Patent Publication Number: US-2020294540-A1

Title: Magnetic disk device and method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2019-048509, filed on Mar. 15, 2019; the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to a magnetic disk device and a method for a magnetic disk. 
     BACKGROUND 
     Magnetic disk devices including two or more magnetic heads and two or more actuators that can move the magnetic heads independently are known. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an exemplary configuration of a magnetic disk device according to a first embodiment; 
         FIG. 2  illustrates a trajectory of a magnetic head according to the first embodiment; 
         FIG. 3  illustrates a recording surface of a magnetic disk according to the first embodiment; 
         FIG. 4  schematically illustrates a method of writing data into each band according to the first embodiment; 
         FIG. 5  schematically illustrates an outline of an update operation according to the first embodiment; 
         FIG. 6  schematically illustrates each operation timing in the update operation according to the first embodiment; 
         FIG. 7  is a flowchart illustrating an exemplary operation of the magnetic disk device according to the first embodiment upon reception of data; 
         FIG. 8  is a flowchart of the update operation according to the first embodiment; 
         FIG. 9  schematically illustrates another exemplary method of accessing a recording surface according to the first embodiment; 
         FIG. 10  schematically illustrates an outline of an update operation according to a second embodiment; 
         FIG. 11  schematically illustrates each operation timing in the update operation according to the second embodiment; and 
         FIG. 12  schematically illustrates a configuration of a control circuit according to a third embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     According to one embodiment, in general, a magnetic disk device includes a magnetic disk, a first magnetic head, a second magnetic head, a buffer memory, and a control circuit. The magnetic disk includes a plurality of first storage regions. The control circuit controls the first magnetic head to read first data from a second storage region of the first storage regions to the buffer memory. The control circuit controls the second magnetic head to write second data to a third storage region concurrently with the reading of the first data. The third region is of the first storage regions, different from the second storage region. The second data corresponds to the first data stored in the buffer memory. 
     Exemplary embodiments of a magnetic disk device and a method for a magnetic disk device will be described below in detail with reference to the accompanying drawings. The following embodiments are merely illustrative and not intended to limit the scope of the present invention. 
     First Embodiment 
       FIG. 1  illustrates an exemplary configuration of a magnetic disk device  1  according to a first embodiment. As illustrated in  FIG. 1 , the magnetic disk device  1  includes, for example, two magnetic disks  101 , two pairs of magnetic heads  102  that read and write data, and two arms  104 . 
     The two magnetic disks  101  include a magnetic disk  101   a  and a magnetic disk  101   b . The two pairs of magnetic heads  102  include a pair of magnetic heads  102   a  and a pair of magnetic heads  102   b . The two arms  104  include an arm  104   a  and an arm  104   b.    
     The two magnetic disks  101  are attached to a rotation shaft  103  of a spindle motor with a given pitch in an axial direction of the rotation shaft  103 . The spindle motor causes the two magnetic disks  101  to rotate together about the rotation shaft  103 . 
     The number of magnetic disks  101  of the magnetic disk device  1  is not limited to two. 
     The magnetic heads  102   a  are attached to a distal end of the arm  104   a . One of the magnetic heads  102   a  opposes a front surface of the magnetic disk  101   a , and the other magnetic head  102   a  opposes a rear surface of the magnetic disk  101   a . Each of the magnetic heads  102   a  reads and writes a signal corresponding to data from and to the magnetic disk  101   a.    
     The magnetic heads  102   b  are attached to a distal end of the arm  104   b . One of the magnetic heads  102   b  opposes a front surface of the magnetic disk  101   b , and the other magnetic head  102   b  opposes a rear surface of the magnetic disk  101   b . Each of the magnetic heads  102   b  writes a signal responsive to data into the magnetic disk  101   b , and reads a signal corresponding to data from the magnetic disk  101   b.    
