Patent Publication Number: US-9842047-B2

Title: Non-sequential write for sequential read back

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims benefit of priority to U.S. Provisional Application No. 62/083,696, entitled “Interlaced Magnetic Recording in HAMR Devices” and filed on Nov. 24, 2014, and also to U.S. Provisional Patent Application No. 62/083,732, entitled “Interlaced Magnetic Recording” and filed on Nov. 24, 2014, and also to U.S. Provisional Patent Application No. 62/108,929, entitled “Non-Sequential Write for Sequential Read Back” and filed on Jan. 28, 2015. All of these applications are specifically incorporated by reference for all that they disclose or teach. 
    
    
     BACKGROUND 
     As requirements for data storage density increase for magnetic media, cell size decreases. A commensurate decrease in the size of a write element is difficult because in many systems, a strong write field gradient is needed to shift the polarity of cells on a magnetized medium. As a result, writing data to smaller cells on the magnetized medium using the relatively larger write pole may affect the polarization of adjacent cells (e.g., overwriting the adjacent cells). One technique for adapting the magnetic medium to utilize smaller cells while preventing adjacent data from being overwritten during a write operation is shingled magnetic recording (SMR). 
     SMR allows for increased areal density capability (ADC) as compared to conventional magnetic recording (CMR) but at the cost of some performance ability. As used herein, CMR refers to a system that allows for random data writes to available cells anywhere on a magnetic media. In contrast to CMR systems, SMR systems are designed to utilize a write element with a write width that is larger than a defined track pitch. As a result, changing a single data cell within a data track entails re-writing a corresponding group of shingled (e.g., sequentially increasing or decreasing) data tracks. 
     Better designs are desired to increase ADC beyond that provided by CMR and SMR systems. 
    
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
         FIG. 1  illustrates a data storage device including a transducer head assembly for writing data on a magnetic storage medium. 
         FIG. 2  illustrates data writes in an interlaced magnetic recording (IMR) system employing an example non-sequential write scheme for sequential data read back. 
         FIG. 3  illustrates a magnetic disc including a number of high-density data bands each separated from one another by one or more low-density data tracks. 
         FIG. 4  illustrates additional example writes in an IMR system employing an example non-sequential write scheme for sequential file read back. 
         FIG. 5  illustrates example operations for performing a non-sequential data write that provides for sequential data read back in an IMR system. 
     
    
    
     SUMMARY 
     Implementations disclosed herein provide a storage device controller configured to receive data addressed to a consecutive sequence of logical block addresses (LBAs) corresponding to a consecutive sequence of data tracks on a storage medium and to write the data to the consecutive sequence of data tracks in a non-consecutive track order. 
     This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. These and various other features and advantages will be apparent from a reading of the following Detailed Description. 
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a data storage device  100  including a transducer head assembly  120  for writing data on a magnetic storage medium  108 . Although other implementations are contemplated, the magnetic storage medium  108  is, in  FIG. 1 , a magnetic storage disc on which data bits can be recorded using a magnetic write pole (e.g., a write pole  130 ) and from which data bits can be read using a magnetoresistive element (not shown). As illustrated in View A, the storage medium  108  rotates about a spindle center or a disc axis of rotation  112  during rotation, and includes an inner diameter  104  and an outer diameter  102  between which are a number of concentric data tracks  110 . Information may be written to and read from data bit locations in the data tracks on the storage medium  108 . 
     The transducer head assembly  120  is mounted on an actuator assembly  109  at an end distal to an actuator axis of rotation  115 . The transducer head assembly  120  flies in close proximity above the surface of the storage medium  108  during disc rotation. The actuator assembly  109  rotates during a seek operation about the actuator axis of rotation  112 . The seek operation positions the transducer head assembly  120  over a target data track for read and write operations. 
     The transducer head assembly  120  includes at least one write element (not shown) including a write pole for converting a series of electrical pulses sent from a controller  106  into a series of magnetic pulses of commensurate magnitude and length. The magnetic pulses of the write pole selectively magnetize magnetic grains of the rotating magnetic media  108  as they pass below the pulsating write element. 
