Abstract:
Modified HDD sector formats have multiple sets of preamble data. The preambles are well separated so that any defect long enough to wipe out both preambles would also overwhelm the ECC.

Description:
I. FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to hard disk drives (HDD).  
       II. BACKGROUND OF THE INVENTION  
       [0002]     A hard error in a hard disk drive (HDD) occurs when the data in a sector cannot be recovered despite repeated attempts. Hard errors are especially important in enterprise storage applications. For example, in RAID 5 systems the most likely mechanism for data loss is that a hard drive fails followed by a subsequent hard error on one of the other (redundant) drives during the rebuild process. For this reason, hard error rate is carefully monitored during the process of qualifying a new enterprise HDD product.  
         [0003]     Soft errors, i.e., misreads due to poor signal-to-noise ratio or disturbances in the read process, can usually be eliminated by repeated re-reads. In contrast, hard errors are usually caused by problems which are repeatable from read to read. Sources of hard errors include scratches and other media defects or disturbances (collectively, “defects”), such as a head-disk contact, occurring when the sector was written. Defects tend to produce bursts of errors which can be corrected very efficiently by the error correction code (ECC) of the HDD. As an example of the power of ECCs, in a HDD with 4 kB sector formats, bursts of errors up to almost 3200 bits in length in the data field can be corrected, assuming that almost all of the ECC redundancy bytes can be used for erasure correction as opposed to error correction.  
         [0004]     As understood by the present invention, each data storage sector of a HDD begins with a preamble consisting of a sync field and one or two sets of sync bytes. The preamble is used in accordance with HDD principles known in the art to coordinate proper reading of the ensuing data field of the sector. Accordingly, if a defect destroys both sets of sync bytes or a large proportion of the sync field then the data in the main body of the sector cannot be read reliably. This means that a relatively small defect, if it occurs in the wrong location, i.e., in the preamble, can cause a hard error that cannot be corrected by the ECC.  
         [0005]     As further understood herein, in present 512 B sector formats the likelihood of sector failure due to a defect compromising the preamble is less than the likelihood of sector failure due to a defect compromising the main data field to the extent that it overwhelms the capacity of the ECC to correct it. In 4 kB sector formats the ECC is more robust than in 512 byte formats, meaning that the likelihood that a defect in the main data field of a 4 kB format sector will overwhelm the ECC is much less than in a 512 byte format sector. As critically observed herein, however, the likelihood that a defect compromises the preamble beyond repair remains almost the same in both 512 byte and 4 kB formats, and thus becomes the dominant mechanism for hard errors particularly in 4 kB formats. That is, for conventional sector formats, even small bursts of errors can cause a sector to fail if the burst occurs around the sync byte at the end of the preamble.  
         [0006]     The disclosure below refers to “burst erasure correction power”. As is understood by those skilled in the art, this is an intrinsic property of an error correcting code. Error correcting codes have a fundamental parameter called minimum (Hamming) distance, which is the smallest number of symbols that must change to go from one valid codeword to another. For uncoded data the minimum distance is one since a single symbol of a codeword can be changed to arrive at another codeword, whereas for data with a parity symbol the minimum distance is two, because a data symbol of a codeword can be changed along with the parity symbol to arrive at a codeword with valid parity. The value of the minimum distance in this latter case is the number of redundant parity symbols plus one. This can be proved to be the theoretic maximum value in all cases. Codes that meet this limit are known as Maximum Distance Separable or MDS codes, one example of which are Reed-Solomon codes. In any case, a code with distance 2 T+1 can always correct T or fewer errors. Furthermore, a code with distance 2 T+1 can always reconstruct 2 T or fewer erased symbols. Regardless of how calculated, this latter characteristic, i.e., of erasure correction power, is referred to herein as “burst erasure correction power”.  
       SUMMARY OF THE INVENTION  
       [0007]     A method for data storage includes rendering, from a data sector, at least two segments, with each segment including a respective sync preamble. The distance in data units between the sync preambles is no greater than a burst erasure correction capability of an error correction code (ECC).  
