Patent Publication Number: US-10770111-B1

Title: Disk drive with efficient handling of off-track events during sequential write

Description:
BACKGROUND 
     Magnetic hard disk drives (HDDs) have been employed in information technology as a low-cost means for providing random access to large quantities of data. As the need for data storage has expanded, the areal density of information stored in HDDs has been continuously increased. In addition to high storage capacity, the ability of an HDD to access stored data quickly is also important. To meet the ever-increasing requirements for high access performance and faster throughput, HDDs have been configured with multiple rotary actuators and associated read/write channels that are designed to operate simultaneously. Thus, each rotary actuator enables the independent positioning of one or more magnetic heads for reading and writing data, thereby greatly increasing the throughput of such HDDs. 
     One drawback to the use of independent rotary actuators is that the mechanical interaction between such actuators can affect positioning accuracy of a magnetic head that is associated with one actuator when another actuator is in motion. For example, when one actuator is seeking to a data track on a first disk surface, the high accelerations and changes in acceleration of the actuator can generate vibrations which will significantly affect the positioning accuracy of the magnetic head of another actuator while the other actuator is track following a target data track on a second disk surface. 
     In some instances, vibrations from one actuator can alter the position of the magnetic head of a track-following actuator to the point that an off-track error occurs, and the magnetic head can no longer read data from or write data to the target data track on the second disk surface. In such instances, a portion of the target data track is not successfully accessed, and an additional revolution of the second disk surface must occur so that the magnetic head can read data from or write data to the portion of the target data track not successfully accessed. Thus, when off-track errors occur with relative frequency, significant delays in completing disk access operations occur, which reduces the normally greater throughput of a multi-actuator HDD. 
     Consequently, there is a need in the art for efficiently recovering from off-track errors that occur in an HDD, particularly in a multi-actuator HDD. 
     SUMMARY 
     One or more embodiments provide systems and methods for efficient recovery in a disk drive when an off-track error occurs during a sequential disk access operation that spans multiple contiguous data tracks. In the embodiments, the disk access operation (e.g., reading from or writing to a disk) is attempted for all sectors of the sequential disk access operation. The disk access operation is then attempted again for sectors associated with any off-track errors that occurred during the disk access operation. As a result, recovery of the off-track errors can generally be completed in fewer additional revolutions of the disk than by recovering from off-track errors using a conventional approach, such as performing an additional revolution for each data track of the sequential disk access operation associated with at least one off-track error. For example, given off-track errors spread across N data tracks of the sequential disk access operation, a conventional approach requires at least N additional revolutions of the disk to complete the sequential disk access operation in the regions of the data tracks associated with off-track errors. By contrast, in the embodiments, the sequential disk access operation is typically completed in N−1 or fewer additional revolutions and, in some instances, as few as one or two revolutions. The embodiments can be implemented for read and/or write operations in a conventional magnetic recording (CMR) disk drive and for read operations in a shingled magnetic recording (SMR) disk drive. 
     One or more embodiments provide systems and methods for efficient recovery in an SMR disk drive when an off-track error occurs during a sequential write operation that spans multiple contiguous data tracks of a disk. In the embodiments, when an off-track error occurs during the sequential write operation and, as a result, a first portion of a data track is not written to, the data originally targeted to be written to the first portion is written to a second portion of the data track that follows the first portion. That is, the data written to the disk after the first portion of the track “slip” to subsequent portions of the disk. Since no additional revolutions of the disk are needed for data associated with the sequential write operation to be written to the disk, there is very little delay in completion of the write operation. 
     According to an embodiment, a method is provided of accessing a recording surface of a magnetic disk in a disk drive. According to the embodiment, the method comprises, moving a head to a first track to start a disk access operation; controlling the head to perform the disk access operation on one or more sectors of the first track beginning at a first sector of the first track; if there are any sectors in a second track to be written as part of the disk access operation after the head has performed the disk access operation on all sectors of the first track to be written as part of the disk access operation, determining whether or not an off-track event occurred while the head was performing the disk access operation on any of the sectors of the first track; and upon determining that the off-track event occurred while the head was performing the disk access operation on any of the sectors of the first track to be written as part of the disk access operation, moving the head to the second track and controlling the head to write sequentially to one or more sectors of the second track beginning at a first sector of the second track. 
     A disk drive, according to another embodiment, comprises a magnetic disk with a recording surface; a head configured to access the recording surface; and a controller. The controller is configured to move the head to a first track to start a disk access operation; control the head to perform the disk access operation on one or more sectors of the first track beginning at a first sector of the first track; if there are any sectors in a second track to be written as part of the disk access operation after the head has performed the disk access operation on all sectors of the first track to be written as part of the disk access operation, determine whether or not an off-track event occurred while the head was performing the disk access operation on any of the sectors of the first track; and upon determining that the off-track event occurred while the head was performing the disk access operation on any of the sectors of the first track to be written as part of the disk access operation, moving the head to the second track and control the head to write sequentially to one or more sectors of the second track beginning at a first sector of the second track. 
     According to an embodiment, a method is provided of writing data to a recording surface of a magnetic disk. According to the embodiment, the method comprises moving a head to a first track on the recording surface to start a write operation on data including first data and second data that are sequential; controlling the head to write the first data to a first sector of the first track; controlling the head to write the second data to a first group of one or more sectors of the first track in a same revolution of the magnetic disk as when the first data was written, wherein the first group of one or more sectors is adjacent to the first sector; and upon detecting that an off-track event occurred while writing the second data to the first group of one or more sectors, controlling the head to write the second data to a second group of one or more sectors of the first track that is adjacent to the first group of one or more sectors in the same revolution of the magnetic disk as when the first data was written and the second data was written to the first group of one or more sectors. 
     A disk drive, according to another embodiment, comprises a magnetic disk with a recording surface; a head configured to access the recording surface; and a controller. The controller is configured to move the head to a first track on the recording surface to start a write operation on data including first data and second data that are sequential; control the head to write the first data to a first sector of the first track; control the head to write the second data to a first group of one or more sectors of the first track in a same revolution of the magnetic disk as when the first data was written, wherein the first group of one or more sectors is adjacent to the first sector; and upon detecting that an off-track event occurred while writing the second data to the first group of one or more sectors, control the head to write the second data to a second group of one or more sectors of the first track that is adjacent to the first group of one or more sectors in the same revolution of the magnetic disk as when the first data was written and the second data was written to the first group of one or more sectors. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of embodiments can be understood in detail, a more particular description of embodiments, briefly summarized above, may be had by reference to the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIG. 1  is a schematic view of an exemplary hard disk drive, according to an embodiment. 
         FIG. 2  schematically illustrates a partial side-view of multiple storage disks and two independent actuator arm assemblies of the hard disk drive of  FIG. 1 , according to an embodiment. 
         FIG. 3  illustrates an operational diagram of the hard disk drive of  FIG. 1 , with some elements of electronic circuits and a motor-driver chip shown configured according to one embodiment. 
         FIGS. 4A-4F  schematically illustrate various stages of an example instance of an off-track event. 
         FIG. 5  schematically illustrates the portion of a storage disk associated with a sequential disk access operation that spans multiple contiguous data tracks. 
         FIGS. 6A-6I  schematically illustrate various steps of the sequential write operation shown in  FIG. 5 , according to an embodiment. 
         FIG. 7  sets forth a flowchart of method steps for recovering from off-track events in a sequential disk access operation, according to an embodiment. 
         FIG. 8  is a schematic illustration of a portion of a recording surface that includes a band of SMR data tracks, according to an embodiment. 
         FIGS. 9A-9C  schematically illustrate various steps of the sequential write operation shown in  FIG. 5 , according to an embodiment. 
         FIG. 10  schematically illustrates a user region of a recording surface and a media-cache region of recording surface, according to an embodiment. 
         FIG. 11  sets forth a flowchart of method steps for recovering from off-track events in a sequential write operation in a shingled magnetic recording drive, according to an embodiment. 
