Disk drive having a high performance access mode and a lower performance archive mode

[0001] 1. Field

[0002] The present invention relates to disk drives for computer systems. In particular, the present invention relates to a disk drive having a high performance access mode and a lower performance archive mode.

[0003] 2. Description of the Related Art

[0004] 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 embedded servo sectors. The embedded 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 velocity of the actuator arm as it seeks from track to track.

[0005] During a write operation, a current is applied to a write element of the head (e.g., a write coil) to create a magnetic field which magnetizes the surface of the disk by orienting the direction of magnetic grains (e.g., horizontally in longitudinal magnetic recording, or vertically in perpendicular magnetic recording). The orientation of the grains exhibits hysteresis thereby generating their own magnetic field when the write magnetic field is removed. During a read operation, a read element of the head (e.g., a magnetoresistive element) transduces the magnetic field emanating from the disk surface into a read signal that is demodulated into an estimated data sequence.

[0006] The hysteresis of the magnetic grains is not permanent meaning that over time the grains will orientate into random directions (magnetic entropy) until the magnetic field is no longer sensed reliably (leading to data errors during reproduction). Magnetic entropy may also be precipitated by various factors, such as increasing ambient temperature. That is, at higher temperatures the uniform alignment of the grains will degrade faster. Another factor that precipitates magnetic entropy is a phenomenon referred to as adjacent track interference (ATI) wherein when writing data to a target track, the fringe field from the write element degrades the uniform alignment of the grains recorded in an adjacent track. The degrading effect of ATI on the adjacent tracks compounds over time with each write operation to the target track. Eventually, the magnetic field emanating from the disk surface will deteriorate to the point that the data is no longer recoverable.

[0007] There is, therefore, a need for an improved reliability disk drive that compensates for magnetic entropy.

[0008] An embodiment of the present invention comprises a disk drive including a disk having a plurality of tracks, and a head actuated over the disk. A write frequency of write data received from a host is monitored. When the write frequency falls below a threshold, the disk drive is configured to operate in a low frequency access mode when accessing the disk, wherein a parameter of the disk drive is configured to increase the reliability of the disk drive. When the write frequency rises above a threshold, the disk drive is configured to operate in a high frequency access mode when accessing the disk, wherein the parameter of the disk drive is configured to increase the performance of the disk drive.

[0009] In one embodiment, a plurality of zones are defined on the disk, including a first zone and a second zone. A write frequency of write data received from the host is monitored for the first and second zones. When the write frequency for the first zone falls below a threshold, the disk drive is configured to operate in the low frequency access mode when accessing the first zone. When the write frequency for the first zone rises above a threshold, the disk drive is configured to operate in the high frequency access mode when accessing the first zone. When the write frequency for the second zone falls below a threshold, the disk drive is configured to operate in the low frequency access mode when accessing the second zone. When the write frequency for the second zone rises above a threshold, the disk drive is configured to operate in the high frequency access mode when accessing the second zone.

[0010] In another embodiment, the write frequency falls below the threshold due to the disk drive being disconnected from the host.

[0011] In yet another embodiment, the parameter comprises a servo control parameter for actuating the head over the disk. In one embodiment, the parameter comprises a seek trajectory, in another embodiment the parameter comprises a repeatable runout compensation, and in another embodiment the parameter comprises a degree of disk revolution for the head to settle on a target track after seeking the head to the target track.

[0012] In still another embodiment, the parameter comprises a write current setting, and in one embodiment, when in the low frequency access mode, the write current setting is decreased and write operations are performed by writing data to an area of the disk at least twice.

[0013] In another embodiment, each track comprises a plurality of data sectors, and the parameter comprises a track level redundancy generated in response to the data sectors stored in a track. In one embodiment, the control circuitry dynamically assigns memory for storing the track level redundancy to the zones.

[0014] In one embodiment, a refresh monitor is maintained for each zone, and data recorded in a zone is refreshed in response to the corresponding refresh monitor. In one embodiment, data recorded in a zone is refreshed at a refresh interval, and when in the low frequency access mode, the refresh interval is increased.