     View C illustrates magnified views  114 ,  116 , and  118  of a same surface portion of the storage media  108  according to different write methodologies and settings of the data storage device  100 . Specifically, the magnified views  114 ,  116 , and  118  include a number of magnetically polarized regions, also referred to herein as “data bits,” along the data tracks of the storage media  108 . Each of the data bits (e.g., a data bit  128 ) represents one or more individual data bits of a same state (e.g., is or Os). For example, the data bit  128  is a magnetically polarized region representing multiple bits of a first state (e.g., “000”), while an adjacent data bit  126  is an oppositely polarized region representing one or more bits of a second state (e.g., a single “1”). The data bits in each of the magnified views  114 ,  116 , and  118  are not necessarily illustrative of the actual shapes or separations of the bits within an individual system configuration. 
     The magnified view  114  illustrates magnetic transitions recorded according to a conventional magnetic recording (CMR) technique. Under CMR, all written data tracks are randomly writeable and have substantially equal written track width. “Written track width” refers to, for example, a width measured in a cross-track direction of a recorded data bit. Each of the individual data tracks is randomly writeable, but a maximum attainable ADC is reduced as compared to the shingled and interlaced magnetic recording technique described below. 
     As used herein, a data track is randomly writeable when the data track can be individually re-written multiple times without significantly degrading data on other adjacent data tracks. An adjacent data track is “significantly degraded” if reading the data track results in a number of read errors in excess of a maximum number of errors that can be corrected by an error correction code (ECC) of the data  100  storage device. 
     The magnified view  116  illustrates magnetic transitions recorded according to a shingled magnetic recording (SMR) technique. Under SMR, consecutive data tracks are written to sequentially with a write element that affects two data tracks at once. Data is written to the magnetic medium  108  in units of “data bands” where each data band includes a series of consecutive data tracks and is separated from adjacent data band(s) by one or more “guard tracks” where no data is stored. Under SMR, no data tracks are randomly writeable, but a resulting ADC is increased as compared to a maximum ADC attainable when the above-described CMR technique is employed. 
     The magnified view  118  illustrates magnetic transitions recorded according to an interlaced magnetic recording (IMR) technique. IMR utilizes alternating data tracks of different written track widths arranged with slightly overlapping edges so that a center-to-center distance between directly adjacent tracks (e.g., the track pitch) is uniform across the surface of the magnetic medium  108 . 
     For example, the IMR technique shown in the magnified view  118  illustrates alternating data tracks of two different written track widths. A first series of alternating tracks (e.g., tracks  158 ,  160 , and  162 ) have a wider written track width than a second series of interlaced data tracks (e.g.,  164  and  166 ). Each wide data track of the first series (e.g., the data track  160 ) is written before the narrow and directly adjacent data tracks of the second series (e.g., the data tracks  164  and  166 ). For example, the data track  160  is written before the data tracks  164  and  165 . The write to data tracks  164  and  166  overwrites outer edge portions of the data track  160 ; however, the data track  160  is still readable. 
     Because each data track of wide written width is written prior to directly adjacent data tracks of narrower width, the data tracks of the wider written width (e.g., data tracks  158 ,  160 , and  162 ) are also referred to herein as “bottom tracks,” while the alternating data tracks of narrower written width (e.g., the data tracks  164  and  166 ) are referred to herein as “top tracks.” According to one implementation, the top tracks are of a lower linear density (e.g., kilo bytes per inch, measured in the down-track direction) than the bottom tracks. Other IMR implementations utilize interlaced data tracks having more than two different linear densities and/or written track widths. 
     By manipulating the linear densities of the top and bottom data tracks and also manipulating an order in which the data tracks are written, IMR systems can be tuned to exhibit a number of characteristics superior to CMR and SMR systems. 
     In one implementation, the IMR technique illustrated in magnified view  118  provides for system performance that is better than the best performing SMR systems. If, for example, the linear density of the top data tracks is selected to be lower than an experimentally-defined threshold, then each of the top data tracks is randomly writeable. Since random writes cannot be performed in SMR systems, an IMR system that provides for random writes exhibits superior performance characteristics as compared to SMR systems. 
     In another implementation, the IMR technique illustrated in magnified view  118  provides for an ADC of the magnetic medium  108  that significantly exceeds the maximum attainable ADC in SMR systems. If, for example, the linear densities of the top and bottom data tracks are pushed to upper limits (e.g., beyond that of systems described above), the data tracks may cease to be randomly writeable but provide for a very high total ADC. For example, a linear density of the top data track  166  may be selected so that the top data track  166  can be written exactly once without significantly degrading data of the adjacent (e.g., previously-written) bottom data tracks  160  and  162 . Many of the implementations described herein elaborate on this type of IMR system that provides a very high ADC by pushing linear densities of the top data tracks and bottom data tracks to upper limits. 