         FIG. 12  sets forth a flowchart of method steps for recovering from off-track events in a sequential write operation in an SMR drive, according to another embodiment. 
     
    
    
     For clarity, identical reference numbers have been used, where applicable, to designate identical elements that are common between figures. It is contemplated that features of one embodiment may be incorporated in other embodiments without further recitation. 
     DETAILED DESCRIPTION 
     System Overview 
       FIG. 1  is a schematic view of an exemplary hard disk drive (HDD)  100 , according to one embodiment. For clarity, HDD  100  is illustrated without a top cover. HDD  100  is a multiple actuator drive, and includes one or more storage disks  110 , each including one or two recording surfaces on which a plurality of concentric data storage tracks are disposed. In  FIG. 1 , only the top recording surface  112 A of storage disk  110  is visible. The one or more storage disks  110  are coupled to and rotated by a spindle motor  114  that is mounted on a base plate  116 . Two or more actuator arm assemblies  120 A and  120 B are also mounted on base plate  116 , and each of the assemblies includes one or more arm-mounted sliders with one or more magnetic read/write heads that read data from and write data to the data storage tracks of an associated recording surface, such as recording surface  112 A. 
     One or more actuator arms  124 A-C are included in actuator arm assembly  120 A, and one or more actuator arms  124 D-F are included in actuator arm assembly  120 B. Actuator arm assembly  120 A and the one or more actuator arms  124 A-C included therein are rotated together about a bearing assembly  126  by a voice coil motor (VCM)  128 A independently from actuator arm assembly  120 B. Likewise, actuator arm assembly  120 B and the one or more actuator arms  124 D-F included therein are rotated together about bearing assembly  126  by a VCM  128 B independently from actuator arm assembly  120 A. Thus, each of VCMs  128 A and  128 B moves a group of the sliders radially relative to a respective recording surface of a storage disk  110  included in HDD  100 , thereby providing radial positioning of a read/write head over a desired concentric data storage track. For example, VCM  128 A moves sliders  121 A-D relative to respective recording surfaces, thereby providing radial positioning of read/write head  127 A over a desired concentric data storage track on recording surface  112 A. Spindle motor  114 , the read/write heads  127 A-D and  127 E-H, and VCMs  128 A and  128 B are coupled to electronic circuits  130 , which are mounted on a printed circuit board  132 . HDD  100  is described above as a drive that employs concentric data storage tracks. Alternatively, a drive that employs a single or a small number of spiral data storage tracks can also benefit from implementation of one or more embodiments described herein. 
     When data are transferred to or from a particular recording surface of HDD  100 , one of the actuator arm assemblies  120 A or  120 B moves in an arc between the inner diameter (ID) and the outer diameter (OD) of the storage disk  110 . The actuator arm assembly accelerates in one angular direction when current is passed in one direction through the voice coil of the corresponding VCM and accelerates in an opposite direction when the current is reversed, thereby allowing control of the position of the actuator arm assembly and the attached read/write head with respect to the particular storage disk  110 . 
     In the embodiment illustrated in  FIG. 1 , four sliders  121 A- 121 -D, three actuator arms  124 A- 124 C, and four read/write heads  127 A- 127 D are shown for actuator arm assembly  120 A and four sliders  121 E- 121 H, three actuator arms  124 D- 124 F, and four read/write heads  127 E- 127 H are shown for actuator arm assembly  120 B. In other embodiments, each of actuator arm assemblies  120 A and  120 B can include more or fewer actuator arms, sliders, and read/write heads. Further, in some embodiments, HDD  100  can include more than two actuator arm assemblies, each rotated about bearing assembly  126  by a respective VCM independently from each other. 
       FIG. 2  schematically illustrates a partial side-view of multiple storage disks  110 A- 110 D and two independent actuator arm assemblies  120 A and  120 B of HDD  100 , according to an embodiment. The recording surfaces of multiple storage disks  110 A and  110 B are each accessed by one of the read/write heads included in the independent actuator arm assembly  120 A (e.g., read/write heads  127 A,  127 B,  127 C, and  127 D), and the recording surfaces of multiple storage disks  110 C and  110 D are each accessed by the read/write heads included in the independent actuator arm assembly  120 B (e.g., read/write heads  127 E,  127 F,  127 G, and  127 H). Thus, in the embodiment illustrated in  FIG. 2 , HDD  100  is configured with multiple storage disks  110 A- 110 D having a total of eight recording surfaces  112 A- 112 H and multiple read/write heads  127 A- 127 H, each corresponding to one of these recording surfaces. Specifically, in the embodiment illustrated in  FIG. 2 , HDD  100  includes: a storage disk  110 A with recording surfaces  112 A and  112 B; a storage disk  110 B with recording surfaces  112 C and  112 D; a storage disk  110 C with recording surfaces  112 E and  112 F; and a storage disk  110 D with recording surfaces  112 G and  112 H. Thus, read/write head  127 A reads data from and writes data to recording surface  112 A, read/write head  127 B reads data from and writes data to corresponding recording surface  112 B, and so on. 
     Read/write heads  127 A- 127 H are disposed on sliders  121 A- 121 H, respectively, and sliders  121 A- 121 H are mounted on actuator arms  124 A- 124 F as shown. Actuator arms  124 A- 124 C are included in actuator arm assembly  120 A, and actuator arms  124 D- 124 F are included in actuator arm assembly  120 B. In an embodiment of the invention, actuator arm assemblies  120 A and  120 B are independently controlled and both rotate about bearing assembly  126  (which includes a same shaft axis  226 ). 
     In some embodiments, HDD  100  includes one or more microactuators for each of read/write heads  127 A- 127 H. In the embodiment illustrated in  FIG. 2 , HDD  100  includes microactuators  129 A- 129 H (collectively referred to herein as microactuators  129 ), each of which is associated with a respective read/write head  127 A- 127 H, and/or microactuators  123 A- 123 H (collectively referred to herein as microactuators  129 ), each of which is associated with a respective read/write head  127 A- 127 H. Each of microactuators  129  and/or  123  compensates for perturbations in the radial position of sliders  121 A- 121 H, so that read/write heads  127 A- 127 H follow the proper data track on recording surfaces  112 . Thus, microactuators  123  and/or  129  can compensate for vibrations of the disk, inertial events such as impacts to HDD  100 , and irregularities in recording surfaces  112 . 
     In some embodiments, each of sliders  121 A- 121 H is mounted on a corresponding flexure arms via a microactuator  129 . For example, a microactuator  129  can include a microactuator (MA) second stage that includes two lead zirconate titanate piezoelectric actuators attached to a baseplate of the corresponding flexure arm. Alternatively, in some embodiments, each of sliders  121 A- 121 H is mounted directly on a corresponding flexure arm. Further, in some embodiments, each of microactuators  123  is disposed near a base of a respective flexure arm, i.e., proximate one of actuator arms  124 A- 124 D. For example, microactuators  123  can each include a pair of piezoelectric strips that are mounted on the corresponding flexure arm near to where that flexure arm is attached to the corresponding actuator arm  124 A- 124 F. When one such piezoelectric strip expands and the other piezoelectric strip contracts, the flexure arm sways to one side, moving a corresponding slider  121 A- 121 H radially. Because the diameter of the circular arc along which the slider moves is approximately equal to the length of the flexure arm (which is generally larger than the length of the slider), microactuators  123  can provide significantly greater range of radial motion of a read/write head  127 A- 127 H than microactuators  129 . 