[0015] Another embodiment of the present invention comprises a disk drive including a disk comprising a plurality of tracks, and a head actuated over the disk. A plurality of zones are defined on the disk, including a first zone and a second zone. The disk drive is configured to operate in a high frequency access mode when accessing the first zone, wherein the disk drive is configured according to a parameter so as to increase the performance of the disk drive. The disk drive is configured to operate in a low frequency access mode when accessing the second zone, wherein the disk drive is configured according to a parameter so as to increase the reliability of the disk drive. A write frequency of write data received from a host for the first zone is monitored, and when the write frequency for the first zone falls below a threshold, data stored in the first zone is copied to the second zone.

[0033] FIG. 1A shows a disk drive according to an embodiment of the present invention including a disk 2 having a plurality of tracks 4, and a head 6 actuated over the disk 2. The disk drive further comprises control circuitry 8 for monitoring a write frequency of write data received from a host, and executing the flow diagram of FIG. 1B. When the write frequency falls below a threshold (step 10), the disk drive is configured to operate in a low frequency access mode when accessing the disk 2 (step 12), wherein a parameter of the disk drive is configured to increase the reliability of the disk drive. When the write frequency rises above a threshold (step 14), the disk drive is configured to operate in a high frequency access mode when accessing the disk 2 (step 16), wherein the parameter of the disk drive is configured to increase the performance of the disk drive.

[0034] The disk drive is configured into a higher reliability mode when the write frequency decreases in order to help protect the data against magnetic entropy. In one embodiment, only the write operations are configured to operate with higher reliability to avoid performance loss during read operations. Although there is a performance loss for write operations, it is less of a concern since the write frequency has decreased. In addition, in an embodiment described in more detail below, a refresh interval may be increased when the disk drive is operating with higher reliability which may improve performance by reducing the frequency of refresh operations.

[0035] In the embodiment of FIG. 1A, the disk 2 comprises a plurality of embedded servo sectors 18₀-18_N that define the tracks 4. Each embedded servo sector 18 comprises coarse head positioning information (e.g., a track address) and fine head positioning information (e.g., servo bursts). The head 6 is attached to a distal end of an actuator arm 20 which is rotated about a pivot by a voice coil motor (VCM) 22 in order to actuate the head 6 radially over the disk 2. The control circuitry 8 processes the read signal 23 emanating from the head 6 in order to demodulate the head position information recorded in the embedded servo sectors 18₀-18_N. After suitable compensation (e.g., filtering), the control circuitry 8 generates a control signal 24 applied to the VCM 22 in order to seek and maintain the head 6 over a target track during write and read operations.

[0036] FIG. 2A shows an embodiment of the present invention wherein a plurality of zones are defined on the disk 2, including a first zone (e.g., ZONE_1) and a second zone (e.g., ZONE_2). The control circuitry 8 monitors a write frequency of write data received from the host for the first and second zones and executes the flow diagram of FIG. 2B. When the write frequency for the first zone falls below a threshold (step 26), the disk drive is configured to operate in the low frequency access mode when accessing the first zone (step 28). When the write frequency for the first zone rises above a threshold (step 30), the disk drive is configured to operate in the high frequency access mode when accessing the first zone (step 32). Similarly, when the write frequency for the second zone falls below a threshold (step 26), the disk drive is configured to operate in the low frequency access mode when accessing the second zone (step 28). When the write frequency for the second zone rises above a threshold (step 30), the disk drive is configured to operate in the high frequency access mode when accessing the second zone (step 32). Partitioning the disk 2 into zones may improve performance by increasing the reliability (and optionally a refresh interval) only for data areas of the disk 2 having a low write frequency rather than for the entire disk surface.