     The magnetic disk device  1  includes two actuators  105 , that is, an actuator  105   a  and an actuator  105   b . Each of the actuator  105   a  and the actuator  105   b  is, for example, a voice coil motor (VCM). The actuator  105   a  and the actuator  105   b  operate independently of each other. 
     The actuator  105   a  causes the arm  104   a  to rotate about a shaft  106  to move the positions of the magnetic heads  102   a  relative to the recording surfaces of the magnetic disk  101   a.    
       FIG. 2  illustrates a trajectory of the magnetic heads  102   a  according to the first embodiment, as viewed from the magnetic disk  101   a  along the shaft  106 . 
     As illustrated in  FIG. 2 , the actuator  105   a  causes the arm  104   a  to rotate about the shaft  106  within a fixed range, so that the magnetic heads  102   a  move along a broken line T. The magnetic heads  102   a  are radially positioned on any track of the magnetic disk  101   a.    
     The actuator  105   b  causes the arm  104   b  to rotate about the shaft  106  to move the positions of the magnetic heads  102   b  relative to the recording surfaces of the magnetic disk  101   b . The magnetic heads  102   b  can thus follow the trajectory similar to that of the magnetic heads  102   a.    
     Referring back to  FIG. 1 , the magnetic disk device  1  further includes a control circuit  20 . 
     The control circuit  20  establishes communications with a host  2  via an interface for external connection, such as a contact pin, attached to a casing (not illustrated) of the magnetic disk device  1 . Examples of the host  2  may include a server device, a mobile computer, and a processor. The control circuit  20  controls the respective components of the magnetic disk device  1  in accordance with, for example, a command from the host  2 . Examples of the command may include a data write command and a data read command. 
     The control circuit  20  includes a preamplifier (PreAmp)  21  and a read channel circuit (RDC)  22  for each actuator  105 . In other words, the control circuit  20  includes a preamplifier  21   a  and an RDC  22   a  for the actuator  105   a . The control circuit  20  also includes a preamplifier  21   b  and an RDC  22   b  for the actuator  105   b.    
     The control circuit  20  also includes a digital signal processor (DSP)  23 , a buffer memory  24 , a hard disk controller (HDC)  25 , a micro processing unit (MPU)  26 , and a memory  27 . 
     The preamplifier  21   a  amplifies a signal read from the magnetic disk  101   a  by each of the magnetic heads  102   a  (read elements), and outputs the amplified signal to the RDC  22   a . The preamplifier  21   a  amplifies the signal sent from the RDC  22   a , and sends the amplified signal to each of the magnetic heads  102   a  (write elements). 
     The RDC  22   a  encodes data to be written into the magnetic disk  101   a , and sends the encoded data as a signal to the preamplifier  21   a . The RDC  22   a  decodes the signal read from the magnetic disk  101   a  and sent from the preamplifier  21   a . The RDC  22   a  outputs the decoded signal as digital data to the HDC  25 . 
     The preamplifier  21   b  amplifies and outputs a signal read from the magnetic disk  101   b  by each of the magnetic heads  102   b  (read elements), and sends the amplified signal to the RDC  22   b . The preamplifier  21   b  amplifies a signal sent from the RDC  22   b , and sends the amplified signal to each of the magnetic heads  102   b  (write elements). 
     The RDC  22   b  encodes data to be written into the magnetic disk  101   b , and sends the encoded data as a signal to the preamplifier  21   b . The RDC  22   b  decodes a signal read from the magnetic disk  101   b  and sent from the preamplifier  21   b . The RDC  22   b  outputs the decoded signal as digital data to the HDC  25 . 
     The DSP  23  controls the spindle motor and the respective actuators  105  to perform positioning control such as seek and following. 
     The buffer memory  24  serves as a buffer for, for example, data to be transferred to and from the host  2 . Specifically, data transmitted from the host  2  is stored in the buffer memory  24 . The data transmitted from the host  2  and stored in the buffer memory  24  is written into each of the magnetic disks  101 . Read from each of the magnetic disks  101 , the data is stored in the buffer memory  24 . Read from each of the magnetic disks  101  and stored in the buffer memory  24 , the data is output to the host  2 . 