     Returning to  FIG. 1 , electronic circuits  130  include read channels  137 A and  137 B, a microprocessor-based controller  133 , random-access memory (RAM)  134  (which may be a dynamic RAM and is used as one or more data buffers) and/or a flash memory device  135 , and, in some embodiments, a flash manager device  136 . In some embodiments, read channels  137 A and  137 B and microprocessor-based controller  133  are included in a single chip, such as a system-on-chip (SoC)  131 . HDD  100  further includes a motor-driver chip  125  that accepts commands from microprocessor-based controller  133  and drives spindle motor  114 , and VCMs  128 A and  128 B. Via a preamplifier (not shown), read/write channel  137 A communicates with read/write heads  127 A-D and read/write channel  137 B communicates with read/write heads  127 E-H. The preamplifier is mounted on a flex-cable, which is mounted on either base plate  116 , one of actuator arms  124 A-D or  124 E-H, or both. Electronic circuits  130  and motor-driver chip  125  are described below in greater detail in conjunction with  FIG. 3 . 
       FIG. 3  illustrates an operational diagram of HDD  100 , with some elements of electronic circuits  130  and motor-driver chip  125  shown configured according to one embodiment. HDD  100  is connected to a host  10 , such as a host computer, via a host interface  20 , such as a serial advanced technology attachment (SATA) bus or a Serial Attached Small Computer System Interface (SAS) bus. As shown, microprocessor-based controller  133  includes one or more central processing units (CPU)  301  or other processors, a hard disk controller (HDC)  302 , a RAM  134 , and read/write channels  137 A and  137 B, while motor-driver chip  125  includes a position control signal generating circuit  313  (e.g., Driver IC), a spindle motor (SPM) control circuit  314 , a first actuator control circuit  315 , and a second actuator control circuit  316 . RAM  134  may be integrated on the same die as the controller  133 , included in a separate die in the same package as the controller  133 , or included in a separate package mounted on circuit board  130 . HDD  100  further includes preamplifiers  320 A and  320 B, which can be each mounted on actuator arm assemblies  120 A and  120 B or elsewhere within the head and disk assembly (HDA) of HDD  100 . Preamplifier  320 A supplies a write signal (e.g., current) to read/write head  127 A-D in response to write data input from read/write channel  137 A, and preamplifier  320 B supplies a write signal (e.g., current) to read/write head  127 E-H in response to write data input from read/write channel  137 B. In addition, preamplifier  320 A amplifies a read signal output from to read/write head  127 A-D and transmits the amplified read signal to read/write channel  137 A, and preamplifier  320 B amplifies a read signal output from to read/write head  127 E-H and transmits the amplified read signal to read/write channel  137 B. 
     CPU  301  controls HDD  100 , for example according to firmware stored in flash memory device  135  or another nonvolatile memory, such as portions of recording surfaces  112 A- 112 H. For example, CPU  301  manages various processes performed by HDC  302 , read/write channels  137 A and  137 B, read/write heads  127 A- 127 H, recording surfaces  112 A- 112 H, and/or motor-driver chip  125 . Such processes include disk access operations, such as a writing process for writing data onto recording surfaces  112 A- 112 H and a reading process for reading data from recording surfaces  112 A- 112 H. The writing process can be performed via conventional magnetic recording (CMR) and/or shingled magnetic recording (SMR). In addition, processes managed by CPU  301  include an off-track recovery process (described in greater detail below). In some embodiments, the off-track recovery process is implemented via an off-track recovery algorithm  303  that can reside in whole or in part in RAM  134  and/or in firmware or an application-specific integrated circuit  301 A included in CPU  301 . 
     In the embodiment illustrated in  FIG. 3 , microprocessor-based controller  133  includes a single CPU  301  incorporated into a single SoC  131 . In alternative embodiments, microprocessor-based controller  133  includes more than one CPU. In such embodiments, HDD  100  can include two CPUs; one devoted to servo/spindle control and the other devoted to a combination of host-based and disk-control activities. Alternatively, in such embodiments, HDD  100  includes a separate SoC for each actuator, where each SoC has two such CPUs. Further, in some embodiments, microprocessor-based controller  133  includes multiple motor driver chips. For instance, in one such embodiment, a first motor driver chip is dedicated for controlling the spindle motor and a first actuator while a second motor driver chip is dedicated for controlling a second actuator. 
     Read/write channels  137 A and  137 B are signal processing circuits that encode write data input from HDC  302  and output the encoded write data to respective preamplifiers  320 A and  320 B. Read/write channels  137 A and  137 B also decode read signals transmitted from respective preamplifiers  320 A and  320 B into read data that are outputted to HDC  302 . In some embodiments, read/write channels  137 A and  137 B each include a single read channel and a single write channel, whereas in other embodiments, read/write channels  137 A and  137 B each include multiple write channels and/or multiple read channels for read/write heads  127 A- 127 H. HDC  302  controls access to RAM  134  by CPU  301 , read/write channels  137 A and  137 B, and host  10 , and receives/transmits data from/to host  10  via interface  20 . In some embodiments, the components of microprocessor-based controller  133  (e.g., CPU  301 , HDC  302 , RAM  134 , and read/write channels  137 A,  137 B) are implemented as a one-chip integrated circuit (i.e., as an SoC). Alternatively, one or more of CPU  301 , HDC  302 , DRAM  134 , and read/write channels  137 A and  137 B can each be implemented as a separate chip. 
     Motor-driver chip  125  drives the spindle motor  114 , a first actuator (that includes VCM  128 A, actuator arms  124 A- 124 D, and bearing assembly  126 A), and a second actuator (that includes VCM  128 B, actuator arms  124 E- 124 H, and bearing assembly  126 B). Specifically, SPM control circuit  314  generates a drive signal  341  (a drive voltage or a drive current) in response to a control signal  351  received from the CPU  301  and feedback from the spindle motor  114 , and supplies drive signal  341  to spindle motor  114 . In this way, spindle motor  114  rotates storage disks  110 A- 110 D. In addition, first actuator control circuit  315  generates a drive signal  342  (drive voltage or drive current) in accordance with a received position control signal  352 , and supplies drive signal  342  to the first actuator (VCM  128 A). In this way, the first actuator positions read/write heads  127 A- 127 D radially relative to a corresponding one of recording surfaces  112 A- 112 D. Further, second actuator control circuit  316  generates a drive signal  343  in accordance with a received position control signal  353 , and supplies the position control signal  343  to the second actuator (VCM  128 B). In this way, the second actuator positions read/write heads  127 E- 127 H radially with respect to a corresponding one of recording surface  112 E- 127 H. Position control signal generating circuit  313  generates position control signals  352  and  353  in response to control signals  361  and  362  (which are control values for VCMs  128 A and  128 B) from CPU  301 , respectively. Control signals  361  enable execution of disk access commands received from host  10  that are to be executed by a first servo system of HDD  100  and control signals  362  enable execution of disk access commands received from host  10  that are to be executed by a second servo system of HDD  100 . CPU  301  generates position control signals  363  and  364 , which are control values for microactuators  123  and/or microactuators  129 , and transmits position control signals  363  and  364  to preamplifiers  320 A and  320 B. In the embodiment illustrated in  FIG. 3 , first servo controller  315  and second servo controller  316  are shown implemented as parts of motor-driver chip  125 . In other embodiments, first servo controller  315  and second servo controller  316  are implemented in whole or in part in firmware running on CPU  301  or elsewhere in microprocessor-based controller  133 . In embodiments in which microprocessor-based controller  133  includes multiple CPUS, such firmware can run on one or more of the multiple CPUs. 
     In an embodiment, the first servo system of HDD  100  (e.g., CPU  301 , read/write channel  137 A, preamplifier  320 A, first actuator control circuit  315 , and voice-coil motor  128 A) performs positioning of a read/write head included in actuator arm assembly  120 A (e.g., read/write head  127 A) over a corresponding recording surface (e.g., recording surface  112 A), during which CPU  301  determines an appropriate current to drive through the voice coil of VCM  128 A. Typically, the appropriate current is determined based in part on a position feedback signal of the read/write head, i.e., a position error signal (PES) and on a target current profile. Similarly, the second servo system of HDD  100  (e.g., CPU  301 , read/write channel  137 B, preamplifier  320 B, second actuator control circuit  316 , and voice-coil motor  128 B) performs positioning of a read/write head included in actuator arm assembly  120 B (e.g., read/write head  127 D) over a corresponding recording surface (e.g., recording surface  112 D), during which CPU  301  determines an appropriate current to drive through the voice coil of VCM  128 B. Although a single CPU  301  is shown here, it is possible that multiple CPUs might be used (for example, one or more CPUs for each actuator). 
