute position for the head is generated corresponding to a first servo wedge (block 26 ). A second write operation is executed to a second track and a second absolute position for the head is generated corresponding to the first servo wedge (block 28 ). An off-track write condition is detected (block 32 ) when executing the second write operation based on the first absolute position and the second absolute position (block 30 ). The first absolute position comprises a track address of the first track and a position error measurement based on the servo bursts in the first servo wedge.

In the embodiment of FIG. 2A , the servo sectors 22 0 - 22 N define a plurality of servo tracks, wherein data tracks are defined relative to the servo tracks at the same or different radial density. The control circuitry 24 processes a read signal 34 emanating from the head 16 to demodulate the servo sectors 22 0 - 22 N and generate a position error signal (PES) representing an error between the actual position of the head and a target position relative to a target track. The control circuitry 24 filters the PES using a suitable compensation filter to generate a control signal 36 applied to a voice coil motor (VCM) 38 which rotates an actuator arm 40 about a pivot in order to actuate the head 16 radially over the disk 18 in a direction that reduces the PES. The servo sectors 22 0 - 22 N may comprise any suitable head position information, such as a track address for coarse positioning and servo bursts for fine positioning. The servo bursts may comprise any suitable pattern, such as an amplitude based servo pattern or a phase based servo pattern.

During a write operation, the head 16 may deviate from the center of the target track resulting in an off-track write condition. In one embodiment shown in FIG. 2C , the off-track write condition is detected relative to the absolute position of the head 16 when writing a current track n+1 and the absolute position of the head 16 when writing a previous track n. For example, if the difference between the absolute positions is less than a threshold at block 30 of FIG. 2B , then an off-track write condition is detected. Referring to the example of FIG. 2C , an off-track write condition is not detected at servo wedge SS[i] whereas an off-track write condition is detected at servo wedge SS[i+1]. In one embodiment, in order to detect the off-track write condition, the absolute position of the head when writing the adjacent track n is saved in a position buffer. In one embodiment, the position buffer stores an absolute position corresponding to each servo wedge so that if the entire track n is written, then each entry of the position buffer is updated with the absolute position generated at each servo wedge. In one embodiment, write operations may be executed to non-adjacent tracks (including partial tracks) such that the position buffer may store the absolute position for each servo wedge that may correspond to different, non-adjacent tracks.

An example of this embodiment is illustrated in FIGS. 3A-3D which illustrate how the position buffer may be updated when executing a number of write operations. This example shows the disk 18 may comprise 14 servo wedges ( 0 - 13 ); however, the disk 18 may comprise any suitable number of servo wedges. FIG. 3A shows a first write operation to a first track n, wherein the first write operation spans servo wedges 2 - 6 and therefore a corresponding absolute position generated at servo wedges 2 - 6 is stored in the position buffer. FIG. 3B shows a second write operation to a second, adjacent track n+1, wherein the second write operation spans servo wedges 4 - 10 and therefore a corresponding absolute position generated at servo wedges 4 - 10 is stored in the position buffer. In this example, the absolute position previously stored in the position buffer for servo wedges 4 - 6 when executing the first write operation ( FIG. 3A ) are overwritten with the absolute position generated for servo wedges 4 - 6 during the second write operation ( FIG. 3B ).

When executing the second write operation, the absolute position generated at servo wedge 4 may be compared to the absolute position saved in the position buffer in order to detect an off-track write condition. If the off-track write condition is not detected, the absolute position generated at servo wedge 4 during the second write operation may replace the absolute position previously stored in the position buffer. In an alternative embodiment, the absolute position generated at each servo wedge during the second write operation may be stored in a scratch buffer. If the second write operation completes successfully without detecting an off-track write condition, then the absolute position values stored in the scratch buffer may replace the absolute position values stored in the position buffer. This embodiment may facilitate rewriting one or more of the data wedges during a retry write operation if an off-track write condition is detected.

FIG. 3C shows a third write operation to a third, non-adjacent track n+7 (non-adjacent to track n+1), wherein the third write operation spans servo wedges 8 - 11 and therefore a corresponding absolute position generated at servo wedges 8 - 11 is stored in the position buffer. When executing the third write operation, the difference between the absolute position stored in the position buffer for servo wedge 8 (which corresponds to writing track n+1) and the absolute position generated for the third write operation will be very large since the third track n+7 is non-adjacent with the previously written track n+1. Therefore an off-track write condition based on the absolute position stored in the position buffer will not be detected. During the fourth write operation to track n+2 as shown in FIG. 3D , an off-track write condition may be detected for servo wedges 4 - 5 since the absolute position stored in the position buffer corresponds to adjacent track n+1. Accordingly, in this embodiment the position buffer stores an absolute position for each servo wedge around the circumference of the disk; however, the absolute positions stored in the position buffer may correspond to different (potentially non-adjacent) tracks. A benefit provided by this embodiment is that an off-track write condition may be detected based on the absolute position of the head during the write operations without having to store the absolute position for every track written. Further, by storing the absolute position (as compared to just storing the PES for each servo wedge) in the position buffer, the case where the written tracks are non-adjacent is automatically taken into account.