     The buffer memory  24  includes, for example, a high-speed operable memory. Type of the memory constituting the buffer memory  24  is not limited to a specific type. The buffer memory  24  may include, for example, a dynamic random access memory (DRAM) or a static random access memory (SRAM). The buffer memory  24  may not be located in the control circuit  20 . The buffer memory  24  may be located outside the control circuit  20 . 
     The HDC  25  is connected to the host  2  via a given interface to establish communications with the host  2 . A standard to which the interface conforms is not limited to a specific standard. The HDC  25  receives data from each of the RDCs  22   a  and  22   b , and stores the received data in the buffer memory  24 . The HDC  25  transfers the data from the RDCs  22   a  and  22   b  to the host  2  from the buffer memory  24 . 
     The HDC  25  receives data together with a write command from the host  2 , and stores the received data in the buffer memory  24 . That is, the buffer memory  24  receives data from the host  2 . The HDC  25  outputs the data from the host  2  to the RDCs  22   a  and  22   b  from the buffer memory  24 . 
     The MPU  26  serves as a processor that executes firmware i.e., a firmware program. The MPU  26  analyzes a command received from the host  2  by the HDC  25 , to monitor the state of the magnetic disk device  1  and control the respective components of the magnetic disk device  1 , for example. 
     The memory  27  functions as, for example, a region in which firmware and various kinds of management information are stored. The memory  27  includes a volatile memory, a nonvolatile memory, or a combination thereof. Examples of the volatile memory may include an SRAM and a DRAM. Examples of the nonvolatile memory may include a flash memory. 
     As described above, the pair of magnetic heads  102   a  and the pair of magnetic heads  102   b  are attached to the different arms  104 . The arms  104  are driven by the different actuators  105 . The preamplifiers  21  and the RDCs  22  are provided for the respective actuators  105 . 
     Thereby, the control circuit  20  can independently control access to the magnetic disk  101   a  using the actuator  105   a  and the pair of magnetic heads  102   a  and access to the magnetic disk  101   b  using the actuator  105   b  and the pair of magnetic heads  102   b . Thus, the control circuit  20  can control the actuator  105   a  and the actuator  105   b  to concurrently access the magnetic disk  101   a  and the magnetic disk  101   b , for example. 
       FIG. 3  illustrates one of the recording surfaces of each magnetic disk  101  according to the first embodiment. The front surface and the rear surface of each magnetic disk  101  include recording surfaces  200 .  FIG. 3  illustrates one of the front surface and the rear surface of the magnetic disk  101 . 
     The recording surface  200  is divided into a plurality of concentric storage regions  210  about the center of rotation of the magnetic disk  101 . The storage regions  210  include one media cache region  220  and a plurality of bands  230 . The recording surface  200  includes, between every two adjacent storage regions  210 , a guard region in which data write operation is inhibited; however,  FIG. 3  omit illustrating such guard regions. Each of the bands  230  is an example of a first storage region. 
     In the example illustrated in  FIG. 3 , the outermost storage region  210  serves as the media cache region  220  in the recording surface  200  of the magnetic disk  101 . However, the location of the media cache region  220  is not limited thereto. The number of bands  230  in the recording surface  200  is four. However, the number of bands  230  is not limited thereto. 
       FIG. 4  schematically illustrates a method of writing data into each of the bands  230  according to the first embodiment. 
     Data is written to each of the bands  230  by shingled magnetic recording (SMR). SMR refers to a data recording method that allows the adjacent tracks to overlap with each other. It is apparent from  FIG. 4  that, according to SMR, a track pitch (TP) is narrower than a core width (WHw) of the write element of each magnetic head  102 . SMR enables decrease in track pitch and improvement in recording density. 