[0037] The write frequency may decrease and fall below the threshold for a number of reasons, for example, if the data written to the disk is archive data (data not frequently overwritten), or if the disk drive is disconnected from the host for a long period of time (e.g., an archive disk drive). In one embodiment, the control circuitry 8 comprises suitable circuitry for monitoring the write frequency (and a refresh interval) while disconnected from the host, and in one embodiment, even while powered down (e.g., a battery powered monitor). As described in more detail below, if a refresh monitor indicates a need for a refresh operation, the control circuitry 8 comprises suitable circuitry to wake up the disk drive and perform the refresh operation independent of the host. If the idle time of the disk drive exceeds the write frequency threshold (meaning that the write frequency falls below the threshold), the control circuitry 8 will configure the disk drive into the low frequency access mode, perform the refresh operation with a higher reliability, and then increase the next refresh interval.

[0038] Any suitable parameter (or parameters) of the disk drive may be adjusted to configure the disk drive into the low or high frequency access mode. In one embodiment, the parameter comprises a servo control parameter for actuating the head 6 over the disk 2. For example, the parameter may comprise a seek trajectory, in another embodiment the parameter may comprise a repeatable runout compensation, and in another embodiment the parameter may comprise a degree of disk revolution for the head 6 to settle on a target track after seeking the head 6 to the target track.

[0039] In another embodiment, the parameter comprises a write current setting for the head 6. For example, in one embodiment, when in the low frequency access mode, the write current setting is decreased and write operations are performed by writing data to an area of the disk at least twice. This embodiment may increase the reliability of other data recorded on the disk by reducing the fringe field from the write element, thereby reducing adjacent track interference. Writing the data over multiple passes also helps ensure the written data saturates the magnetic grains along the written track.

[0040] FIG. 3A shows an embodiment of the present invention wherein each track 4 comprises a plurality of data sectors 34₀-34_N, and the parameter comprises a track level redundancy 36 generated in response to the data sectors 34₀-34_N stored in a track (FIG. 3C). Any suitable technique for generating the track level redundancy 36 may be employed in the embodiments of the present invention, such as generating a parity sector over the data sectors in the track. The track level redundancy 36 may cover one or more tracks, or it may cover a partial track. If a data sector is unrecoverable using the sector level redundancy (e.g., Reed-Solomon ECC), the track level redundancy 36 may be used to recover the data sector. The control circuitry 8 executes the flow diagram of FIG. 3B, wherein when the write frequency falls below a threshold (step 38), the disk drive is configured to utilize the track level redundancy (step 40). When the write frequency rises above a threshold (step 42), the disk drive is configured to disable the track level redundancy (step 44). Utilizing track level redundancy 36 increases reliability, but may also reduce performance during write operations since generating or regenerating the track level redundancy may require additional rotational latency.

[0041] The track level redundancy 36 may be stored in any suitable non-volatile memory, such as on the disk 2 or in a FLASH semiconductor memory implemented as part of the control circuitry 8. Storing the track level redundancy 36 in FLASH may avoid the latency of seeking to tracks on the disk 2 that store the track level redundancy. In one embodiment, the control circuitry 8 dynamically assigns memory for storing the track level redundancy to zones. An example of this embodiment is illustrated in FIG. 4A wherein the disk 2 is partitioned into five zones (Z1-Z5) each comprising two tracks, with non-volatile memory 38 for storing track level redundancy for up to three of the zones (FIG. 4C). The control circuitry 8 executes the flow diagram of FIG. 4B wherein when the write frequency for a zone falls below a threshold (step 40), the memory 38 for storing track level redundancy is assigned to that zone (step 42). In one embodiment, the control circuitry 8 generates the track level redundancy 36 for the data already recorded in the zone either immediately, or during an idle mode of the disk drive. When the write frequency for a zone rises above the threshold (step 44), the control circuitry 8 de-allocates the memory 38 for the zone so that it may be used for a different zone (step 46). In one embodiment, the track level redundancy 36 stored in the memory 38 for a de-allocated zone may still be used to recover a data sector for the zone if the track level redundancy 36 for that zone is still valid.

