Patent Publication Number: US-6907499-B2

Title: Interrupting disc write operations to service read commands

Description:
RELATED APPLICATIONS 
     This application claims priority of U.S. provisional application Ser. No. 60/353,699, filed Jan. 31, 2002. 
    
    
     FIELD OF THE INVENTION 
     This application relates generally to systems and methods for improving data storage device performance and more particularly to systems and methods for improving data storage device performance by interrupting disc write operations to service read commands. 
     BACKGROUND OF THE INVENTION 
     Modern hard disc drives comprise one or more rigid discs that are coated with a magnetizable medium and mounted on a hub of a spindle motor for rotation at a constant high speed about a rotational axis. Information is stored on one or more surfaces of the disc or discs in a plurality of concentric circular tracks. Data is written to, and/or read from, sectors on the tracks via transducers (“heads”) mounted to a radial actuator that positions the heads relative to the discs. The read and write elements are typically positioned over specific sectors of the disc in accordance with read and write commands, respectively, received from a host connected to the disc drive. These commands, as well as their associated data, are transferred between the disc drive and the host via a communication interface, such as, for example, various versions of the Advanced Technology Attachment (ATA) interface or the Small Computer Systems Interface (SCSI) interface. 
     One of the primary performance characteristics of a disc drive is overall data transfer rate of the drive. The overall data transfer rate of the disc drive generally includes two main components, the external data transfer rate and the internal data transfer rate. The external data transfer rate, sometimes referred to as the interface, host, or burst transfer rate, relates to the speed at which data can be transferred between the disc drive and the host. The internal data transfer rate, sometimes referred to as the sustained transfer rate, relates to the speed at which a disc drive can read and write data to and from the tracks, once that data has been received or requested from the host. The internal data transfer rate typically includes the time required for the disc drive to process the read or write command, as well as the time required to move the transducer to the appropriate track to read or write the data. 
     While the internal and external data transfer rates of a disc drive are defined separately, they are often interrelated. For example, in accordance with many communication interface specifications, after sending a command to a disc drive, the host must wait for the disc drive to respond that the command has been serviced before the host may issue another command. That is, the host must wait for the data it requested until it can request more data. As such, the external transfer rate of the disc drive is limited, to some extent, by the time required by the disc drive to service commands. As will be appreciated, the time required by the disc drive to service the commands is primarily related to the internal data transfer rate. 
     A common way to increase both the internal and external data transfer rates involves providing some sort of disc caching mechanism in the disc drive to temporarily store data that is being written to or retrieved from the disc drive. Disc caching typically involves using a fast solid state memory in the disc drive as a buffer for data transferred between the disc drive and the host. 
     In accordance with one embodiment, a method for managing read commands and disc write operations in a disc drive is contemplated. In accordance with this embodiment, if the disc drive receives a read command from the host (or other some other computer process external to the disc drive) while the disc drive is conducting a disc write operation, then the disc write operation is discontinued to service the read command. By discontinuing the disc write operation to service the read command, the internal and/or external data transfer rates of the disc drive may be improved. 
     In disc drives employing write caching, when a host sends a write command to the disc drive, the data is immediately stored in the write cache for later writing to the disc. The disc drive then immediately informs the host that the data has been written to the disc. By informing the host immediately that the data has been written, rather than waiting until the data is actually written to the disc, the host is freed to prepare and send other read or write commands and/or to perform other data transfer functions, thus speeding up the external transfer rate of the disc drive. 
     In addition to speeding up the external transfer rate, the write cache may also be used to speed up the internal transfer rate of the disc drive. For example, in a process known as write coalescing, data received at the disc drive, in accordance with various write command from a host, is first stored in the write cache and then examined to locate blocks of data that are to be written to the disc in contiguous data blocks. That is, contiguous data blocks are blocks of data that have contiguous logical block addresses or blocks of data that can be written to the disc in a single disc write operation. Once located, these contiguous data blocks are then combined or coalesced and written to the disc in a single disc write operation. By writing this data in a single disc write operation, rather than a number of individual write operations, the internal data transfer rate of the disc drive may be improved. 
     Additionally, a write cache may also be used as a temporary buffer for future read operations. For example, when a command is received from the host to read a particular block of data, the write cache may first be searched to determine if the requested data is still present in the write cache. If the data is still present, the data can be then be sent back to the disc drive directly from the write cache, without requiring the disc drive to spend additional time accessing the disc for the data. 
     In disc drives employing read caching, data that is read from the discs of the disc drive in response to a read command from the host, is stored in the read cache before it is sent off to the host. Additionally, read caching may also involve storing “extra” data, sometimes referred to as look-ahead data, that was not requested by the host in the read command, but which is located on the same track as the requested data on the disc drive. This is done because hosts tend to request data from the disc drive in a sequential or predictably ordered manner. As such, when the host sends the next read command, it is likely that at least part of the data requested will have been read into the cache in the look-ahead operation. Again, since data retrieval from the read cache is typically several thousand times faster than data retrieval from the disc, data transfer rates can be greatly increased by using the read caching. 