     The embodiments illustrated in  FIGS. 1-3  depict a multiple-actuator drive. However, various embodiments can also be beneficially implemented in any single-actuator drive that is subject to mechanical disturbances. 
     Off-Track Event Overview 
     One or more embodiments provide systems and methods for efficient recovery in a disk drive when an off-track error occurs during a sequential disk access operation.  FIGS. 4A-4F  schematically illustrate various stages of an example instance of an off-track event. 
       FIG. 4A  illustrates a portion of a data storage track  411  on a recording surface and a read/write head  427  positioned over data storage track  411  prior to detection of an off-track event. As shown, data storage track  411  includes a plurality of data sectors  421 - 424  (referred to collectively herein as data sectors  420 ) and servo sectors  431 - 434  (referred to collectively herein as servo sectors  430 ). Each data sector  420  is configured to store a certain quantity of user data, e.g., 512B or 4 KB), and is disposed along data storage track  411  between two servo sectors  430 . Each servo sector  430  includes information indicating one or more of track number, position signals, and track offset signals. Thus, when read/write head  427  passes over a particular servo sector  430 , a position-error signal (PES) is generated that indicates the radial position (indicated by arrow  401 ) of read/write head  427  relative to data storage track  411 . 
     In  FIGS. 4A-4F , servo sectors  430  corresponding to four servo wedges N−1, N, N+1, and N+2 are shown, but typically the number of servo sectors included in a data storage track is on the order of about 400 to 600 or more. In the portion of data storage track  411  illustrated in  FIGS. 4A-4F , servo sector  431  is included in servo wedge N−1, servo sector  432  is included in servo wedge N, servo sector  433  is included in servo wedge N+1, and servo sector  434  is included in servo wedge N+2. For purposes of description, data sectors  420  of data storage track  411  are considered contiguous with each other, even though subgroups of data sectors  420  are separated by a servo sector  430 . For example, while data sectors  421  are separated from data sectors  422  by servo sector  432 , the group of data sectors  420  formed by data sectors  421  is considered adjacent to the group of data sectors  420  formed by data sectors  422 . Further, while four data sectors  420  are shown between each servo sector  420  in  FIGS. 4A-4F , in other embodiments, as few as a single data sector  420  is disposed between two servo sectors  430 . Alternatively, in other embodiments, more than four data sectors  420  are disposed between two servo sectors  430 . 
     In  FIG. 4A , read/write head  427  is disposed between the servo sector  431  and servo sector  432 . Due to rotation of the recording surface on which data storage track  411  is disposed, read/write head is moving relative to the recording surface from left to right. As shown, read/write head  427  is substantially centered on data storage track  411 , and therefore is not “off-track.” As a result, read/write head  427  is performing a disk access operation on data sectors  421 , since data sectors  421  are between the servo sector  430  most recently crossed by read/write head  427  (i.e., servo sector  431 ) and the next servo sector  430  to be crossed by read/write head  427  (i.e., servo sector  432 ). 
     In  FIG. 4B , read/write head  427  is disposed between servo sector  432  and  433  and has just passed over servo sector  432 . Thus, a PES associated with servo sector  432  is generated and the radial position of read/write head  427  can be determined. Although read/write head  427  is shown radially displaced from data storage track  411 , read/write head  427  is considered “on-track” and performs the sequential disk access operation on one or more of data sectors  422  until an off-track condition is determined. 
     In  FIG. 4C , read/write head  427  is still disposed between servo sector  432  and  433 , but sufficient time has elapsed for the off-track position of read/write head  427  to be determined based on the PES generated from servo sector  432 . As a result, the sequential disk access operation to be performed by read/write head  427  is terminated until read/write head  427  is determined to again be on-track. Consequently, the sequential disk access operation does not continue to be performed on some or all of data sectors  422 . Alternatively, in some embodiments, when the sequential disk access operation is a read operation, the read operation may continue even after read/write head  427  is determined to be off-track, since cyclic redundancy check (CRC) or error check code (ECC) information included in each data sector  420  indicates if a data read from the data sector  420  has been corrupted due to the off-track condition. 
     In some instances, one or more of the initial data sectors  422  may have a disk access operation performed thereon before the off-track condition is detected, while in other instances, the off-track condition (or even an impending off-track condition) is detected prior to the disk access operation being performed on any of data sectors  422 . Further, in response to the PES generated from servo sector  432 , the servo system controlling the radial position of read/write head  427  implements appropriate commands to begin returning read/write head  427  to an on-track position. 
     An off-track condition for read/write head  427  is generally based on a current radial position of read/write head  427 , as indicted by a PES generated from one of servo sectors  430 . For example, in some embodiments, read/write head  427  is considered to be off-track when radially offset from data storage track  411  by a radial distance that exceeds a predetermined threshold. In some embodiments, the predetermined threshold can have a different value depending on whether the sequential disk access operation is a read operation or a write operation. In such embodiments, the predetermined threshold for a write operation is generally a lower value than the predetermined threshold for a read operation. Furthermore, in some embodiments, the drive may halt writing with read/write head  427  when a determination is made indicating that read/write head  427  might go off-track over the next wedge time, i.e., before crossing the next servo wedge  430 . In such embodiments such a determination may be based upon recent measured position signals and/or control signals associated with read/write head  427 . 
     In  FIG. 4D , read/write head  427  is disposed between servo sectors  433  and  434 , and has just passed over servo sector  433 . Thus, a PES associated with servo sector  433  is generated and the radial position of read/write head  427  can be determined. In  FIG. 4D , the radial position of read/write head  427  has been corrected and read/write head  427  is substantially centered on or radially proximate data storage track  411 . However, read/write head  427  is still considered “off-track” and does not perform the sequential disk access operation on one or more of data sectors  423  until being on-track has been determined. 
     In  FIG. 4E , read/write head  427  is still disposed between servo sectors  433  and  434 , but sufficient time has elapsed for the on-track position of read/write head  427  to be determined based on the PES generated from servo sector  433 . As a result, disk access operations by read/write head  427  are again begun via read/write head  427 . In some instances, some or most of data sectors  423  have the sequential disk access operation performed thereon after read/write head  427  is determined to be on-track, and in other instances all of data sectors  423  may have the sequential disk access operation performed thereon after read/write head  427  is determined to be on-track. Alternatively, in some embodiments, read/write head  427  is not determined to be on-track until a PES from multiple sequential servo sectors  430  indicate that read/write head  427  in within a threshold distance of data storage track  411 . In such embodiments, none of data sectors  423  have the sequential disk access operation performed thereon, even though read/write head  427  is determined to be on-track while over data sectors  423 . 
     In  FIG. 4F , read/write head  427  has just passed over servo sector  434  and remains substantially centered on or radially proximate data storage track  411 . Thus, a PES associated with servo sector  434  is generated and the radial position of read/write head  427  can be determined. In embodiments in which read/write head  427  is considered on-track after the PES of two consecutive servo sectors  420  indicate read/write head  427  is within the predetermined threshold distance from data storage track  411 , read/write head  427  begins performing the sequential disk access operation on some or all of data sectors  424 . 