     The direction of track generation is not limited to a specific direction. The tracks may be sequentially set from radially outside to inside in each of the magnetic disks  101 . Alternatively, the tracks may be sequentially set from radially inside to outside in each of the magnetic disks  101 . 
     According to the SMR, the track pitch is narrower than the core width WHw of the write element. Consequently, in updating part of continuously written data in the tracks by the SMR, data in the tracks adjacent to the track storing the data to update may be damaged. For this reason, data is updated in unit of band  230 . 
     For example, while certain data (referred to as old data) is written to a band  230 , new data corresponding to the old data is sent. In this case, the new data is temporarily stored in a storage region, e.g., the media cache region  220 , different from the band in question. Upon satisfaction of a given condition, all the data in the band  230  is transferred to a different band  230 . In the transfer, the old data is replaced with the new data. This completes an update of the old data to the new data. 
     Each of the bands  230  includes a large number of tracks. As described above, the data update involves data transfer in unit of band  230 . The data update therefore takes a large amount of time. 
     In view of this, according to the first embodiment, the control circuit  20  controls the different actuators  105  to read data from a band  230  being a transfer source and write data to a band  230  being a transfer destination. The control circuit  20  controls a data read operation from a source band  230  and a data write operation to a destination band  230  concurrently, which leads to reduction in a length of data update time. 
     Concurrent data read and write operations refer to starting a data write operation before completion of a data read operation. That is, there is a period in which data is read and written concurrently. 
     In the following, data update operation is referred to as update operation. Data in unit of band  230  is referred to as band data. 
       FIG. 5  schematically illustrates an outline of the update operation according to the first embodiment. 
     For example, to update part (old data  310 ) of band data  300  to new data  320  in a band  230   a  of the magnetic disk  101   a  through an update operation, the control circuit  20 , e.g., the HDC  25  controls a read operation of the band data  300  from the band  230   a  to the buffer memory  24 . The HDC  25  uses the actuator  105   a  to read the band data  300 . The HDC  25  stores the read band data  300  in the buffer memory  24 . 
     The control circuit  20 , e.g., the MPU  26  selects a band  230  as a transfer destination of the band data  300 , from the bands  230  accessible by the actuator  105   b , that is, from the bands  230  in the magnetic disk  101   b . The band  230  thus selected in the magnetic disk  101   b  is referred to as a band  230   b.    
     The control circuit  20 , e.g., the HDC  25 , reflects the new data  320 , pre-stored in the buffer memory,  24 , on the band data  300  stored in the buffer memory  24 . Specifically, the control circuit  20  replaces the old data  310  of the band data  300  with the new data  320 . The control circuit  20  writes, to the band  230   b , the band data  300  including the new data  320  replacing the old data  310  as band data  300 ′. 
     The new data  320  is sent from the host  2  after the band data  300  is stored in the band  230   a , and is overwritten to the old data  310 . In other words, the new data  320  is equivalent to an update to the band data  300  stored in the band  230   a.    
       FIG. 6  schematically illustrates band data read timing and band data write timing in the update operation according to the first embodiment. 
     For example, at time t 0 , a read operation of band data  300  with the actuator  105   a  starts. A write operation of the band data  300  may start as long as the band data  300  is partially stored in the buffer memory  24 . Thus, a write operation of the band data  300  with the actuator  105   b  starts before completion of reading the band data  300 . In the example illustrated in  FIG. 6 , the write operation of the band data  300  starts immediately after the start of reading the band data  300  (time t 1 ). 
     In writing the band data  300 , new data  320  is appropriately reflected on the band data  300 . For example, the control circuit  20  transfers part of the band data  300 , excluding old data  310 , from the buffer memory  24  to the band  230   b . The control circuit  20  then transfers the new data  320  to the band  230   b  in place of the old data  310 . The control circuit  20  thus reflects the new data  320  on the band data  300 . 
     After the start of a write operation of the band data  300 , the read operation of the band data  300  terminates at time t 2 . At time t 3 , the write operation of the band data  300  terminates. The update operation thus ends. 