     As described, in a typical disc drive employing data caching, when a read command arrives at the disc drive, the disc drive first checks to see whether the requested data is in the read and/or write cache. If the read and/or write cache contains the data, the requested data is sent immediately to the host from the cache. If, however, the cache does not contain the requested data, the drive must retrieve the requested data from the discs. If the disc drive is currently busy, such as performing a write operation, the disc drive, and thus the host, must wait until the write operation is complete before retrieving the requested data from the disc. In this situation, disc caching does nothing to speed up the retrieval of the requested information from the disc (internal data transfer rate) or the transmission of the data back to the host (external transfer rate). As such, both the internal and external data transfer rates of the disc drive will suffer. This situation is made worse when a number of write commands have been combined or coalesced into one long disc write operation, thus increasing the time for the disc write operation to complete. 
     As such, there is a need in the art for systems and/or methods that allow for the fast servicing of read-commands, even in situations where a lengthy disc write operation is being carried out in the disc drive. It is against this backdrop that the present invention has been developed. 
     SUMMARY OF THE INVENTION 
     In general, the various embodiments of the present invention described herein relate to systems and methods for processing read and write commands in a disc drive. More particularly, the various embodiments described herein relate to systems and methods for discontinuing a disc write operation in a disc drive to service a read command from a host, so as to improves the internal and external data transfer rates of the disc drive. 
     In accordance with one embodiment, a method for managing read commands and disc write operations in a disc drive is contemplated. In accordance with this embodiment, if the disc drive receives a read command from the host (or other some other computer process external to the disc drive) while the disc drive is conducting a disc write operation, then the disc write operation is discontinued to service the read command. By discontinuing the disc write operation to service the read command, the internal and/or external data transfer rates of the disc drive may be improved. 
     In accordance with another embodiment, data is written from a write cache to a disc in the disc drive in accordance with a disc write operation. If, during the disc write operation, a read command is received by the disc drive, a determination is made as to whether it is appropriate or desired to interrupt the disc write operation to service the read command. If it is determined that it is appropriate or desired to interrupt the disc write operation, the disc write operation is interrupted, and the read command is serviced. Additionally, the remaining unwritten part of the data that was to be written to the disc during the disc write operation may be reestablished in the write cache after the disc write operation has been interrupted. 
     These and various other features as well as advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of a disc drive incorporating an embodiment of the present invention. 
         FIG. 2  is a functional block diagram generally showing the main functional components used to control the disc drive of FIG.  1 . 
         FIG. 3  illustrates an operation flow including various operations for handling commands received by the disc drive shown in  FIG. 1 , in accordance with an embodiment of the present invention. 
         FIG. 4  illustrates an operational flow of a handle write in progress operation of  FIG. 3 , in accordance with an embodiment of the present invention. 
         FIG. 5  illustrates an operational flow of the handle write in progress operation of  FIG. 3 , in accordance with another embodiment of the present invention. 
         FIG. 6  illustrates an operational flow of the handle write in progress operation of  FIG. 3 , in accordance with yet another embodiment of the present invention. 
         FIG. 7  illustrates an operational flow of the handle write in progress operation of  FIG. 3 , in accordance with another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In general, the various embodiments of the present invention described herein relate to systems and methods for processing read and write commands in a disc drive. More particularly, the various embodiments described herein relate to systems and methods for discontinuing a disc write operation in a disc drive to service a read command from a host, so as to improve the internal and external data transfer rates of the disc drive. 
     An exemplary disc drive  100  in which embodiments of the present invention may be incorporated is shown in FIG.  1 . The disc drive  100  includes a base  102  to which various components of the disc drive  100  are mounted. A top cover  104 , shown partially cut away, cooperates with the base  102  to form an internal, sealed environment for the disc drive in a conventional manner. The components include a spindle motor  106  that rotates one or more magnetic discs  108  at a constant high speed. Information is written to and read from nominally circular, concentric tracks  109  on the discs  108  through the use of an actuator assembly  110 , which rotates during a seek operation about a bearing shaft assembly  112  positioned adjacent the discs  108 . The actuator assembly  110  includes a plurality of actuator arms  114  that extend towards the discs  108 . One or more flexures  116  extending from each of the actuator arms  114 . Mounted at the distal end of each of the flexures  116  is a head  118  that includes an air bearing slider enabling the head  118  to fly in close proximity above the corresponding surface of the associated disc  108 . 
     As shown in  FIG. 1 , the disc drive  100  includes a voice coil motor (VCM)  124 , which typically includes a coil  126  attached to the actuator assembly  110 , as well as one or more permanent magnets  128  that establish a magnetic field in which the coil  126  is immersed. The controlled application of current to the coil  126  causes magnetic interaction between the permanent magnets  128  and the coil  126 , so that the coil  126  moves in accordance with the well known Lorentz relationship. As the coil  126  moves, the actuator assembly  110  pivots about the bearing shaft assembly  112 , and the heads  118  are caused to move across the surfaces of the discs  108 . 