[0042] FIG. 5A shows another embodiment of the present invention wherein the disk 2 comprises a plurality of zones, including a first zone (e.g., ZONE_1) and a second zone (e.g., ZONE_2). The disk drive is configured to operate in a high frequency access mode when accessing the first zone, wherein the disk drive is configured according to a parameter so as to increase the performance of the disk drive. The disk drive is configured to operate in a low frequency access mode when accessing the second zone, wherein the disk drive is configured according to the parameter so as to increase the reliability of the disk drive. The control circuitry 8 monitors the write frequency of write data received from the host for the first zone and executes the flow diagram of FIG. 5B. When the write frequency for the first zone falls below a threshold (step 48), data stored in the first zone is copied to the second zone (step 50). When the write frequency for the second zone rises above a threshold (step 52), data stored in the second zone is copied to the first zone (step 54).

[0043] In one embodiment, the disk drive is configured according to a parameter corresponding to a different format between the zones. In one embodiment, the parameter comprises a servo control parameter for actuating the head 6 over the disk 2, such as a format of the servo sectors. This embodiment is illustrated in FIG. 6A wherein the servo sectors of ZONE_1 comprise a first format for the servo data, and the servo sectors of ZONE_2 comprise a second extended format for the servo data. For example, in one embodiment the servo data of ZONE_1 comprises no (or reduced) repeatable runout compensation data, whereas the servo data of ZONE_2 comprises repeatable runout information. The extended repeatable runout compensation improves the compensation for repeatable runout due to eccentricity in the rotation of the disk 2 which in turn improves the reliability of write/read operations. However, the extended servo sectors reduce the capacity of the disk 2 to store user data.

[0044] In another embodiment, the parameter comprises an error correction setting. For example, in an embodiment illustrated in FIG. 6B, the parameter comprises a sector level redundancy (e.g., Reed-Solomon ECC) generated in response to user data stored in a data sector, wherein the data sector format of ZONE_2 utilizes more sector level redundancy as compared to ZONE_1. The increased sector level redundancy increases the reliability of the disk drive, but decreases the capacity to store user data. In another embodiment, the parameter comprises a track level redundancy generated in response to the data sectors stored in a track. This embodiment is illustrated in FIG. 6C wherein the tracks of ZONE_1 comprise no track level redundancy (all sectors are data sectors), whereas the tracks of ZONE_2 comprise one or more sectors of track level redundancy.

[0045] In another embodiment, the parameter comprises a radial tracks per inch (TPI) recording density. For example, the TPI of ZONE_1 may be greater than the TPI of ZONE_2 in order to increase the capacity of ZONE_1 and increase the reliability of ZONE_2. A lower TPI may increase the reliability by reducing adjacent track interference during write operations. In another embodiment, the parameter comprises a linear bits per inch (BPI) recording density. For example, the BPI of the data sectors recorded in ZONE_1 may be greater than the BPI of the data sectors recorded in ZONE_2 in order to increase the capacity of ZONE_1 and increase the reliability of ZONE_2. A lower BPI may increase the reliability by increasing the number of magnetic grains per bit cell.

[0046] Any suitable number of zones may be employed in the embodiments of the present invention, as well as any suitable format for the zones. For example, each zone may comprise an entire disk surface, or part of a disk surface. Each zone may comprise a single track or a plurality of tracks of a disk surface, and a zone may span more than one disk surface.

[0047] In one embodiment, a refresh monitor is maintained for each zone, and data recorded in a zone is refreshed (read and rewritten) in response to the corresponding refresh monitor. In the embodiment of FIG. 2A, the control circuitry 8 may refresh the data recorded in a zone at a refresh interval, wherein the refresh interval may be increased when in the low frequency access mode. In other words, when the zone is accessed with higher reliability while in the low frequency access mode, the refresh interval can be increased since the data will not degrade as fast compared to when the zone is accessed in the high frequency access mode. Similarly in the embodiment of FIG. 5A, the refresh interval for ZONE_1 may be less than the refresh interval for the ZONE_2, since ZONE_2 is accessed with higher reliability.