     In one implementation, portions of incoming data addressed to a sequence of consecutive logical block addresses (LBAs) are written to a series of consecutive data tracks (e.g., the data tracks  158 ,  164 ,  160 ,  166 , and  162 ). Consequently, the data may be read back from the consecutive data tracks in order of the consecutive LBA sequence without lifting the actuator arm  109  to perform a seek operation. This type of read operation is referred to herein as a “sequential read.” Notably, the nature of IMR does not allow for sequential writes (e.g., writing to consecutive data tracks in a consecutive order) without significantly degrading one or more of the data tracks. In one implementation, the technology disclosed herein provides for sequential reads of stored data by directing incoming data addressed to a consecutive sequence of LBAs to a series of consecutive data tracks in a non-consecutive track order. 
       FIG. 2  illustrates data writes to a magnetic disc in an IMR system employing an example non-sequential write scheme  200  for sequential data read back. A storage device controller receives data addressed to a consecutive sequence of LBAs corresponding to a consecutive sequence of data tracks ( 201 - 207 ) on the magnetic disc and writes the data to the consecutive sequence of data tracks in a non-consecutive track order. 
     In  FIG. 2 , the data addressed to the consecutive sequence of LBAs is annotated with the letters A-G, where alphabetical order indicates a sequential LBA order. In at least one implementation, the data addressed to the consecutive sequence of LBAs is a file with the data portions sequentially ordered A-G, from beginning to end. 
     In the following description, the terms “first portion”, “second portion”, “third portion”, etc. refer to portions of the data addressed to various consecutive LBAs. If, for example, data is addressed to LBAs 0-299, a “first portion of the data” may refer to LBAs 0-99; a “second portion of the data” may refer to LBAs 100-199; and a “third portion of the data” may refer to LBAs 200-299. In  FIG. 2 , the letters A-G represent portions of the data addressed to consecutive LBA sequences. For example, the letter ‘A’ may refer to data addressed to LBAs 0-99; the letter ‘B’ may refer to data addressed to LBAs 100-199; the letter ‘C’ may refer to data addressed to LBAs 200-299, etc. 
     In writing the data to the magnetic disc, the storage device controller first writes the first and third portions of the data (A and C) to alternating data tracks  201  and  203  separated by an interlaced data track  202  (e.g., a blank data track). After writing portions A and C, the storage device controller writes a second portion (B) to the interlaced data track  202 . Then, in chronological order, the storage device controller writes a fifth portion of the data (E) to a data track  205 ; a fourth portion of the data (D) to a data track  204 , a seventh portion of the data (G) to a data track  207 , and a sixth portion of the data (F) to a data track  206 . 
     To summarize the above-described write scheme: if a most-recently written data track is referred to as index N, the next data track written has an index of N+2 if the data track of index N−1 already stores data. However, if the data track of index N−1 is blank and does not already store data, the data track of index N−1 is written next immediately following the write to the data track of index N. This trend continues so long as some portion of the data remains to be written to the magnetic disc. 
     After all portions of the data are written to the magnetic disc, the series of consecutive data tracks  201 - 207  stores the consecutive portions of the data, A-G, in a consecutive order. To read the data back from the data tracks  201 - 207  in the consecutive LBA order, the storage device controller can perform a sequential read of the data tracks  201 - 207 . 
     As shown in  FIG. 2 , the odd-numbered data tracks (e.g.,  201 ,  203 ,  205 ,  207 , etc.) are bottom data tracks having a larger written track width than the top, even-numbered data tracks (e.g.,  202 ,  204 ,  206 , etc.). Each of the top data tracks is written after a data write to the immediately adjacent bottom data tracks. For example, the data track  204  is not written to until after data is written to the immediately adjacent data tracks  203  and  205 . Each of the top data tracks (e.g.,  204 ) overlaps and overwrites edge data of the immediately adjacent bottom data tracks (e.g.,  203 ,  205 ). 
     In one implementation, two different linear densities are employed to write the data to the magnetic disc. Data stored in the bottom data tracks is written at a first linear density that is higher than the linear density of data stored in the top data tracks. In one implementation, linear densities selected for use in the top and bottom data tracks are low enough that the top data tracks are randomly writeable. 