     A flex assembly  130  provides the requisite electrical connection paths for the actuator assembly  110  while allowing pivotal movement of the actuator assembly  110  during operation. The flex assembly includes a printed circuit board  132  to which head wires (not shown) are connected; the head wires being routed along the actuator arms  114  and the flexures  116  to the heads  118 . The printed circuit board  132  typically includes circuitry for controlling the write currents applied to the heads  118  during a write operation and a preamplifier for amplifying read signals generated by the heads  118  during a read operation. The flex assembly terminates at a flex bracket  134  for communication through the base deck  102  to a disc drive printed circuit board (not shown) mounted to the bottom side of the disc drive  100 . 
     Referring now to  FIG. 2 , shown therein is a generalized functional block diagram of the disc drive  100  of  FIG. 1 , illustrating some of the functional components of the disc drive  100 . Some of these components may be resident on a disc drive PCB (not shown) and may be used to control the operation of the disc drive  100 . As shown in  FIG. 2 , the disc drive includes an interface application specific integrated circuit (interface)  202 . In operation, the interface  202  receives commands, such as read and write commands, from a host  200 . The host may be any computer, computer network, or computer process that is operable to send commands to the disc drive. The interface  202  includes an associated buffer or cache, such as a write cache  210  and/or a read cache  211 , to facilitate high speed data transfer from a host  200 . 
     Data to be written to the disc  108  of the disc drive  100  are passed via one or more data paths  206  to the interface  202  and then into the write cache  210 . When deemed appropriate by the disc drive, the data are passed, either alone or in combination with other data stored in the write cache  210 , to a read/write channel  212 , which encodes and/or serializes the data and provides the requisite write current signals to the write elements of the head  118 , to cause the data to be written to appropriate data sectors on the disc(s)  108  of the disc drive  100 . After data has been written to the disc  108 , it may then be read back by passing the read element of the head  118  over the data to produce a corresponding read current, which is then decoded and/or deserialized by the read/write channel  212  and sent to the interface  202  for transmission to the host  200 . Additionally, before being sent, the data may be held in the read cache  211 . 
     As also shown in  FIG. 2 , a microprocessor  216  is operably connected by electrical path  220  to the interface  202 . The microprocessor  216  provides top level communication and control for the disc drive  100  in conjunction with programming for the microprocessor  216  that may be stored in a non-volatile microprocessor memory (MEM)  224 . The MEM  224  can include random access memory (RAM), read only memory (ROM) and other sources of resident memory for the microprocessor  216 . Additionally, the microprocessor  216  provides control signals for spindle control  226 , and servo control  228 . 
     Each track  109  of the disc drive  100  shown in  FIG. 1  preferably includes a number of servo sectors that are periodically interspersed with data sector along the track  109 . As is typical, the data sectors are used to store user data and the servo sectors are used to store prerecorded servo information that is employed by a disc drive servo system to control the position of the read/write heads over the disc(s)  108 . 
     Data to be written to or stored in the data sectors on the discs of the disc drive are typically received in conjunction with a write command from the host  200 . As used herein, the term “write command” refers to a command that is received by the disc drive  100  requesting that data associated with the write command be written to data sectors on a disc  108  of the disc drive  100 . The data associated with the write command is preferably stored in the write cache  210  of the disc drive for later writing to disc(s) of the disc drive. At some later point, this data is then written to data sectors on the disc(s) of the disc drive in accordance with a disc write operation. As used herein the term “disc write operation” refers to the operation of actually writing blocks of data that have been stored in the write cache  210  to data sectors on a disc  108  of the disc drive  100 . The data that is written to the data sectors in accordance with a disc write operation may be data that was transmitted to the disc drive along with a single write command from the host. Alternatively, the data that is written to the data sectors in accordance with a disc write operation may be data that was transmitted to the disc drive in accordance with other write commands from the host. 
     To retrieve data from the disc drive  100 , the host may issue a read command to the disc drive requesting specified data. As used herein, the term “read command” refers to a command received by the disc drive  100  requesting that specified data stored in the disc drive  100  is read and transmitted to the disc drive. The data that is requested in the read command is then located in the disc drive and transmitted to the host  200 . In some cases, the data will be located in a cache on the disc drive, such as read cache  211  and/or write cache  210 . In such cases, the data is transmitted directly from the cache to the host, without requiring the data to be read from data sectors. In other cases, the data must first be read from the data sectors on the disc(s) of the disc drive, and then sent to the host  200 . Additionally, after the data is read from the data sectors, in some cases it may be temporarily stored in a read cache on the disc drive before being sent to the host  200 . 
       FIGS. 3 ,  4 ,  5 ,  6  and  7  illustrate operational flows exemplifying various methods related to servicing commands in the disc drive  100  from a computer or computing process located remotely from the disc drive  100 . The operations shown in  FIGS. 3 ,  4 ,  5 ,  6  and  7  may be implemented (1) as a sequence of microprocessor implemented acts or program modules running on one or more microprocessors and/or (2) as interconnected machine logic circuits or circuit modules within the disc drive. For example, as will be described, various of the operations shown in  FIGS. 3 ,  4 ,  5 ,  6  and  7  may be performed by the microprocessor  216  and/or the interface  202 . Alternatively, the operations shown in  FIGS. 3 ,  4 ,  5 ,  6  and  7  may be implemented performed by other hardware, software or firmware in the disc drive  100 . The implementation of the operations shown in  FIGS. 3 ,  4 ,  5 ,  6  and  7  is a matter of choice dependent on performance requirements and/or hardware and software capabilities of the disc drive  100 . While some of the operations shown are preferably implemented as software stored on a computer readable medium, it will be recognized by one skilled in the art that these operations may be implemented in software, in firmware, in special purpose digital logic, or any combination thereof without deviating from the spirit and scope of the present invention, as recited in the claims attached hereto. As used herein, computer readable medium may be any available medium that can store or embody computer-readable instructions. 