     As a result of the off-track event described above, certain data sectors  420  of data storage track  411  are associated with the off-track event. In some embodiments, the sequential disk access operation is assumed not to have been performed on such data sectors  420 . Consequently, the sequential disk access operation for those data sectors  420  is retried to complete the sequential disk access operation. In some embodiments, some or all of the data sectors  420  immediately following a servo sector  430  whose PES indicates an off-track condition are associated with the off-track event, such as data sectors  422  in  FIGS. 4A-4F . In some embodiments, some or all of the data sectors  420  immediately following the servo sector  430  that precedes the servo sector  430  whose PES indicates an off-track condition are associated with the off-track event, such as data sectors  421  in  FIGS. 4A-4F . In some embodiments, some or all of the data sectors  420  immediately following one or more servo sector  430  whose PES again indicates read/write head  427  is no longer off-track are still associated with the off-track event, such as data sectors  423  and  424  in  FIGS. 4A-4F . 
     Delayed Off-Track Event Recovery 
       FIG. 5  schematically illustrates the portion of a storage disk  510  associated with a sequential disk access operation that spans multiple contiguous data tracks  511 - 513 . In the instance illustrated in  FIG. 5 , the sequential disk access operation spans three data tracks of storage disk  510 . Storage disk  510  rotates counterclockwise (indicated by arrow  501 ) and, as a result, a read/write head (not shown) performing the sequential disk access operation moves clockwise (indicated by arrow  502 ) relative to storage disk  510 . 
     When the sequential disk access operation is a write operation, data are written to the portions of contiguous data tracks  511 - 513  starting at a first sector  521  of the sequential disk access operation and ending at a final sector  522  of the disk access operation. It is noted that final sector  522  of the disk access operation is generally a different data sector than an end sector (not shown) of the current data track (in this case data track  513 ). Similarly, when the sequential disk access operation is a read operation, data are read from the portions of contiguous data tracks  511 - 513  starting at first sector  521  on data track  511  and ending at final sector  522  of the disk access operation. 
       FIGS. 6A-6I  schematically illustrate various steps of the sequential disk access operation shown in  FIG. 5 , according to an embodiment. In  FIG. 6A , a read/write head  627  begins performing the sequential disk access operation at first sector  631  on data track  511 . First sector  631  on data track  511  corresponds to first sector  521  in  FIG. 5 . Also shown in  FIG. 6A , read/write head  627  moves clockwise relative to storage disk  510  and continues until a first off-track condition for read/write head  627  is determined at first off-track sector  632 . In some embodiments, the off-track condition is determined based on the PES associated with one or more servo sectors (not shown) of data track  511 . 
     In  FIG. 6B , read/write head  627  continues to move clockwise relative to storage disk  510  until read/write head  627  is determined to again be on-track. Thus, read/write head  627  moves past a group of off-track sectors  633 . Off-track sectors  633  are data sectors of data track  511  that are associated with the first off-track condition. Therefore, the sequential disk access operation currently underway is assumed not to have been performed on off-track sectors  633 . Off-track sectors  633  are included in a single data track of storage disk  510  and typically span multiple servo sectors (not shown) of the single data track. 
     In  FIG. 6C , read/write head  627  continues to move clockwise relative to storage disk  510  until a second off-track condition for read/write head  627  is determined at a first off-track sector  634  on data track  512 . In the instance illustrated in  FIG. 6C , prior to reaching first off-track sector  634 , read/write head  627  moves clockwise to an end sector  635  of data track  511 , is radially repositioned over a first data sector  636  of data track  512 , and continues to move clockwise along data track  512 . As shown in  FIG. 6C , end sector  635  of data track  511  is circumferentially proximate, i.e., in approximately the same circumferential location, as first data sector  636  of the next data track (data track  512 ) of the sequential disk access operation. To facilitate transition of read/write head  627  from end sector  635  to first data sector  636 , first data sector  636  is typically offset a short distance circumferentially from end sector  635 . In some embodiments, such a circumferential offset, or “single-track skew,” can be significantly greater than that depicted in  FIGS. 5 and 6A-6I . For example, in a 7200 rotations-per-minute drive, such a circumferential offset between end sector  635  and first data sector  636  can be on the order of about 1/20 th  to 1/10 th  of a revolution of storage disk  510 . 
     In  FIG. 6D , read/write head  627  continues to move clockwise relative to storage disk  510  until read/write head  627  is determined to again be on-track. Thus, read/write head  627  moves past a group of off-track sectors  637 . Off-track sectors  637  are data sectors of data track  512  that are associated with the second off-track condition. Therefore, the sequential disk access operation currently underway is assumed not to have been performed on off-track sectors  637 . Off-track sectors  637  are included in a single data track of storage disk  510 , typically span multiple servo sectors (not shown) of the single data track, and are associated with the second off-track condition. 
     In  FIG. 6E , read/write head  627  completes the disk access operation by continuing to move clockwise along data track  512  to an end sector  638  of data track  512 , is radially repositioned over a first data sector  639  of data track  513 , and continues to move clockwise along data track  513  to a final data sector  640  of the sequential disk access operation. Thus, in  FIG. 6E , read/write head  627  is shown completing a first attempt to complete the sequential disk access operation. However, due to the occurrence of first off-track condition and the second off-track condition, the sequential disk access operation is not completed until the portions of the sequential disk access operation associated with off-track sectors  633  and off-track sectors  637  are successfully performed. 
     In  FIG. 6F , read/write head  627  begins a delayed off-track event recovery. That is, read/write head  627  is controlled to complete the remaining portions of the sequential disk access operation associated with off-track conditions. Thus, in the instance illustrated in  FIG. 6F , read/write head  627  is controlled to complete the sequential disk access operation for off-track sectors  633  and off-track sectors  637 . To that end, immediately upon completion of the first attempt to complete the sequential disk access operation, read/write head  627  seeks to the group of off-track sectors that can first be reached. In the instance illustrated in  FIG. 6F , read/write head  627  seeks to data track  511  while moving clockwise from final data sector  640  to first off-track sector  632 . 
     In  FIG. 6G , read/write head  627  performs the portion of the sequential disk access operation associated with off-track sectors  633 , such as reading data from off-track sectors  633  or writing data to off-track sectors  633 . To that end, read/write head  627  moves clockwise along data track  511  to the last of off-track sectors  633  while performing the sequential disk access operation. 
     In  FIG. 6H , read/write head  627  seeks to the next group of off-track sectors that can be reached soonest. In the instance illustrated in  FIG. 6H , read/write head  627  seeks to data track  512  while moving clockwise from the last sector of off-track sectors  633  to first off-track sector  634 . 
     In  FIG. 6I , read/write head  627  performs the portion of the sequential disk access operation associated with off-track sectors  637 , such as reading data from off-track sectors  637  or writing data to off-track sectors  637 . To that end, read/write head  627  moves clockwise along data track  512  to the last of off-track sectors  637  while performing the sequential disk access operation. Read/write head  627  then begins to perform the next disk access operation for storage disk  510 . 
     According to embodiments described herein, off-track sectors associated with a sequential disk access operation can be completed in a small number of additional revolutions of storage disk  510 . For example, as shown in  FIGS. 6A-6I , read/write head  627  performs the portion of the sequential disk access operation associated with off-track sectors  633  and  637  in a single revolution of storage disk  510 . More generally, in instances in which M off-track events occur in a single sequential disk access operation, read/write head  627  can typically perform the portion of the sequential disk access operation associated with the M groups of off-track sectors in a number of revolutions of storage disk  510  that is significantly smaller than M. By contrast, conventional approaches for recovering from off-track events during a sequential disk access operation generally require an additional revolution of the storage disk for each data track on which there is a group of off-track sectors associated with the off-track events. Thus, for the instance illustrated in  FIGS. 6A-6I , a conventional approach for recovering from the first and second off-track events requires two additional revolutions of storage disk  510 . 