     In reading and writing the band data  300  using the same actuator  105 , the band data  300  are read and written serially. In this case, the update operation requires a length of time exceeding a sum of the time for reading the band data  300  and the time for writing the band data  300 . 
     In the example illustrated in  FIG. 6 , the band data  300  is read and written concurrently in the period from the time t 1  to the time t 2 . This results in reduction in the length of time for the update operation as compared with reading and writing the band data  300  serially. 
     Each of the bands  230  contains data of the plurality of tracks. The band data  300  is therefore considerably large in size. The capacity of the buffer memory  24  may be smaller than the size of the band data  300 , and the band data  300  may be read and written with the same actuator  105 . In such a case, the band data  300  is divided into regions of a size smaller than the capacity of the buffer memory  24 , and each division is repeatedly subjected to read and write operations. 
     In the first embodiment, a write operation of the band data  300  can start before the completion of a read operation of the band data  300 . Thus, in the case of the buffer memory  24  with a smaller capacity, the update operation can be performed without the band data  300  divided. 
     Next, a description will be given of the operation of the magnetic disk device  1  according to the first embodiment. 
       FIG. 7  is a flowchart illustrating an exemplary operation of the magnetic disk device  1  according to the first embodiment in response to reception of data. 
     First, data is received from the host  2  and stored in the buffer memory  24 . When the buffer memory  24  stores the data received from the host  2  (Yes in S 101 ), the control circuit  20  controls a write operation of the data to one of the media cache regions  220  (S 102 ). 
     Any media cache region  220  is appropriately selected as a write destination. The control circuit  20  can select, as a write destination, one of the media cache regions  220  in the recording surfaces  200  on the front and rear surfaces of the magnetic disk  101   a  or the magnetic disk  101   b.    
     When the buffer memory  24  stores no data received from the host  2  (No in S 101 ) or after S 102 , the control circuit  20  determines whether a given update condition is satisfied (S 103 ). 
     The update condition may be set to any condition. For example, the update condition may be such that the amount of written data in the media cache region  220  reaches a given amount. Alternatively, the update condition may be such that no receipt of commands from the host  2  continues for a given period or more. 
     After satisfaction of the update condition (Yes in S 103 ), the control circuit  20  performs the update operation (S 104 ). Upon no satisfaction of the update condition (No in S 103 ) or after S 104 , S 101  is carried out again. 
       FIG. 8  is a flowchart of the update operation according to the first embodiment. 
     First, the control circuit  20  selects a band  230  as a transfer source (S 201 ). The band  230  selected in S 201  is referred to as a first band. 
     Next, the control circuit  20  specifies an actuator  105  for use in accessing the first band (S 202 ). For example, when the first band is of the recording surfaces  200  of the magnetic disk  101   a , the control circuit  20  determines the actuator  105   a  as an actuator  105  for use in accessing the first band. When the first band is of the recording surfaces  200  of the magnetic disk  101   b , the control circuit  20  determines the actuator  105   b  as an actuator  105  for use in accessing the first band. The actuator  105  specified in S 202  is referred to as a first actuator. Each magnetic head  102  to be moved by the first actuator is referred to as a first magnetic head. 
     Next, the control circuit  20  selects a band  230  as a transfer destination from free bands  230  accessible by an actuator different from the first actuator (S 203 ). The free bands  230  refer to bands  230  in which band data is storable. For example, the free bands  230  are bands  230  storing no written data or from which band data has been deleted. 
     The actuator different from the first actuator and selected in S 203  is referred to as a second actuator. Each magnetic head  102  to be moved by the second actuator is referred to as a second magnetic head. The destination band  230  selected in S 203  is referred to as a second band. 
     Next, the control circuit  20  controls a read operation of data, equivalent to an update to the band data stored in the first band, from the media cache region  220  to the buffer memory  24  (S 204 ). 