     In another implementation, the linear densities of the top and bottom data tracks are selected to be so high that individual tracks within the series (e.g., data tracks  201 - 207 ) are not randomly writeable. For example, a linear density of the data track  204  is so high that the data portion D cannot be re-written to the data track  204  without significantly degrading data on the immediately adjacent data tracks  203  and  205 . In this case, individual tracks of the data tracks  201 - 207  cannot be updated without re-writing each of the other data tracks. For example, updating data of the portion D may entail writing the following portions in the following order: re-writing portion A to track  201 ; re-writing portion C to track  203 ; re-writing portion B to track  202 , re-writing portion E to track  205 ; updating data of the portion D in track  204 ; re-writing portion G to track  207 ; and re-writing portion F to track  206 . 
     Maximizing linear densities (as described above) can provide for a large increase in total ADC as compared to implementations where each individual data track has a low enough linear density and can be written to at random. However, the use of very high linear densities implicates a write scheme under which groupings of data tracks (e.g., two or more adjacent data tracks) are treated as individual write units. 
     For example, the data tracks  201 - 207  may represent a high-density data band  220  that is re-written in its entirety each time any data track within the high-density data band  220  is updated. The high-density data band  220  may be used to store a single file (as described above), or multiple files. Different high-density data bands on the magnetic disc can be of uniform or variable size. 
     In one implementation, each high-density data band on the magnetic disc is separated by an adjacent high-density data band by one or more low-density data tracks. For example, the data track  208  may be a low-density data track that stores data at a lower linear density than any of the data tracks within the high-density data band  220 . Each of the low-density data tracks is randomly writeable and thus can be updated without updating or re-writing data of any other data track on the magnetic disc. 
     Designated low-density data tracks, such as a data track  208 , can be used to separate adjacent high-density data bands. In one simplistic implementation, low-density data tracks separating high-density data bands are left blank until such a time that the magnetic disc becomes so full that user data cannot be directed elsewhere. In another implementation, one or more of the low-density data tracks store a portion of a file including other data stored in one of the high-density data bands. In yet another implementation, the low-density data tracks are used to store infrequently accessed data, such as system data or infrequently accessed user data. 
       FIG. 3  illustrates a magnetic disc  300  including a number of high-density data bands (e.g., high density data bands  302 ,  304 ,  306 ) each separated from one another by one or more low-density data tracks (e.g., low-density data tracks  308 ,  310 , and  312 ). Each of the high-density data bands  302 ,  304 , and  306  includes multiple data tracks updated as a single write unit. For example, updating data in a single data track within the high density data band  304  may entail reading all other data tracks in the high density data band  304  to a temporary memory location and then re-writing the entire band to include the updated data in the affected data track. 
     In one implementation, each of the high density data bands  302 ,  304 , or  306  includes data corresponding to (e.g., originally addressed to) a consecutive LBA sequence spanning the data tracks of the associated data band so as to allow for a sequential read of the data from the magnetic disc  300  according to the consecutive LBA sequence. 
     To record the data on the magnetic disc  300 , a storage device controller writes to data tracks of a high-density data band in a non-consecutive track order. In one implementation, the high density data bands store data according to an IMR technique where alternating data tracks have varying written track width and/or linear densities. 
     Each of the low density data tracks (e.g.,  308 ,  310 , and  312 ) are randomly writeable and store data at a lower linear density than the data tracks in any of the high density data bands. A written track width of the low density data tracks may be the same or different from one another and one or more data tracks located in a high density data band. 
       FIG. 4  illustrates example writes in an IMR system employing an example non-sequential write scheme  400  for sequential file read back. In one implementation, the write scheme is used to write a file to a high-density data band of IMR data. The example writes  400  illustrate options for writing portions of a first file (e.g., A1-F1) and a second file (e.g., A2-I2) to data tracks located within a same high-density data band. In  FIG. 4 , consecutive file portions are annotated by alphabetically arranged letters indicating a sequential ordering of the file from beginning to end. 