     It is noted that implementation of the embodiment illustrated in  FIG. 6A-6I  results in portions of a sequential disk access operation being performed out of order. Specifically, groups of off-track data sectors are read from (or written to) after portions of the sequential disk access operation that normally occur before the off-track data sectors. For example, in a sequential read operation, data are read from off-track sectors  633  (as shown in  FIG. 6G ) after data are read from the portion of data storage track  511  between off-track sectors  633  and first off-track sector  634  (as shown in  FIG. 6C ). In embodiments in which host  10  accepts out-of-order data for a sequential read operation, data read normally from normal (i.e., not off-track) portions of the sequential read operation can be transmitted to the host as they are read, even though these normally read portions occur later in the sequential read operation than one or more of the off-track data sectors. In such embodiments, data read from the off-track data sectors is transmitted to the host after the normally read portions of the sequential read operation. By contrast, in embodiments in which host  10  does not accept out-of-order data for a sequential read operation, once an off-track event is detected during the sequential read operation, data read after the off-track event are stored, e.g. in RAM  134 , and transmitted to the host later in the sequential read operation. For example, in an embodiment, data from the normally read portions of the sequential read operation can be transmitted to host  10  once the preceding off-track data are read from the appropriate off-track sectors. 
       FIG. 7  sets forth a flowchart of method steps for recovering from off-track events in a sequential disk access operation, according to an embodiment. In some embodiments, the method steps can be employed for sequential reading and/or writing operations using CMR. Alternatively or additionally, in some embodiments, the method steps can be employed for sequential reading operations using SMR. Although the method steps are described in conjunction with HDD  100  of  FIGS. 1-6I , persons skilled in the art will understand that the method steps may be performed with other types of systems. The control algorithms for the method steps can reside in CPU  301  as off-track recovery algorithm  303 . Off-track recovery algorithm  303  can be implemented in whole or in part as software- or firmware-implemented logic, and/or as hardware-implemented logic circuits. Prior to the method steps, microprocessor-based controller  133  receives a request from host  10  for a sequential disk access command, such as a read or write command directed to data sectors spanning two or more contiguous data storage tracks. 
     A method  700  begins at step  701 , when microprocessor-based controller  133  causes a read/write head (e.g., read/write head  127 A) to move to a first data storage track (e.g., on recording surface  112 A) of the sequential disk access operation. In step  702 , microprocessor-based controller  133  begins the sequential disk access operation. For example, in some embodiments, microprocessor-based controller  133  begins the disk access operation when the read/write head reaches the first data sector of the disk access operation. In step  703 , microprocessor-based controller  133  causes the disk access operation to be performed on the current data sector that is proximate the read/write head. 
     In step  704 , microprocessor-based controller  133  determines whether the data sector on which the disk access operation is performed in step  703  is the last data sector of the current data storage track. If no, method  700  returns to step  703  and the disk access operation is performed on the next data sector of the disk access operation; if yes, method  700  proceeds to step  705 . 
     In step  705 , microprocessor-based controller  133  detects, logs, or otherwise determines whether one or more off-track events occurred on the current data storage track. In some embodiments, microprocessor-based controller  133  performs step  705  during steps  703  and  704 , i.e., throughout the time that the disk access operation is performed on the data sectors of the current data storage track. 
     In step  706 , microprocessor-based controller  133  determines whether there is a remaining data storage track of the disk access operation. That is, microprocessor-based controller  133  determines whether the disk access operation references one or more data sectors on a remaining data storage track. If yes, method  700  proceeds to step  711 ; if no, method  700  proceeds to step  707 . 
     In step  707 , microprocessor-based controller  133  determines whether there is a remaining group of off-track data sectors associated with an off-track event that occurred during the disk access operation. If yes, method  700  proceeds to step  708 ; if no, method  700  proceeds to step  720 . In step  720 , microprocessor-based controller  133  begins execution of the next disk access operation. 
     In step  708 , microprocessor-based controller  133  selects a group of off-track data sectors associated with an off-track event that occurred during the disk access operation. In some embodiments, microprocessor-based controller  133  selects the group on which the read/write head can begin execution of the disk access command soonest, such as the group that the read/write head can seek to the soonest. Alternatively, in some embodiments, microprocessor-based controller  133  first determines a sequence of the groups of off-track data sectors that can be performed efficiently, and then selects the first group in that sequence. 
     In step  709 , microprocessor-based controller  133  causes the read/write head to perform the disk access operation on the selected group of off-track data sectors. Method  700  then returns to  707 . 
     Step  711  is performed in response to microprocessor-based controller  133  determining there is a remaining data storage track of the disk access operation. In step  711 , microprocessor-based controller  133  causes the read/write head to move radially to the data track that includes the next data sector of the sequential disk access operation. 
     Slipping Sectors in an SMR Write Operation to Recover from an Off-Track Event 
     In some embodiments, efficient recovery from an off-track event is enabled for a write operation in an SMR drive. In such embodiments, HDD  100  of  FIG. 1  is configured as an SMR drive and includes an SMR region on one or more of recording surfaces  112 A- 112 H. In such embodiments, the SMR region(s) of recording surfaces  112 A- 112 H include data storage tracks that are arranged in groups, or “bands” of data storage tracks. Each band of data storage tracks is separated from adjacent bands by guard regions, which are inter-band gaps in which no data tracks are formed. Further, the data storage tracks formed in an SMR region are written in an SMR format, and therefore overlap adjacent data tracks in the same band. Thus, each band in an SMR region includes a plurality of overlapping data tracks that each have a width that is significantly narrower than a width of the write element included in the corresponding read/write head. One embodiment of such a band is illustrated in  FIG. 8 . 
       FIG. 8  is a schematic illustration of a portion  800  of recording surface  112  that includes a band  820  of SMR data tracks, according to an embodiment. Band  820  includes a plurality of SMR data tracks  821 - 828 , and is separated from adjacent bands (not shown) by guard regions  830 . As shown, each of SMR data tracks  821 - 828  overlaps and/or is overlapped by at least one adjacent SMR data track. As a result, each of SMR data tracks  821 - 828  has a readable width  801  that is significantly less than an as-written width  802 . It is noted that in the embodiment illustrated in  FIG. 8 , band  820  only includes nine SMR data tracks, whereas in practice band  820  may include up to one hundred or more SMR data tracks. 
     Also shown in  FIG. 8  is a read/write head  827 , which is configured with a write head  803  and a read head  804  that are formatted for SMR. As such, read head  804  is configured with a width that is approximately equal to or less than readable width  801 , and is positioned within a read/write head  827  to facilitate reading of SMR data tracks  821 - 828 . Furthermore, write head  803  is positioned within read/write head  827  to facilitate writing of SMR data tracks  821 - 828  with as-written width  802 . In accordance with the principle of SMR, as-written width  802  exceeds readable width  801 , for example by a factor of two. Thus, as a particular one of SMR data tracks  821 - 828  is written, write head  803  is positioned to overlap a significant portion of the preceding SMR data track. 
     According to some embodiments, efficient recovery from an off-track event is enabled in an SMR disk drive during a sequential write operation that spans multiple contiguous data tracks. In the embodiments, when an off-track error occurs during the sequential write operation and a first portion a first portion of a data track is not written to, the data originally targeted to be written to the first portion is written to a second portion of the data track that follows the first portion. That is, the data written to the disk after the first portion of the track “slip” to subsequent portions of the disk. Since no additional revolutions of the disk are needed for all data associated with the sequential write operation to be written to the disk, there is very little delay in completion of the write operation. Implementation of one such embodiment is illustrated in  FIGS. 9A-9C . 
       FIGS. 9A-9C  schematically illustrate various steps of the sequential write operation shown in  FIG. 5 , according to an embodiment. In  FIG. 9A , a read/write head  927  begins performing the sequential write operation at first sector  931  on data track  511 . First sector  931  on data track  511  corresponds to first sector  521  in  FIG. 5 . Also shown in  FIG. 9A , read/write head  927  moves clockwise relative to storage disk  510  and continues until a first off-track condition for read/write head  927  is determined at first off-track sector  932 . In some embodiments, the off-track condition is determined based on the PES associated with one or more servo sectors (not shown) of data track  511 . 