     Turning now to  FIG. 3 , shown therein is an operation flow  300  including various operations for handling commands from a computer or computing process located remotely from the disc drive  100 , such as the host  200 . At the start of the operational flow  300 , a new command operation  302  determines if a new command has been received from the host  200  by the disc drive  100 . The commands received from the host  200  may be write commands, read commands, or other commands, such as a status command, etc. If it is determined by the new command operation  302  that a new command has not been received from the host  200 , a cache determination operation  304  determines if the write cache  210  contains any uncommitted data waiting to be written to the disc(s)  108 . That is, the cache determination operation  304  determines if the write cache  210  contains any unwritten data not associated with a disc write operation that is currently writing data to the disc, but which is to be written to the disc  108 . 
     If the cache determination operation  304  determines that the write cache  210  does not contain uncommitted data, the operational flow  300  ends. However, if the cache determination operation  304  determines that the write cache  210  does contains uncommitted data waiting to be written to the disc(s)  108 , a disc write operation  306  writes all, or a part of, the uncommitted data to the disc. For example, if uncommitted data in the cache are associated with a number of disc write operations, the data associated with one of the disc write operations will be written to the disc in a disc write operation, and the operational flow  300  will end. 
     If it is determined in the new command operation  302  that a new command has been received from the host  200 , a write command determination operation  310  then determines if the command received from the host  200  is a write command. If the write command determination operation  310  determines that the command received from the host  200  is a write command, the data associated with the write command is placed in the write cache  210  by caching operation  312 , and the operational flow  300  ends. If, however, the write command determination operation  310  determines that the command received from the host  200  is not a write command, a read command determination operation  314  determines if the command received from the host  200  is a read command. 
     If the read command determination operation  314  determines that the command received from the host is not a read command, a command handling operation  316  handles the command, and the operational flow  300  ends. As will be appreciated, commands handled by the command handling operation  316  are not read commands or a write commands, but rather are other type of commands received by the disc drive from the host  200 , such as a status command, etc. As such, the command handling operation  316  handles the command in accordance with the appropriate operational procedures of the disc drive  100  for handling the specific type of command received. 
     If the read command determination operation  314  determines that the command received from the host  200  is a read command, a disc write determination operation  318  determines if a disc write operation is in progress. That is, the disc write determination operation  318  determines if a disc write operation is currently writing data to a disc of the disc drive  100 . As described above, the data written to the disc in accordance with the disc write operation may be stored in the write cache. If the disc write determination operation  318  determines that a disc write operation is not in progress, a seek operation  320  performs a seek to the first data sector of the data requested in the read command. A disc read operation  322  then reads the data requested by the read operation  322  from the appropriate data sector(s). The read data is then processed by the disc drive  100  and sent to the host  200 , and the operational flow  300  ends. 
     If the disc write determination operation  318  determines that a disc write operation is in progress, a handle write in progress operation  324  is performed. Various embodiments of the handle write in progress operation  324 , and the operations that may take place therein, are described in more detail below with respect to  FIGS. 4 ,  5 ,  6 , and  7 . However, in general, the handle write in progress operation  324  determines if it is appropriate or desired to interrupt or abort the disc write operation in progress to service the read command. Following the handle write in progress operation  324 , the operational flow  300  continues to seek operation  320 , and then to the read operation  322 , as previously described. 
     As will be noted,  FIG. 3  includes a dashed line  326  extending from the end of the operational flow  300  to the start of the operation flow  300 . This dashed lines is intended to indicate that the entire operational flow  300  comprises an operational loop that may be performed repeatedly in the disc drive  100 . As will be appreciated by those skilled in the art, this operational loop will preferably be performed repeatedly while the disc drive  100  is in its normal operational state, such as when commands are being received from the host  200 . 
     Turning now more particularly to the handle write in progress operation  324 , as will be described, various embodiments of the present invention relate particularly to the operations that take place therein. For example, and without limitation,  FIGS. 4 ,  5 ,  6  and  7  each illustrate operations that take place in accordance with embodiments of the handle write in progress operation  324 . In general, the handle write in progress operation  324  determines if it is appropriate or desired to discontinue a disc write operation that is in progress to service a read command from the host  200 . In this context, discontinuing the disc write operation comprises either aborting or interrupting the disc write operation. 