     The data is received from the host  2  and stored in the media cache region  220  in S 102  of  FIG. 7 . In S 204 , the control circuit  20  specifies, from the data stored in the media cache region  220 , the data equivalent to the update to the band data stored in the first band. 
     For example, all the items of data received from the host  2  are correlated with logical addresses. The logical address refers to information indicating a location in a logical address space to be provided from the magnetic disk device  1  to the host  2 . The logical addresses are correlated with data in units of sector. The data in units of sector is referred to as sector data. 
     The control circuit  20  stores therein a correspondence between data and a logical address for each sector data stored in the magnetic disks  101 . When the media cache region  220  stores sector data correlated with the same logical address as that of sector data in the first band, the control circuit  20  regards the sector data stored in the media cache region  220 , as the update to the band data stored in the first band. The control circuit  20  retrieves the sector data correlated with the same logical address as that of the sector data stored in the first band, thereby specifying the update to the band data stored in the first band. 
     The foregoing specifying method is merely illustrative. The control circuit  20  can specify the update to the band data stored in the first band by any method. For example, in storing, in the media cache region  220  in S 102  of  FIG. 7 , the sector data correlated with the same logical address as that of the sector data written to any of the bands  230 , the control circuit  20  may record this fact as management information, and specify the update to the band data stored in the first band on the basis of the management information in S 204 . 
     Subsequent to S 204 , the control circuit  20  starts controlling a read operation of the band data from the first band to the buffer memory  24  (S 205 ). The control circuit  20  allows the first actuator and the first magnetic head to read the band data from the first band. 
     Next, the control circuit  20  starts controlling a reflection of the updated part on the band data  300  stored in the buffer memory  24  and a write operation of the band data, on which the updated part has been reflected, into the second band (S 206 ). The control circuit  20  allows the second actuator and the second magnetic head to write the band data, on which the updated part has been reflected, into the second band. 
     After completion of the read operation of the band data, the reflection of the updated part, and the write operation of the band data on which the updated part has been reflected (S 207 ), the control circuit  20  controls deletion of the band data  300  from the first band  230  (S 208 ). The update operation thus ends. 
     The foregoing embodiment has described the example that the pair of magnetic heads  102   a  and the pair of magnetic heads  102   b  are independently moved by the different actuators  105 . However, the number of independently movable magnetic heads  102  is not limited to two. The magnetic disk device  1  may include three or more magnetic heads  102  and actuators  105  for the respective magnetic heads, and the magnetic heads  102  are movable independently of one another. For example, the control circuit  20  may use any two of the three or more magnetic heads  102  to implement the operation described above. 
     The foregoing embodiment has described the magnetic disk device  1  including the arm  104   a  and the arm  104   b  with the common rotation shaft, by way of example. In order to enable the different actuators  105  to concurrently access the same recording surface  200 , the magnetic disk device  1  may include the arm  104   a  with a rotation shaft  106   a  and the arm  104   b  with a rotation shaft  106   b  as illustrated in, for example,  FIG. 9 . In this case, a destination band  230  and a source band  230  are selectable from the same recording surface  200 . 
     The foregoing embodiment has described the example that the control circuit  20  controls a read operation of the updated part of the band data from the media cache region  220  to the buffer memory  24  before starting a read operation of the band data. However, the updated-part read timing is not limited to such an example. For example, the control circuit  20  may interrupt a read operation or a write operation of the band data, and resume the interrupted read or write operation after reading the updated part from the media cache region  220  to the buffer memory  24 . Which one of the read and write operations of the band data is to be interrupted is determined depending on the actuator  105  used in reading the updated part. When the recording surface  200 , accessible by the actuator  105  for use in reading the band data, stores the updated part, the control circuit  20  interrupts a read operation of the band data. When the recording surface  200 , accessible by the actuator  105  for use in writing the band data, stores the updated part, the control circuit  20  interrupts a write operation of the band data. 