     In  FIG. 9B , read/write head  927  continues to move clockwise relative to storage disk  510  until read/write head  927  is determined to again be on-track. Thus, read/write head  927  moves past a group of off-track sectors  933 . Off-track sectors  933  are data sectors of data track  511  that are associated with the first off-track condition. As a result, data are not written to most or all of off-track sectors  933  during the sequential write operation. Further, any data that might have been written to the initial sectors of off-track sectors  933  before the off-track condition is determined are disregarded and not considered valid. Therefore, the sequential write operation currently underway is assumed not performed on off-track sectors  933 . Off-track sectors  933  are included in a single data track of storage disk  510  and typically span multiple servo sectors (not shown) of the single data track. 
     In  FIG. 9C , read/write head  927  performs the portion of the sequential write operation associated with off-track sectors  933  on one or more slip sectors  934  that follow off-track sectors  933 . In some embodiments, the first data sector of slip sectors  934  is first data sector of data track  511  that follows the last data sector of off-track sectors  933 . As shown, data to be written to off-track sectors  933  are instead written to one or more slip sectors  934 . To that end, read/write head  927  moves clockwise along data track  511  to the last of slip sectors  934  while performing the sequential write operation originally targeted for off-track sectors  933 . 
     In the embodiment described above in conjunction with  FIGS. 9A-9C , off-track events result in portions of storage disk  510  not being written to, such as off-track sectors  933 . Consequently, additional data sectors  935  from the last data track (in this case data track  513 ) of the sequential write operation are used for completion of the sequential write operation. In the instance illustrated in  FIGS. 9A-9C , a single off-track event occurs during the sequential write operation. Therefore, the number of data sectors in additional data sectors  935  is equal to the number of data sectors in off-track sectors  933 . In an instance in which multiple off-track events occur during the sequential write operation, the number of data sectors in additional data sectors  935  is at least equal to the total number of data sectors in all off-track sectors of the sequential write operation. 
       FIGS. 9A-9C  illustrate that, according to various embodiments, recovery from an off-track event occurs before read/write head  927  has stopped following the current data track of storage disk  510 . As a result, an additional revolution of storage disk  510  is generally not needed for writing the data associated with an off-track event. 
     In some embodiments, an SMR band can be nominally configured to store a fixed quantity of data, e.g., 100 MB, but includes a number of addition data sectors or data storage tracks. As a result, in practice, the SMR band includes more than the number of data sectors needed to store the nominal quantity of data associated with the SMR band. Instead, the SMR band can effectively have a storage capacity that is 0.1-2% larger than the nominal storage capacity of the band. In such embodiments, the number of extra data sectors included in a particular SMR band can be employed as additional data sectors  935  shown in  FIG. 9C . However, the number of such extra data sectors is generally limited for each SMR band. As a result, in some instances the number of additional data sectors  935  for a particular sequential write operation exceeds the number of existing extra data sectors remaining in the SMR band on which the particular sequential write operation is being performed. According to some embodiments, data sectors included in a media cache of the storage disk are then employed as additional data sectors  935 . In such embodiments, when the extra data sectors of a particular SMR band are exhausted and additional storage capacity for the SMR band is needed to compensate for off-track sectors in the SMR band, data to be stored in the SMR band is stored in the media cache. One such embodiment is illustrated in  FIG. 10 . 
       FIG. 10  schematically illustrates a user region  1010  of a recording surface  1012  and a media-cache region  1020  of recording surface  1012 , according to an embodiment. For clarity, data storage tracks formed on recording surface  1012  are not shown in  FIG. 10 . In the embodiment illustrated in  FIG. 10 , media-cache region  1020  is disposed proximate OD  1002  of recording surface  1012  and user region  1010  includes the remainder of recording surface  1012 . In other embodiments, media-cache region  1020  may be disposed in any other region of recording surface  1012 , for example proximate ID  1001 , or a middle diameter (MD) region of recording surface  1012 . In other embodiments, the media cache may be split into multiple regions at multiple radii across the stroke of the disk. Generally, user region  1010  includes the majority of the storage capacity of recording surface  1012  and is an SMR region of recording surface  1012 . 
     Media-cache region  1020  is configured to store media-cache entries during normal operation of HDD  100 . As such, media-cache region  320  can include a combination of SMR data storage tracks and/or CMR data storage tracks. CMR data storage tracks are not written in an SMR format, and therefore are not overlapping. Thus, data storage tracks in the CMR portion of media-cache region  1020  can be used to store random block writes without an entire band of shingled tracks being re-written for each write command received. Alternatively, in some embodiments, a media cache region employed to store slip overflow data from SMR bands can also exist in non-volatile RAM (e.g. flash). In such embodiments, a sequential write operation can be completed more quickly since no seek and rotational delays are involved in the storage of the slip overflow data. Further, some non-volatile RAM (e.g., flash) is written sequentially, which is well-suited for such use. 
     In some embodiments, when the remaining data sectors included in a particular SMR band are insufficient for storing some or all data included in a write command to be performed in the SMR band, the write command is completed by storing some or all such data in physical locations (such as sectors) of media-cache region  1020 . For example, when a number of off-track events of sufficient size occur during write operations to a particular SMR band, the remaining data sectors included in the SMR band can be exhausted even while data for one or more additional write commands are targeted to be stored in the SMR band. In such embodiments, once the last available data sector of the SMR band is used to store data, remaining write command data for the SMR band are written to media-cache region  1020 . In some embodiments, media-cache region  1020  is configured with a circular buffer, and the remaining write command data are written into the circular buffer. In other embodiments, the remaining write command data are written to any appropriate media-cache data sectors of media-cache region  1020 . Conventional mapping techniques known in the art can be employed to associate the data written in this way to media-cache region  1020  with the SMR band that was targeted to store such data. 
       FIG. 11  sets forth a flowchart of method steps for recovering from off-track events in a sequential write operation in an SMR drive, according to an embodiment. Although the method steps are described in conjunction with HDD  100  of  FIGS. 1-10 , persons skilled in the art will understand that the method steps may be performed with other types of systems. The control algorithms for the method steps can reside in CPU  301  as off-track recovery algorithm  303 . Off-track recovery algorithm  303  can be implemented in whole or in part as software- or firmware-implemented logic, and/or as hardware-implemented logic circuits. 
     A method  1100  begins at step  1101 , when microprocessor-based controller  133  receives a sequential write command from host  10  directed to data sectors spanning two or more contiguous data storage tracks in an SMR band on a recording surface of HDD  100  (e.g., one of recording surfaces  112 A- 112 H). 
     In step  1102 , microprocessor-based controller  133  causes a read/write head (e.g., one of read/write heads  127 A- 127 H) to move to the next servo sector of the sequential write operation. Thus, in the initial iteration of step  1102 , the read/write head seeks to the data track of the recording surface that includes the first data sector of the sequential write command. In addition, the recording surface is rotated until the servo sector preceding the next data sector of the sequential write command passes under the read/write head. In instances in which the previous servo sector passed over by the read/write head is the last servo sector of the current data track, the read/write head first seeks to the next data track of the sequential write operation (i.e., the data track that includes the next data sector of the sequential write operation). 
     In step  1103 , microprocessor-based controller  133  determines if an off-track event has occurred, based, at least in part, on the PES generated by the servo sector. For example, in some embodiments, the off-track event may be determined based on previous PES values, current VCM or microactuator commands, an input from a shock sensor or an accelerometer, and the like. If yes, method  1100  proceeds to step  1104 ; if no, method  1100  proceeds to step  1110 . 
     In step  1104 , microprocessor-based controller  133  recovers the position of the read/write head from the off-track event. Specifically, a servo system of HDD  100  determines and applies an appropriate current to the actuator and voltage to any associated microactuator controlling the radial position of the read/write head to position the read/write head radially over the current data track. In addition, in step  1104  data are not written to the data sectors passing under the read/write head while microprocessor-based controller  133  recovers the position of read/write head. As a result, off-track sectors  933  are present in the SMR band. 