     In writing a first file to the high-density data band, a storage device controller initially writes first and third portions of the first file (A1 and C1) to alternating data tracks  401  and  404  separated by a blank, interlaced data track  402 , followed by a second portion (B1) of the first file to the interlaced data track  402 . Then, in chronological order, the storage device controller writes a fifth portion of the first file (E1) to a data track  405 ; a fourth portion of the first file (D1) to a data track  404 , and a sixth (e.g., a final) portion of the first file (F1) to a data track  407 . Notably, the sixth and final portion (F1) of the first file is separated from the remaining portions of the file by a blank data track  406 . Thus, so long as the data tracks  406  and  408  do not include any data, the sixth portion (F1) is randomly writeable to the data track  407 . 
     Once the storage device controller receives and stores the data of the first file (e.g., A1-F1, as shown), the controller follows one of two possible addressing schemes to address and store portions of the second file to the same high-density data band. Under a first option, (option 1), the storage device controller moves the sixth portion (F1) of the first file from the data track  407  to the adjacent data track  406  so that all consecutive portions (A1-F1) of the first file are stored in consecutive data tracks  401 - 406 . Thereafter, the storage device controller addresses consecutive portions (A2-I2) of the second file to consecutive data tracks  407  through  415 . The consecutive portions A2-I2 are written to the data tracks  407  through  415  according to the following non-consecutive track order: portion A2 to  407 , portion C2 to  409 , portion B2 to  408 , portion E2 to  411 , portion D2 to  410 , portion G2 to  413 , portion F2 to  412 , portion I2 to  415 , and portion H2  414 . 
     Under Option 2, the storage device controller allows the first portion (A2) of the second file to be interlaced between the two final portions (E1 and F1) of the first file. The last portion (F1) of the first file is not (e.g., as in option 1) moved from the data track  407  to  406  prior to writing the portions (A2-I2) of the second file. Rather, the storage device controller addresses the first portion (A2) of the second file to the data track  406  so that it is separated from other portions of the second file by the data track  407 , which stores the sixth portion (F1) of the first file. 
     Remaining portions of the second file are written to the high density data band using a write methodology similar to that described above. For example, portions of the second file are written according to the following non-consecutive track order: portion A2 to  405 , portion C2 to  409 , portion B2 to  408 , portion E2 to  411 , portion D2 to  410 , portion G2 to  413 , portion F2 to  412 , portion I2 to  415 , and portion H2 to  414 . 
       FIG. 5  illustrates example operations  500  for performing a non-sequential data write that provides for sequential LBA and/or file read back in an IMR system. A receiving operation  505  receives data addressed to a consecutive sequence of LBAs. In one implementation, the consecutive data tracks are included in a high-density data band generally written to as a single write unit and separated from an adjacent high-density data band on the disc by one or more randomly writeable data tracks storing data at a lower linear density. 
     A first writing operation  510  writes a first consecutively-addressed portion (e.g., a portion 1) of the data to a corresponding data track having an index N. A determination operation  515  then determines whether a data track of index N−1 already stored data of the file. If the data track of index N−1 does not already store data, a writing operation  520  writes a next consecutively-addressed portion (e.g., a portion 2) of the data to the data track of index N−1. If, however, the data track N−1 does already store data, another writing operation  525  writes the next consecutively-addressed portion (e.g., the portion 2) to the data track of index N+1. A re-defining operation  530  re-defines a most recently written data track (e.g., N−1 or N+1) as “N”, and the operations  515  and  520  or  525  repeat until the entire data file is written to the magnetic disc. 
     The embodiments of the disclosed technology described herein are implemented as logical steps in one or more computer systems. The logical operations of the presently disclosed technology are implemented (1) as a sequence of processor-implemented steps executing in one or more computer systems and (2) as interconnected machine or circuit modules within one or more computer systems. The implementation is a matter of choice, dependent on the performance requirements of the computer system implementing the disclosed technology. Accordingly, the logical operations making up the embodiments of the disclosed technology described herein are referred to variously as operations, steps, objects, or modules. Furthermore, it should be understood that logical operations may be performed in any order, adding and omitting as desired, unless explicitly claimed otherwise or a specific order is inherently necessitated by the claim language. 
     The above specification, examples, and data provide a complete description of the structure and use of exemplary embodiments of the disclosed technology. Since many embodiments of the disclosed technology can be made without departing from the spirit and scope of the disclosed technology, the disclosed technology resides in the claims hereinafter appended. Furthermore, structural features of the different embodiments may be combined in yet another embodiment without departing from the recited claims.