     The data received from the host  2  is not necessarily written to the media cache region  220 . The control circuit  20  may hold the data received from the host  2  in the buffer memory  24 , thereby omitting reading the updated part from the media cache region  220  to the buffer memory  24  in the update operation. 
     The foregoing embodiment has described the example of writing data by SMR. The first embodiment is applicable to a magnetic disk device that writes data by conventional magnetic recording (CMR). 
     For example, stored data may be transferred from a magnetic disk to another region for some reason, irrespective of SMR or CMR writing. To transfer data, as with the data update operation described above, different actuators serve to read data from the current region to the buffer memory and write data from the buffer memory to another region concurrently. This makes it possible to reduce the data transfer time. 
     According to the first embodiment, thus, a control circuit (e.g., the control circuit  20 ) concurrently controls a read operation of first data from a certain region to a buffer memory (e.g., the buffer memory  24 ), and a write operation of second data corresponding to the first data from the buffer memory to another region. The second data corresponding to the first data may be equal to the first data or may be first data on which an updated part has been reflected, such as the band data  300 ′. 
     The control circuit  20  may not constantly control the different actuators  105  for use in reading band data and writing band data. The control circuit  20  may determine whether to perform such control in accordance with, for example, a command from the host  2 . 
     The foregoing embodiment has not specifically described a writing method of data to a media cache region  220 . The writing method to a media cache region  220  is not limited to a specific method. For example, data is written into a media cache region  220  by CMR. 
     As described above, according to the first embodiment, the control circuit  20  controls a write operation of band data from the buffer memory  24  to a destination band  230  concurrently with a read operation of band data from a source band  230  to the buffer memory  24 . 
     This can reduce a length of time taken for the update operation. In other words, the performance of the magnetic disk device  1  can be improved. 
     In addition, the control circuit  20  controls storing of an updated part of band data in the buffer memory  24 , a reflection of the updated part on the band data stored in the buffer memory  24 , and a write operation of the band data, on which the updated part has been reflected, to a destination band  230 . 
     This enables reduction in the length of time for the update operation in the magnetic disk device  1  that adopts the SMR. 
     The control circuit  20  controls a write operation of data received from the host  2  to a media cache region  220 . In the update operation, the control circuit  20  controls a read operation of data equivalent to an updated part of band data, of the data written to the media cache region  220 , from the media cache region  220  to the buffer memory  24 . 
     This enables reduction in the length of time for the update operation in the magnetic disk device  1  that adopts the SMR. 
     Second Embodiment 
     The first embodiment describes reading band data from a source band  230  to the buffer memory  24  and writing band data from the buffer memory  24  to a destination band  230  concurrently, by way of example. 
     The magnetic disk device  1  may include three or more actuators  105  that are operable independently of one another. In such a case, the control circuit  20  may be configured to concurrently control a read operation of an updated part from a media cache region  220  to the buffer memory  24 , a read operation of band data from a source band  230  to the buffer memory  24 , and a write operation of the band data from the buffer memory  24  to a destination band  230 . 
       FIG. 10  schematically illustrates an outline of an update operation according to a second embodiment. 
     A magnetic disk device  1  includes a magnetic disk  101   c  in addition to magnetic disks  101   a  and  101   b . The magnetic disk device  1  also includes an arm  104   c  in addition to arms  104   a  and  104   b . The arm  104   c  is driven by an actuator  105   c  different from actuators  105   a  and  105   b . A pair of magnetic heads  102   c  is attached to a distal end of the arm  104   c , opposing the recording surfaces  200  of the magnetic disk  101   c . A control circuit  20  drives the actuator  105   c  to move the magnetic heads  102   c . Specifically, the magnetic disk device  1  can allow the actuator  105   a , the actuator  105   b , and the actuator  105   c  to concurrently access the magnetic disk  101   a , the magnetic disk  101   b , and the magnetic disk  101   c , respectively. 
     The control circuit  20  controls a write operation of band data and updating of the band data to the recording surfaces  200  accessible by the different actuators  105 . In the update operation, thus, the different actuators  105  can serve to concurrently read the band data and the update. 