In one embodiment shown in FIG. 8 , the disk may be divided into a number of radial zones, wherein a separate position buffer (ABP buffer) may be maintained for each radial zone. This embodiment may improve the chances of detecting an off-track write condition since the content of each position buffer will more likely correspond to an adjacently written track. For example, in one embodiment one or more of the radial zones may correspond to a “shingled zone” wherein the tracks may be written as a circular buffer using log structured writes. In this embodiment, the tracks of a “shingled zone” are written consecutively such that the corresponding position buffer will typically comprise the absolute position information of an adjacent, previously written track. Even in radial zones where the writes may not always be consecutive, at least part of the position buffer may store absolute position information of a previously written, adjacent track such that an off-track write condition may be detected using the position buffer as described above.

In the embodiment described above, the absolute position may comprise a track address and a position error measurement based on the servo bursts in the servo wedge. In another embodiment, the absolute position may further include a head number corresponding to the disk surface being written in a multi-disk surface disk drive. In this embodiment, an off-track write condition may be detected based on the absolute position stored in the position buffer when writing to the same disk surface.

FIG. 4 is a flow diagram according to an embodiment for executing a write operation (block 42 ), wherein when the head reaches servo sector i (block 44 ) an absolute position ABPw is generated (block 46 ) corresponding to the servo wedge. A difference between the absolute position ABPw and the corresponding absolute position ABP[i] stored in the position buffer is compared to a threshold (block 48 ). If the difference is greater than the threshold, then an off-track-write condition is not detected and the absolute position ABPw is stored in the position buffer at ABP[i] (block 50 ). The write operation then continues (block 52 ) until the write operation finishes (block 54 ) or until an off-track write condition is detected at block 48 . If it is not completed at block 54 , the next servo sector is processed (increments i) and the process returns to block 44 . In the embodiment of FIG. 4 , when an off-track write condition is detected at block 48 , the write operation is aborted (block 56 ). In an alternative embodiment, the absolute positions generated at block 46 may be saved in a scratch buffer, and the content of the scratch buffer copied to the position buffer after the write operation has successfully completed. This embodiment may facilitate a retry write operation when a write operation is aborted due to detecting an off-track write condition.

FIG. 5 is a flow diagram according to an embodiment which extends on the flow diagram of FIG. 4 , wherein when an off-track write condition is not detected at block 48 based on the position buffer, an off-track write condition may still be detected at block 58 if the absolute position ABPw deviates sufficiently from a reference absolute position (ABRw) which corresponds to the center of the track being written. That is, if the difference between the absolute position ABPw and the reference position ABRw exceeds a threshold (block 58 ) an off-track write condition is detected and the write operation is aborted at block 56 .

In one embodiment, the control circuitry 24 of FIG. 2A is configured to define a data buffer corresponding to a circumference of the disk. When a first write operation is executed to write first write data to a first track, the first write data is stored to a first part of the data buffer. When second write operation is executed to write second write data to a second track, at least part of the second write data is stored to a second part of the data buffer. When a third write operation is executed to write third write data to a third track, at least part of the third write data is stored to a third part of the data buffer. The data buffer simultaneously stores at least part of the first write data, the second write data, and the third write data, wherein the third track is non-adjacent to the first track. An example of this embodiment may be understood with reference to FIGS. 3A-3D described above, except that each block of the buffer corresponds to a data wedge of a write operation rather than an absolute position generated for each servo wedge.

FIG. 6 is a flow diagram illustrating this embodiment which extends on the flow diagram of FIG. 5 , wherein when executing a write operation the write data written to a data wedge of the track is stored in a data buffer (block 60 ) similar to storing the absolute position information in the position buffer at block 50 . When writing data to different data tracks, the write data overwrites the previously stored write data in the data buffer at block 60 . Accordingly, the data buffer may store data wedges for different tracks, including for non-adjacent tracks. In this manner, when an off-track write condition is detected at block 48 based on the position buffer when writing to a current track, the write data stored in the data buffer may be used to refresh the data written to a previously written, adjacent track (block 62 ). When an off-track write condition is detected at block 58 based on the absolute reference position, the write operation is aborted (block 64 ) without refreshing a previously written, adjacent track since the data stored in the data buffer does not correspond to a previously written, adjacent track.

FIG. 8 illustrates an embodiment wherein the control circuitry may maintain a data buffer for each radial zone similar to maintaining a position buffer for each radial zone. Accordingly, if an off-track write condition is detected based on the position buffer for a radial zone, the corresponding data buffer may be used to refresh the data written in the previously written, adjacent track before retrying the current write operation to the current track.