     In step  1105 , microprocessor-based controller  133  determines a size (e.g., in data sectors) of the off-track event. That is, microprocessor-based controller  133  determines how many off-track sectors are associated with the off-track event. 
     In step  1106 , microprocessor-based controller  133  determines whether the remaining data sectors in the SMR band is greater than the number of data sectors needed to complete the sequential write command. Generally, the remaining data sectors in the SMR band include the unused extra sectors associated with the SMR band and the unused normal data sectors, where the data sectors associated with an off-track event are not considered to be unused normal sectors. If yes, method  1100  proceeds to step  1107 ; if no, method  1100  proceeds to step  1112 . 
     In step  1107 , microprocessor-based controller  133  continues to perform the sequential write operation using slip sectors  934  that follow the off-track sectors  933  associated with the off-track event determined in step  1103 . Thus, data originally targeted for being written to off-track sectors  933  are written to slip sectors  934  in the same revolution of the recording surface. By contrast, conventional approaches for recovering from off-track write events generally involve rotating the recording surface for an additional revolution so that the read/write head can attempt to retry writing to off-track sectors  933 . 
     In some embodiments, as a part of the sequential write operation, information about the writing of data to slip sectors  934  is stored into a table in RAM  134 , so that a subsequent read of the SMR band will return the proper data. When the drive is subjected to a normal power off sequence (preceded by commands to the drive to anticipate such a power off), that information is saved to a system area of the drive, in one or more copies located on one or more surfaces of disks  112 A-F. In such embodiments, the portion of RAM  134  into which such information is written is also protected by circuitry that writes it to a non-volatile memory in the event of an unexpected loss of power. Thus, the drive does not need to interrupt the sequential write operation in order to commit such information to a disk surface. Systems and methods for implementing such embodiments is described in greater detail in U.S. Pat. No. 9,070,417, filed Oct. 24, 2014. 
     In step  1108 , microprocessor-based controller  133  determines whether there is remaining data to be written in the sequential write operation. If yes, method  1100  returns to step  1102 ; if no, method  1100  proceeds to step  1120  and terminates. 
     Step  1110  is performed in response to microprocessor-based controller  133  determining that no off-track event has occurred. In step  1110 , microprocessor-based controller  133  determines whether there is remaining data to be written in the sequential write operation. If yes, method  1100  proceeds to step  1111 ; if no, method  1100  proceeds to step  1120  and terminates. 
     Step  1111  is performed in response to microprocessor-based controller  133  determining that no off-track event has occurred and data still remain to be written in the current sequential writing operation. In step  1111 , microprocessor-based controller  133  causes the read/write head to write data to some or all of the data sectors following the servo sector that passed under the read/write head in step  1102 . Method  1100  then returns to step  1102 . 
     Step  1112  is performed in response to microprocessor-based controller  133  determining that there are fewer remaining data sectors in the SMR band than the number of data sectors needed to complete the sequential write command. That is, there are insufficient unused extra and/or unused normal data sectors remaining in the SMR band to store the remaining unwritten data of the current sequential write operation. In step  1112 , microprocessor-based controller  133  causes the read/write head to write data from the sequential write command to remaining data sectors of the SMR band (if any), including the extra data sectors of the SMR band. It is noted that, upon completion of step  1112 , at least some of the data of the current sequential write operation remains to be written. Method  1100  then proceeds to step  1113 . 
     In step  1113 , microprocessor-based controller  133  causes an alternative off-track recovery procedure to be performed. In some embodiments, some or all of the data targeted for being written to the SMR band as part of the sequential write operation are written to a media-cache region (e.g., media-cache region  1020 ) of the recording surface. Upon completion of step  1113 , method  1200  proceeds to step  1108 . 
       FIG. 12  sets forth a flowchart of method steps for recovering from off-track events in a sequential write operation in an SMR drive, according to another embodiment. Although the method steps are described in conjunction with HDD  100  of  FIGS. 1-11 , persons skilled in the art will understand that the method steps may be performed with other types of systems. The control algorithms for the method steps can reside in CPU  301  as off-track recovery algorithm  303 . Off-track recovery algorithm  303  can be implemented in whole or in part as software- or firmware-implemented logic, and/or as hardware-implemented logic circuits. 
     Method  1200  begins at step  1103  of method  1100 . Thus, method  1200  is an alternative embodiment for recovering from off-track events in a sequential write operation in an SMR drive. In step  1103 , microprocessor-based controller  133  determines if an off-track event has occurred, based, at least in part, on the PES generated by the last servo sector passed over by the read/write head. If yes, method  1200  proceeds to step  1201 ; if no, method  1200  proceeds to step  1110  and continues in a similar fashion as method  1100  described above in conjunction with  FIG. 11 . 
     Step  1201  is performed in response to microprocessor-based controller  133  determining that an off-track event has occurred while a sequential write operation is performed on an SMR band in the SMR drive. In step  1201 , microprocessor-based controller  133  determines whether a number of remaining spare sectors in the SMR band is below a predetermined threshold value. If no, method  1200  proceeds to step  1104  and continues in a similar fashion as method  1100  described above in conjunction with  FIG. 11 ; if yes, method  1200  proceeds to step  1202 . 
     In some embodiments, the threshold value is a fixed number of spare sectors, such as a particular fraction of the original number of spare sectors included in the SMR band when the SMR band stores no data (e.g., one quarter of the original number of spare sectors). In some embodiments, the threshold value is a variable number of spare sectors. In one such embodiment, the threshold value is based on the fraction of the data sectors of the SMR band that are currently available. In such an embodiment, data sectors of the SMR band that are currently available can include the data sectors that currently do not have data written thereto and that have not been associated with an off-track event. For example, in one such an embodiment, the threshold value can be the product of the original number of spare sectors and the fraction of the data sectors of the SMR band that are currently available. Therefore, in the embodiment, as the number of data sectors that are currently available in the SMR band decreases, the threshold value decreases. Alternatively or additionally, the threshold value can be a function of remaining storage space in media-cache region  1020 . In some embodiments, the threshold can be a function of the amount of data that has already been written to the SMR band, relative to the total number of user data sectors allocated to the SMR band. For example, in one such embodiment, the threshold could be a fraction of the product of the original number of spare sectors and the difference between the total user capacity of the SMR band and the number of sectors that have already been written to the SMR band. 
     In step  1202 , microprocessor-based controller  133  performs a reduced-slip off-track write recovery procedure, in which a number of slip sectors  934  employed to recover from the off-track event determined in step  1103  is reduced or eliminated. Consequently, in response to an off-track event, spare sectors for the SMR band are employed at a reduced rate or are not employed at all during recovery from the off-track event. Method  1200  then proceeds to step  1108  and continues in a similar fashion as method  1100  described above in conjunction with  FIG. 11 . 
     In some embodiments, the reduced-slip off-track write recovery procedure includes attempting to re-try writing to the off-track sectors instead of using slip sectors  934  for writing data associated with the off-track event (as described above in conjunction with method  1100 ). In some embodiments, the reduced-slip off-track write recovery procedure includes alternating between two different off-track write recovery procedures. For example, in one such embodiment, for every odd-numbered off-track event, an off-track write recovery procedure is employed that includes writing to slip sectors  934  and, for every even-numbered off-track event, an off-track write recovery procedure is employed that includes re-trying writing to the off-track sectors (and using no slip sectors  934 ). As a result, spare sectors included in the SMR are utilized at a reduced rate. 
     In some embodiments, the method used to determine the threshold is modified if the drive has suffered many offtrack events during recent operation (suggesting either that the drive may be in a high-vibration environment or that another actuator of a split-actuator drive is causing frequent disturbances to the actuator writing). For example, the method used to determine the threshold might be modified to increase the threshold if the drive has suffered many offtrack events recently, even if the drive has not yet written many sectors to the current SMR band. In this way, the drive might achieve a better tradeoff between increasing sequential write performance and increasing later sequential reads of the data and reducing the use of the media cache. 
     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.