     A disc write operation is defined as being “in progress,” if the disc write operation is currently writing data to data sectors on a disc(s) of the disc drive. Aborting the disc write operation is defined as ending the disc write operation before all data associated with the disc write operation has been written to data sectors of the disc drive and reestablishing all of the data associated with disc write operation in the write cache  210 . In contrast to aborting the disc write operation, interrupting the disc write operation is defined as ending the disc write operation before all data associated with the disc write operation has been written to data sectors of the disc drive and reestablishing only a portion of the data associated with the disc write operation in the write cache  210 . Preferably, the portion of data associated with the disc write operation that is reestablished in the write cache when the disc write operation is interrupted will be that portion of the data that was not written to data sectors in the disc(s) as a result of the interruption of the disc write operation, that is, the left over data. 
     As will be appreciated by those skilled in the art, reestablishing data in a cache, such as the write cache  210 , typically involves reestablishing in a cache access table, or the like, that the data being reestablished is to remain in the cache. However, in general, and as used herein, reestablishing data in the cache refers to causing the data to be recognized by the disc drive as being contained in the cache, regardless of the mechanism used to achieve this recognition. 
     As previously noted,  FIGS. 4 ,  5 ,  6  and  7  illustrate operational flows  400 ,  500 ,  600 , and  700 , respectively, of various embodiments of the handle write in progress operation  324  of the operational flow  300 , shown in FIG.  3 . It should be noted that while the handle write in progress operation  324  has been, and will be, described with respect to the command handling operations illustrated in operational flow  300 , the various embodiments of the handle write in progress operation  324  illustrated in  FIGS. 4 ,  5 ,  6  and  7  may alternatively be performed in association with other command handling operations. That is, the various embodiments of the handle write in progress operation  324  illustrated in  FIGS. 4 ,  5 ,  6  and  7  may be performed in association with any other command handling operations that initiate or call the handle write in progress operation  324  when it has been determined by the disc drive  100  that a read operation has been received from the host  200  and that a disc write operation is in progress in the disc drive  100 . 
     Turning now to  FIG. 4 , illustrated therein is an operational flow  400  of one embodiment of the handle write in progress operation  324 . As previously described, the handle write in progress operation  324 , and thus the operational flow  400 , occurs after it has been determined that a read command has been received from the host, and that a disc write command is in progress. 
     As shown, at the beginning of the operational flow  400 , a determination operation  402  determines if the disc write operation should be, or is to be, aborted. This determination may be made in a number of ways by the determination operation  402 , as is described in detail below. If the determination operation  402  determines that the disc write operation in progress is not to be aborted, a wait operation  404  causes the operational flow  400  to be halted while the disc write operation is completed, and then the operational flow  400  ends. If, however, the determination operation  402  determines that the disc write operation in progress is to be aborted, the operational flow  400  continues to the abort operation  406 , which causes the write operation to be aborted. Next, a reestablish operation  408  reestablishes all of the data that was to be written in the aborted disc write operation back in the write cache  210 , and the operational flow  400  ends. 
     As mentioned, the determination operation  402  may determine whether the disc write operation should be aborted in a number of ways. For example, in a first embodiment the determination operation  402  is “hard wired” or programmed to always determine that the disc write operation in progress should be aborted. In this first embodiment, the operational flow  400  always proceeds to an abort operation  406  following the determination operation  402 . However, in other embodiments, the determination operation  402  determines whether the disc write operation in progress should be aborted, based on some predetermined criteria, as will now be described. 
     In a second embodiment of the determination operation  402 , the determination as to whether the disc write operation in progress should be aborted is made by checking a disc write operation abort indicator, such as a logical bit. In accordance with this second embodiment of the determination operation  402 , the disc write operation abort indicator may be set by the host  200 , by the user of the disc drive, or by the disc drive itself. For example, and without limitation, the disc write operation abort indicator may be set by the host  200  based on decisions or selections made by the host related to the need or desire to abort a disc write operation in progress in the disc drive. Alternatively, the disc write operation abort indicator may be set manually by the disc drive user, such as by setting a jumper or a switch on the disc drive, or by issuing the disc drive a command via a computer process executing outside of the disc drive. The disc write operation abort indicator may also be set by processes or operations in the disc drive itself. Regardless of how the disc write operation abort indicator is set, if the disc write operation abort indicator indicates that the disc write operation should be aborted, the operational flow  400  proceeds to the abort operation  406  following the determination operation  402 . 
     In accordance with a third embodiment of the determination operation  402 , the determination as to whether the disc write operation should be aborted is based on whether time will be saved in servicing the read command from the host  200  if the disc write operation is aborted. With respect to determining time periods in the disc drive, as is known in the art, many disc drives include mechanisms or algorithms that are operable to determine the time required to move the transducer  118  from one location to another location, or to access a given location on the disc from another location on the disc. This time may include rotational as well as seek time. Additionally, this time may be measured and/or specified as a unit of time, such as milliseconds or clock pluses, or it may be measured and/or specified in units related to the rotation of the disc(s) in the disc drive, such as the number of servo bursts, data sectors, or servo and data sectors encountered by the transducer  118  as the disc rotates and/or as the transducer is radially moved from one location on the disc to access another location on the disc. 