     In addition, the control circuit  20  selects a band  230  to be a transfer destination of band data from the bands  230  accessible by an actuator  105  different from an actuator  105  for use in accessing a band  230  being a transfer source of the band data and an actuator  105  for use in reading an updated part of the band data. Thus, the different actuators  105  can serve to read the band data and the updated part, and write the band data concurrently. 
     In the example illustrated in  FIG. 10 , a media cache region  220  (referred to as a media cache region  220   a ) of the magnetic disk  101   c  stores new data  320  corresponding to an update to the band data  300  stored in a band  230   a . In other words, the actuator  105   a  is used for reading the band data  300 , and the actuator  105   c  and the magnetic heads  102   c  are used for reading the updated part. 
     The control circuit  20  selects, as a transfer destination of the band data  300 , a band  230   b  being the band  230  accessible by the other actuator  105 , i.e., the actuator  105   b.    
       FIG. 11  schematically illustrates each operation timing in the update operation according to the second embodiment. 
     For example, at time t 10 , a read operation of band data  300  with the actuator  105   a  starts. Next, a write operation of the band data  300  starts immediately after the start of reading the band data  300  (time t 11 ). 
     In writing the band data  300 , new data  320  is appropriately reflected on the band data  300 . The new data  320  is read at any timing before the new data  320  is written. In the example illustrated in  FIG. 11 , the new data  320  is read with the actuator  105   c  after time t 12 . The actuator  105   a  and the actuator  105   c  operate independently of each other. Thus, the new data  320  may be read at or before the time t 10 . 
     In the example illustrated in  FIG. 11 , the band data  300  is read and written concurrently even after the new data  320  is read. At time t 13 , the read operation of the band data  300  ends. At time t 14 , the write operation of the band data  300  ends. The update operation thus ends. 
     As illustrated in  FIG. 11 , at time t 12 , the new data  320  (i.e., the updated part) is read from the media cache region  220  to the buffer memory  24 , the band data  300  is read from the source band  230  to the buffer memory  24 , and the band data  300  is written from the buffer memory  24  to the destination band  230  concurrently. 
     According to the second embodiment, the updated part can be read with no interruption of the read and write operations of the band data  300 , which can further reduce the length of time for the update operation. 
     Third Embodiment 
     In the first embodiment, among the constituent elements of the control circuit  20 , the preamplifiers  21  and the RDCs  22  are multiplexed. However, constituent elements to be multiplexed are not limited to the preamplifiers  21  and the RDCs  22 . 
       FIG. 12  schematically illustrates a configuration of a control circuit  20  according to a third embodiment. The control circuit  20  includes preamplifiers  21   a  and  21   b , a DSP  23 , a buffer memory  24 , and a memory  27 . The control circuit  20  also includes two systems-on-a-chip (SoCs), that is, an SoC  28   a  and an SoC  28   b.    
     The SoC  28   a  and the SoC  28   b  have the same hardware configuration. Specifically, the SoC  28   a  includes an HDC  25   a , an RDC  22   a , and an MPU  26   a . The SoC  28   b  includes an HDC  25   b , an RDC  22   b , and an MPU  26   b.    
     Depending on mode settings, the SoC  28   a  functions as a host device, and the SoC  28   b  functions as a slave device to the SoC  28   a.    
     Specifically, the SoC  28   a  causes the SoC  28   b  to perform the access control over the actuator  105   b , among the functions of the HDC  25  and MPU  26  of the control circuit  20  according to the first embodiment. The SoC  28   a  performs access control over an actuator  105   a , exchange of information with a host  2 , and control of a spindle motor, for example. 
     In the case of magnetic disk device  1  including three or more independently operable actuators  105 , the number of SoCs  28  may be three or more. 
     As described above, of the constituent elements of the control circuit  20 , any constituent elements may be appropriately multiplexed. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.