FIG. 7 is a flow diagram illustrating an embodiment which extends on the flow diagram of FIG. 6 , wherein when an off-track write condition is not detected at block 48 based on the position buffer, and an off-track write condition is not detected at block 58 based on the absolute reference position ABRw, an off-track write condition may still be predicted at block 66 . In this embodiment, a predicted absolute position (ABPpw) corresponding to a next servo wedge is generated using any suitable algorithm. For example, in one embodiment the predicted absolute position is generated based on:

ABPpw ( k )=2· ABPw ( k )− ABPw ( k− 1)

If a difference between the ABPpw and the absolute position stored in the position buffer APB[i+1] for the next servo wedge exceeds a threshold at block 66 , an off-track write condition is detected. In one embodiment, when an off-track write condition is detected based on the predicted absolute position of a next servo wedge, the write operation is aborted at block 64 without refreshing the data of the previously written, adjacent track.

FIG. 9A shows a data storage device in the form of a disk drive according to an embodiment comprising a head 16 actuated over a disk 18 , and control circuitry 24 configured to execute the flow diagram of FIG. 9B . A write throughput and a read throughput of the disk drive are measured for a plurality of different off-track write thresholds (block 67 ). An operating off-track write threshold is selected based on the measurements (block 68 ), and a write operation to the disk is aborted based on the operating off-track write threshold (block 70 ).

FIG. 10 shows an example of this embodiment wherein the write throughput of the disk drive increases as the off-track write threshold increases since there are fewer off-track write conditions detected and fewer write aborts. However, increasing the off-track write threshold eventually causes the read throughput of the disk drive to decrease since the reliability of the recorded data decreases which increases the data recovery time due, for example, to executing more retry reads and/or other heroic error recovery procedures. In one embodiment, the control circuitry 24 is configured to select the operating off-track write threshold that substantially maximizes the write throughput biased by a first weighting and the read throughput biased by a second weighting. For example, in one embodiment the control circuitry 24 is configured to select the operating off-track write threshold as the minimum of the measured off-track write thresholds that substantially maximizes the write throughput biased by a first weighting and the read throughput biased by a second weighting. Any suitable first weighting and second weighting may be selected to bias the write/read throughputs, for example, to achieve a target ratio between the write/read throughputs. For example, in one embodiment the first weighting may be different than the second weighting so that the disk drive exhibits better read throughput performance as compared to write throughput.

Any suitable operating off-track write threshold may be calibrated by executing the flow diagram of FIG. 9B , such as the position buffer threshold at block 48 of FIG. 7 , the absolute reference position ABRw threshold at block 58 of FIG. 7 , and/or the predictive threshold at block 66 of FIG. 7 . In one embodiment, two or more off-track write thresholds may be calibrated relative to the write/read throughput of the disk drive.

Any suitable control circuitry may be employed to implement the flow diagrams in the above embodiments, such as any suitable integrated circuit or circuits. For example, the control circuitry may be implemented within a read channel integrated circuit, or in a component separate from the read channel, such as a disk controller, or certain operations described above may be performed by a read channel and others by a disk controller. In one embodiment, the read channel and disk controller are implemented as separate integrated circuits, and in an alternative embodiment they are fabricated into a single integrated circuit or system on a chip (SOC). In addition, the control circuitry may include a suitable preamp circuit implemented as a separate integrated circuit, integrated into the read channel or disk controller circuit, or integrated into a SOC.

In one embodiment, the control circuitry comprises a microprocessor executing instructions, the instructions being operable to cause the microprocessor to perform the flow diagrams described herein. The instructions may be stored in any computer-readable medium. In one embodiment, they may be stored on a non-volatile semiconductor memory external to the microprocessor, or integrated with the microprocessor in a SOC. In another embodiment, the instructions are stored on the disk and read into a volatile semiconductor memory when the disk drive is powered on. In yet another embodiment, the control circuitry comprises suitable logic circuitry, such as state machine circuitry.

In various embodiments, a disk drive may include a magnetic disk drive, an optical disk drive, etc. In addition, while the above examples concern a disk drive, the various embodiments are not limited to a disk drive and can be applied to other data storage devices and systems, such as magnetic tape drives, solid state drives, hybrid drives, etc. In addition, some embodiments may include electronic devices such as computing devices, data server devices, media content storage devices, etc. that comprise the storage media and/or control circuitry as described above.

The various features and processes described above may be used independently of one another, or may be combined in various ways. All possible combinations and subcombinations are intended to fall within the scope of this disclosure. In addition, certain method, event or process blocks may be omitted in some implementations. The methods and processes described herein are also not limited to any particular sequence, and the blocks or states relating thereto can be performed in other sequences that are appropriate. For example, described tasks or events may be performed in an order other than that specifically disclosed, or multiple may be combined in a single block or state. The example tasks or events may be performed in serial, in parallel, or in some other manner. Tasks or events may be added to or removed from the disclosed example embodiments. The example systems and components described herein may be configured differently than described. For example, elements may be added to, removed from, or rearranged compared to the disclosed example embodiments.

While certain example embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions disclosed herein. Thus, nothing in the foregoing description is intended to imply that any particular feature, characteristic, step, module, or block is necessary or indispensable. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the embodiments disclosed herein.