     The determination of whether time will be saved in servicing the read command from the host  200  in this third embodiment of the determination operation  402  may be made, for example, by comparing the overall time required to complete the disc write operation and to access a specified sector associated with the data requested in the read command, with another specified time period. The specified sector associated with the data requested may be a data sector associated with the read command or a servo sector associated with the read command. For example, and without limitation, the specified sector may be the first data sector that is to be read in a sequence of data sectors specified by the read command. Alternatively, and with out limitation, the specified sector may be a servo sector that will be read while or before accessing the data specified in the read command, such as the first servo sector preceding the first data sector that is to be read in a sequence of data sectors specified by the read command. As will be appreciated by those skilled in the art, the overall time required to complete the disc write operation and to access the specified sector may include rotational as well as seek time. 
     In this third embodiment, the specified time period may be a static predetermined time period, such as, the time required to complete some fraction of a rotation of the disc, or some other predetermined static time period. Alternatively, the predetermined time period may be a dynamically determined time period that is determined in the disc drive itself, or by a process outside of the disc drive, such as a process in the host  200 . For example, and without limitation, the predetermined time period may be the time required to position the transducer so as to read the first sector of the data requested in the read command, without completing the disc write operation. 
     Turning now to  FIG. 5 , illustrated therein is an operational flow  500  of another embodiment of the handle write in progress operation  324 . As previously described, the handle write in progress operation  324 , and thus the operational flow  500 , occurs after it has been determined that a read command has been received from the host, and that a disc write operation is in progress. As shown in  FIG. 5 , at the beginning of the operational flow  500 , an interrupt determination operation  502  determines if the disc write operation is to be interrupted. This determination may be made in a number of ways in accordance with this embodiment of the interrupt determination operation  502 . However, in general, the interrupt determination operation  502  in this embodiment will determine if some predetermined criteria is met. For example, in one embodiment, the interrupt determination operation  502  determines if time will be saved in servicing the read command if the disc write operation is interrupted. This determination may be made in any number of ways in the disc drive. 
     As previously described, many disc drives include mechanisms and/or algorithms that are operable to determine the time required to move the transducer  118  from one location to another location, or to access one location on the disc from another location on the disc. Using these mechanisms and/or algorithms, in one embodiment, the interrupt determination operation  502  determines or calculates the time (T 1 ) required to access a particular sector associated with the read command from a specified location, such as the data sector currently being written to by the disc write operation, or a servo sector in close proximity to the data sector currently being written to. T 1  may including the rotational as well as seek time required to access the first sector of the data requested. 
     The interrupt determination operation  502  also determines or calculates the time (T 2 ) required to complete the disc write operation in progress. The interrupt determination operation  502  also determines or calculates the time (T 3 ) required to seek from the current track, where the write operation is writing data, to the track containing the first sector of data requested in the read operation. The sum of T 2  and T 3  (T 2 +T 3 ) is then calculated as T 4  (T 4 =T 3 +T 2 ). If T 4  is less than T 1 , the disc write operation is not interrupted. Rather, a wait operation  510  causes the operational flow  500  to be halted while the disc write operation is completed. Following the wait operation  510 , the operational flow  500  ends. However, if T 4  is less than T 1 , the operational flow  500  proceeds to a determination operation  504 . In calculating T 4 , a constant (C) may be added to the sum T 4  to compensate or account for various process delays (T 1 +T 2 +C=T 4 ). The constant (C) may be predetermined, such as, for example, the time necessary to complete a fraction of a rotation of a disc in the disc drive. Alternatively, the constant (C) may be dynamically determined. 
     If it is determined in the interrupt determination operation  502  that the disc write operation is to be interrupted, an interruption point determination operation  504  then determines the precise point or location where the disc write operation is to be interrupted. In general, the interruption point determination operation  504  determines a point in the disc write operation from which a seek can be made to the data requested in the read command, without requiring an additional revolution of the disc to be made. Stated another way, the interruption point determination operation  504  will select a point to interrupt the disc write operation, such that a revolution will not be slipped in accessing the data requested in the read command. 
     In one embodiment, the point selected in the interruption point determination operation  504  is determined from the previously calculated values of T 1 , T 2 , and/or T 3 . For example, in one embodiment, a time T 5  will be calculated as follows, T 5 =T 1 −(T 3 +K), where K is a predetermined constant. That is, T 5  equals the time (T 1 ) required to position the transducer to access the data requested in the read command, including the rotational as well as seek time, minus the sum of the time (T 3 ) required to seek from the current track to the track containing the data requested in the read operation, and the constant K. A determination is then made as to the sector over which the transducer will be positioned at time T 5 . The determined sector is then designated as the interruption point of the disc write operation. As will be appreciated, the determined sector may be a specific data sector or servo sector, depending on disc drive positioning functionality. 
     Following the interruption point determination operation  504 , an interrupt operation  506  interrupts the disc write operation in progress at the interruption point determined in interrupt determination operation  502 . Next, a reestablish operation  508  reestablishes a portion of the data associated with the disc write operation in the cache, and the operational flow  500  ends. In one embodiment, the portion of the data that is reestablished in the cache is the data that was specified to be written in accordance with the disc write operation, but which was not written due to the interruption of the disc write operation. 