[0048] FIG. 7A shows control circuitry that may be implemented by control circuitry 8 according to an embodiment of the present invention that may be used for detecting the access mode. Although not illustrated, in the embodiments comprising multiple zones, the circuitry of FIG. 7A may be duplicated for each zone. An interval counter 58 is clocked at a frequency f 60 in order to track a write frequency interval, and a write counter 62 is clocked by a write signal 64 in order to count the number of write operations during the write frequency interval. When the output 66 of the interval counter 58 exceeds a threshold 68, the output 70 of comparator 72 enables comparator 74 which compares the output 76 of the write frequency counter 62 to a first threshold 78. If the output 76 is less than the threshold 78, a low frequency mode signal 80 is activated to configure the zone into the low frequency access mode. If the output 76 of the write frequency counter 62 exceeds a second threshold 82 any time during the write frequency interval, then comparator 84 activates a high frequency mode signal 86 to configure the zone into the high frequency access mode. When the low frequency mode signal 80 is activated, the zone remains in the low frequency access mode until the high frequency mode signal 86 is activated, and vice-versa. The output 70 of comparator 72 may reset the interval counter 58 at the end of the interval on the rising edge, and may also reset the write frequency counter 62 on the falling edge.

[0049] Any suitable threshold 68 may be selected to generate a suitable write frequency interval. In one embodiment, a shorter write frequency interval may be selected so that the access mode tracks the real time write frequency of the zone. Alternatively, a longer write frequency interval may be selected so that the access mode changes less frequently (e.g., to ensure a zone is in an archive mode before changing to the low frequency access mode). In one embodiment, the thresholds 78 and 82 are selected to implement a suitable hysteresis to prevent the disk drive from toggling between the access modes. In addition, the first and second threshold 78 and 82 may be selected to achieve any suitable performance/reliability balance as determined, for example, during lifetime testing for a family of disk drives. In another embodiment, the first and second thresholds 78 and 82 may be configured in-the-field relative to a particular application or operating environment, and in another embodiment the thresholds 78 and 82 may be adaptable, for example, based on the access frequency (read and write) of the host.

[0050] FIG. 7B shows control circuitry that may be implemented by control circuitry 8 that may be used for implementing a refresh monitor for refreshing a zone according to an embodiment of the present invention. A refresh interval counter 88 is clocked at a frequency f 60 in order to track a refresh interval. When the output 90 of the refresh interval counter 88 exceeds a threshold 92 at comparator 94, a refresh signal 96 is activated to refresh the zone. After refreshing the zone, the refresh interval counter 88 is reset by signal 98.

[0051] In one embodiment, when in the low frequency access mode, the data recorded in the zone is refreshed using high reliability (written back to the same zone using high reliability parameters or copied to a high reliability zone), and the next refresh interval for the zone is increased. The next refresh interval for the zone may be increased in any suitable manner, such as by increasing the threshold 92 for comparator 94 by selecting an appropriate threshold, such as low frequency threshold 104, through multiplexer 100 in response to a mode signal 102. Alternatively, the frequency f 60 or the increment value of the refresh interval counter 88 may be decreased in order to increase the next refresh interval. Conversely, when in the high frequency access mode, the data recorded in the zone is refreshed using high performance (written back to the same zone using high performance parameters or copied to a high performance zone), and the next refresh interval for the zone may be decreased, such as by selecting high frequency threshold 106.

[0052] Any suitable control circuitry 8 may be employed in the embodiments of the present invention, such as any suitable integrated circuit or circuits. For example, the control circuitry 8 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 steps 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 a 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 an SOC.

[0053] In one embodiment, the control circuitry 8 comprises a microprocessor executing instructions, the instructions being operable to cause the microprocessor to perform the steps of 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 system on a chip (SOC). In another embodiment, the instructions are stored on the disk 2 and read into a volatile semiconductor memory when the disk drive is powered on. In yet another embodiment, the control circuitry 8 comprises suitable logic circuitry, such as state machine circuitry.