The claimed invention relates generally to the field of data handling systems and more particularly, but not by way of limitation, to a method and apparatus for assigning logical track addresses in a disc drive.
A disc drive is a data handling system used to store digital data. A typical disc drive includes a number of rotatable magnetic recording discs which are axially aligned and mounted to a spindle motor for rotation at a high constant velocity. A corresponding array of read/write heads are supported by a rotary actuator and used to access fixed sized data blocks (sectors) on tracks of the discs to write data to and to read data from the discs.
Disc drives are provided with servo control circuitry to move the heads to the various tracks, read/write channel circuitry to write data to and read data from the discs, and interface control circuitry to facilitate communication and data transfer with a host device. A disc drive is typically configured to operate in accordance with an industry standard interface protocol, such as Small Computer Systems Interface (SCSI). Communications and data transfers are carried out between host and drive in accordance with the designated protocol.
The available data storage of a typical disc drive is identified at the host level in terms of a sequence of consecutively numbered logical block addresses (LBAs). Each LBA corresponds to a unique data block (sector) at a physical location within the disc stack. A present generation drive can have several million LBAs available to store data.
During a data write command in which the host device writes a computer file to the disc drive, the host operating system uses a file allocation table (FAT) to identify a corresponding number of LBAs across which the file is to be distributed. The host transfers the data to be written to a data buffer of the drive and identifies the target LBAs to which the data are to be written. The disc drive determines the physical locations of the target LBAs in terms of physical tracks and sectors, moves the appropriate head or heads to the respective tracks, and proceeds to write the data to the appropriate sectors.
A data read command is carried out in a similar fashion; once the host determines need for a previously stored computer file, the host checks the FAT to identify the LBAs in which the file is stored and instructs the disc drive to retrieve the data from the associated LBAs. The drive identifies the physical sectors associated with the target LBAs, schedules movement of the head or heads to the appropriate sectors, and transfers the data to the buffer and then on to the host.
Of particular interest is the manner in which the disc drive identifies the physical locations of the target LBAs during a read or write (data access) operation. To explain this more fully, it will be helpful to first review the manner in which tracks are defined and addressed in a typical disc drive.
Tracks are defined by servo data recorded to the discs during disc drive manufacturing. The servo data are typically arranged in a number of radially spaced apart servo data fields. Data sectors are subsequently defined in regions between adjacent servo fields during a disc drive formatting operation. The servo data fields on each track typically include a track address field that stores a physical track address (PTA) for that track, typically in Gray code format. All of the tracks at a given radius (i.e., all the tracks having the same PTA) make up a cylinder.
Logical track addresses (LTAs) are assigned to the tracks over the user data recording areas of the disc surfaces. It is common to leave relatively small guard bands of tracks near the innermost and outermost diameters of the disc surfaces to allow storage of control parameters and information as well as to provide operational margin for the drive. An exemplary LTA scheme involves assigning the first logical track address (logical track zero) to all of the tracks in a first cylinder near the outermost diameter of the discs (such as at PTA=50) and then incrementing the logical track addresses across the discs toward the innermost diameter. Thus, logical track one would correspond to physical track 51, and so on.
When an access command is received, the interface circuit consults a conversion table or otherwise performs the necessary calculations to identify head, logical track and sector for each LBA associated with the access command. The interface circuit directs the servo control circuitry to carry out a seek to the logical track.
In order to do so, the servo control circuitry converts the logical track address to a physical track address; using the example above, a command to move a selected head to logical track 0 is interpreted by the servo control circuitry to require movement of the head to physical track 50 (in terms of Gray code). The servo circuit proceeds to execute a seek to move the selected head to the target track. Once the servo control circuit reports that the head is on track, the interface circuit directs the read/write channel to read or write the data to the target sector(s) on that track.
As will be appreciated, computer files are often much larger than a single sector and a given data access operation can involve accessing multiple sectors on multiple disc surfaces. This can be especially true in high throughput sustained accesses such as video streaming. While the heads are nominally aligned, small radial offsets (variations in radial location) will typically occur from head to head due to a number of factors such as manufacturing variations, deflection during handling, etc. Thus, if a first head is over physical track 50 and a head switch is made to a second head having a xe2x88x922 track offset with respect to the first head, the second head will actually be over physical track 48 when the head switch operation is completed. A two track corrective seek will be necessary to move the second head to cylinder 50 to continue the data access operation.
It is generally known in the prior art to measure head offsets and account for such during servo operation. Using the above example, if the first head is over physical track 50 and the servo circuit is directed to move the second head over physical track 60, the second head will actually need to move 12 tracks (not just 10) to get to the desired track 60, and adjustments can be made accordingly. However, during an access operation to a given logical cylinder where multiple physical tracks are sequentially accessed by different heads, knowing the head-to-head offsets does nothing to eliminate the need to proceed with small seeks after head switch operations to maintain the heads over the tracks in the selected logical cylinder.
As track densities continue to increase, the magnitudes of corrective seeks (in terms of tracks) after head switch operations will continue to increase and such seeks will generally tend to decrease effective data transfer rate performance of a disc drive. Accordingly, there is a need for improvements to address these and other limitations of the prior art.
In accordance with preferred embodiments, a data handling system (disc drive) is provided in communication with a host device. The disc drive has a plurality of transducing heads adjacent a corresponding plurality of data recording surfaces. Concentric tracks are defined on each of the recording surfaces and are provided with a physical track address determined by servo data written to each track.
Logical track addresses are assigned to the tracks by positioning a first head adjacent a selected location of the data recording surface corresponding to the first head, measuring a head offset value for each of the remaining heads, and assigning logical track addresses to the tracks on each data recording surface in relation to the measured head offset values.
In this way, each logical cylinder will tend to include tracks with different physical track addresses, but the tracks in each logical cylinder will nonetheless be nominally aligned with the heads. This reduces the need for corrective seeks after head switching within the same logical cylinder, thus improving data transfer performance of the drive.
These and various other features and advantages which characterize the claimed invention will be apparent from a reading of the following detailed description and a review of the associated drawings.