     Turning now to  FIG. 6 , illustrated therein is an operational flow  600  of another embodiment of the handle write in progress operation  324 . As previously described, the handle write in progress operation  324 , and thus the operational flow  600 , occurs after it has been determined that a read command has been received from the host, and that a disc write command is in progress. As shown in  FIG. 6 , at the beginning of the operational flow  600 , a calculate T 1 ′ operation  602  calculates the time (T 1 ′) required to access a first data sector associated with the data requested in the read operation while still completing the disc write operation. Alternatively, the calculate T 1 ′ operation  602  may calculate the time (T 1 ) required to access a servo sector immediately preceding the first data sector associated with the data requested in the read operation while still completing the disc write operation. As will be appreciated by those skilled in the art, there are a number of ways in which the time T 1 ′ may be calculated during the calculate T 1 ′ operation  602 . 
     By way of example, and not limitation, the time T 1 ′ may be generally calculated as the sum of the time required to write the remaining sectors of the disc write operation, the time required to seek from the current track to the track containing the data that is to be read, and the rotational time needed to access the first data sector (or a servo sector preceding the first data sector) once the seek has been completed. In other embodiments, T 1 ′ may be calculated in other ways known in the art. T 1 ′ may be specified as a unit of time. However T 1 ′ may also be specified in units more commonly used in disc drives, such as units related to the rotation of the disc(s) in the disc drive, as previously described. 
     Following the calculate T 1 ′ operation  602 , a calculate T 2 ′ operation  604  calculates the time required to reach the location of the first data sector that is being requested in the read operation from the sector currently being written by the disc write operation, or from a predetermined data sector occurring after the sector currently being written. Alternatively, the calculate T 2 ′ operation  602  may calculate the time (T 2 ′) required to access a servo sector immediately preceding the first data sector associated with the data requested in the read operation, or from a predetermined servo sector occurring after the sector currently being written. By way of example, and not limitation, the time T 2 ′ may be generally calculated by determining the seek and rotational time necessary to access the first sector of the data that is to be read in accordance with the read operation from the host from the current location of the transducer. As with T 1 ′, T 2 ′ may also be specified as a unit of time or as a unit associated with the rotation of the discs in the disc drive, as described above. However, the units of measure used for T 1 ′ and T 2 ′ should be identical. Furthermore, in the case where T 1 ′ is calculated relative to a servo sector rather than a data sector, T 2 ′ should likewise be calculated relative to a servo sector. 
     Next, a determination operation  606  determines if T 1 ′ is greater than T 2 ′. If the determination operation  606  determines that T 1 ′ is greater than T 2 ′, an abort write operation  608  aborts the disc write operation. A reestablish operation  610  then reestablishes all of the data associated with aborted disc write operation back into the write buffer  210  for later writing to the disc. Following the reestablish operation  610 , the operational flow  600  then ends. If, however, the determination operation  606  determines that T 1 ′ is not greater than T 1 ′, a wait operation  610  causes the operational flow  600  to be halted while the disc write operation is completed. Following the wait operation, the operational flow  600  ends. 
     Turning now to  FIG. 7 , illustrated therein is an operational flow  700  of another embodiment of the handle write in progress operation  324 . As previously describe, the handle write in progress operation  324 , and thus the operational flow  700 , occurs after it has been determined that a read command has been received from the host, and that a disc write command is in progress. As shown in  FIG. 7 , at the start of operational flow  700 , a calculate operation  704  calculates the last data sector to which data is to be written in the disc write operation, or the last servo sector that is to be accessed before the disc write operation is ended, using a rotationally optimized seek initiation (ROSI) algorithm. In general, a ROSI algorithms calculates the optimum point in the disc write operation at which a seek should be made to the track containing the first sector of data associated with the read operation from the host. In particular, the ROSI algorithm determines an access amount representative of the time required for the transducer to access the first data sector containing data associated with the read command from the host, from the transducers current position. The access amount may be specified in time, or in the number of servo bursts or sectors. 
     Next, the ROSI algorithm calculates the seek amount required to move the transducer from the track over which the transducer is currently located to the track on which the first data sector containing data associated with the read command from the host is located. The seek amount may be specified in time, or in the number of servo bursts or sectors. A latency amount is then calculated from the access amount and the seek amount. The latency may be specified in time, or in the number of servo bursts or sectors. For example, the latency amount may be computed by subtracting the seek amount from the access amount. The last data sector to which data is to be written in the disc write operation before seeking to the first sector specified in the read command is then determined using the latency amount. For example, last data sector to which data is to be written in the disc write operation may be determined as the sector over which the transducer will be positioned after the time specified in the latency amount has passed, or the number of servo bursts or sectors specified by the latency amount have passed under the transducer. 
     It should be understood that in some instances, the ROSI algorithm may determine that the last data sector to which data is to be written (or the last servo sector to be read) in accordance with the disc write operation is, in fact, the present data sector. In such a case, the ROSI algorithm also acts as a determination operation that determines whether the disc write operation should be interrupted to service the read command. Further details regarding calculating the last data sector to which data is to be written in the disc write operation, using a rotationally optimized seek initiation (ROSI) algorithm, may be had by reference to U.S. Pat. No. 6,339,811 issued Jan. 15, 2002 to Gaertner et al., which is hereby incorporated by reference. 
     Following the calculate operation  704 , a determination operation  706  determines if the transducer is currently at or over the last data sector to which data is to be written (or last servo sector that is to be accessed) in the disc write operation, as calculated in the calculate operation  704 . If the transducer is currently at or over the last data sector (or last servo sector), a write complete determination operation  708  determines if the disc write operation has completed. If the write complete determination operation  708  determines that the disc write operation has completed, the operational flow ends. If the write complete determination operation  708  determines that the disc write operation has not completed, the operational flow returns to the determination operation  706 . 
     If the determination operation  706  determines that the transducer is currently at or over the last data sector (or last servo sector), an interrupt write operation interrupts the disc write operation. Next, a reestablish operation  710  reestablishes the data associated with the disc write operation that was not yet written to the disc back into the write cache, and the operational flow  700  ends. That is, the data that was intended to be written in the disc write operation, but which was not written due to the interruption of the disc write operation, is reestablished in the write cache. Again, in most cases, reestablishing the data back in the write cache will involve reestablishing in a cache access table, or the like, that the data is to remain in the cache. 
     In summary, in view of the foregoing discussion it will be understood that various embodiments of the present invention relate to systems and methods for discontinuing a disc write operation in a disc drive  100  to service a read command from the host  200 , so as to improve the internal and external data transfer rates of the disc drive. In accordance with one embodiment, a first method for servicing read commands in a disc drive (such as  100 ) includes the steps of: receiving in the disc drive a read command requesting data (such as  314 ); determining that a disc write operation is in progress in the disc drive (such as  318 ); and discontinuing the disc write operation (such as  406 ,  506 ,  608 , and/or  708 ) to service the read command (such as  320  and  322 ). 
     In accordance with this method, discontinuing the disc write operation may comprise aborting the disc write operation (such as  406  or  608 ). In such a case, the step of discontinuing the disc write operation may comprise determining if the disc write operation is to be aborted (such as  402  or  606 ) and, if it is determined that the disc write operation is to be aborted, aborting the disc write operation (such as  406  or  608 ) to service the read command (such as  320  and  322 ). 
     Alternatively, in accordance with this method, discontinuing the disc write operation may comprise interrupting (such as  506  or  710 ) the disc write operation. In such a case, the step of discontinuing the operation may include determining an optimal point during the disc write operation to interrupt the disc write operation (such as  504  or  704 ) and then interrupting the disc write operation at the optimal point (such as  506  or  710 ). Additionally, where discontinuing the disc write operation comprises interrupting the disc write operation, determining an optimal point during the disc write operation to interrupt the disc write operation may comprise using a rotationally optimized seek initiation (ROSI) algorithm (such as  704 ) to determine the optimal point during the disc write operation to interrupt the disc write operation. 
     In accordance with another embodiment, a method for managing read commands and write operations in a disc drive (such as  100 ) comprises steps of: receiving a read command in the disc drive (such as  314 ); determining that a disc write operation is occurring in the disc drive (such as  318 ); determining if the disc write operation is to be discontinued to service the read command (such as  402 ,  502 ,  606 , or  704 ); and discontinuing the disc write operation to service the read command, if it is determined in determining step (c) that the disc write operation is to be discontinued (such as  406 ,  506 ,  608 , or  710 ). 
     In accordance with this method, the a disc write operation may comprise reading data from a write cache in the disc drive (such as  210 ) and writing the data read from the write cache to specified data sectors of the disc drive. In accordance with this method, the determination as to whether the disc write operation is to be discontinued to service the read command may include determining a time (T 1 ) required to complete the disc write operation and to access a predetermined sector in the disc drive associated with the read command; (such as  602 ) and determining a time (T 2 ) required to access the predetermined sector if the disc write operation is discontinued (such as  604 ). Once these to time values T 1  and  12  are known, a determination may then be made as to whether the disc write operation is to be discontinued, based on these time values (such as  606 ). 
     In accordance with another embodiment, a disc drive (such as  100 ) comprises a disc (such as  108 ) on which data associated with a disc write operation are written and from which data associated with a read command are read. In accordance with this embodiment, the disc drive also includes write discontinuation means in the disc drive for discontinuing a disc write operation to service a read command (such as  400 ,  500 ,  600 , or  700 ). 
     It will be clear that the present invention is well adapted to attain the ends and advantages mentioned as well as those inherent therein. While a presently preferred embodiment has been described for purposes of this disclosure, various changes and modifications may be made that are well within the scope of the present invention. For example, while the various embodiments of the present invention are described herein particularly with respect to disc drives, it will be understood to those skilled in the art that the methods and systems described herein are also applicable to other types of data storage devices. Furthermore, many of the operations shown and described with respect to operational flows  400 ,  500 ,  600 , and  700  may be combined with one another and/or substituted with one another to accomplish the function of determining if a disc write operation should be interrupted to service a read operation, and/or to determine an optimal point to discontinue the disc write operation if it is determined that the disc write operation should be discontinued. Numerous other changes may be made that will readily suggest themselves to those skilled in the art and that are encompassed in the spirit of the invention disclosed and as defined in the appended claims.