         [0008]     In one embodiment, two and only two segments are established, and the segments have different sizes from each other. In another embodiment, a multiple “n” of two segments are established, with each with its own respective sync preamble. Here, “n” may be one or it may be an integer greater than one. For instance, at least four segments per sector may be established. The segments may have equal sizes. If desired, segments of different data sectors may be interleaved with each other on the disk, and sector sizes may be different in different radial locations of the disk.  
         [0009]     In another aspect, a hard disk drive includes logic rendering plural sync preambles in at least one sector having a size of 512 bytes or 4 kB such that the spacing between sync preambles is keyed to a burst erasure correction capability of an ECC executed by the HDD to recover from errors.  
         [0010]     In still another aspect, a data storage system includes at least one data storage disk defining plural sectors, and means for rendering at least one sector into at least two segments. Each segment has a respective preamble containing sync data useful for coordinating reading of a data field of the segment. The locations of the preambles relative to each other are based at least in part on the burst erasure correction capability of the ECC.  
         [0011]     The details of the present invention, both as to its structure and operation, can best be understood in reference to the accompanying drawings, in which like reference numerals refer to like parts, and in which: 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a block diagram of an illustrative non-limiting hard disk drive employing the present sector layout;  
         [0013]      FIG. 2  is a schematic view of a data sector using a first layout scheme, referred to herein as a “Titanic” scheme; and  
         [0014]      FIG. 3  is a schematic view of a data sector using a second layout scheme. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0015]     Referring initially to  FIG. 1 , a hard disk drive (HDD) is shown, generally designated  10 , having a housing  11  holding a hard disk drive controller  12  that can include and/or be implemented by a microcontroller. The controller  12  may access electronic data storage in a computer program device or product such as but not limited to a microcode store  14  that may be implemented by a solid state memory device. The microcode storage  14  can store microcode embodying the logic discussed further below.  
         [0016]     The HDD controller  12  controls a read/write mechanism  16  that includes one or more heads for writing data onto one or more disks  18 . Non-limiting implementations of the HDD  10  include plural heads and plural disks  18 , and each head is associated with a respective read element for, among other things, reading data on the disks  18  and a respective write element for writing data onto the disks  18 .  
         [0017]     The HDD controller  12  communicates with solid state cache. In non-limiting implementations, the cache may be embodied by solid state volatile memory such as but not limited to a Dynamic Random Access Memory (DRAM) device  20 , and/or by solid state non-volatile memory such as but not limited to a flash memory device  22  over an internal HDD bus  24 . The HDD controller  12  also communicates with an external host computer  25  through a host interface module  26  in accordance with HDD principles known in the art. The host computer  25  can be a portable computer that can be powered by a battery, so that the HDD  10  can be a mobile HDD.  
         [0018]     At least portions of the logic disclosed below may be contained in a code storage  14  that is separate from the HDD controller  12 , or the storage  14  may be integrated into the controller  12 . Or, it may be contained in the read/write mechanism  16 , or on the DRAM  20  or flash memory device  22 . The logic may be distributed through the components mentioned above, and may be implemented in hardware logic circuits and/or software logic circuits.  
         [0019]     Now referring to  FIG. 2 , in a first embodiment a data sector  40  of the disk  18  is rendered into segments  42 . The sector  40  may contain 4 kB of user data but without limitation may contain 512 bytes, 1024 bytes, or any other value. Each segment  42  includes an associated preamble  44 , with a preamble including a sync field and at least one sync word. Each preamble  44  is independent of the other preambles, and each preamble  44  contains sync information useful for coordinating the reading of the data field that constitutes the remainder of the segment  42 . In many cases a single data sector can be split by one or more servo sectors that occur at fixed, regularly spaced locations around the track in accordance with servo principles known in the art.  
         [0020]     In accordance with present principles, the size of each segment  42  in data units is smaller than the burst erasure correction power of the ECC of the HDD.  
         [0021]     In non-limiting implementations an even number of segments  42  is established for the sector  40 , i.e., the number of segments into which the sector  40  is divided is 2n, wherein n is an integer selected such that the size of each segment  42  is smaller than the burst erasure correction power of the ECC and such that no partial segment is left over. The segments  42  preferably have identical sizes as each other.  
         [0022]     Because each segment  42  has its own preamble  44 , if a defect compromises the preamble the respective segment will be lost. However, if this happens the segments  42  are sufficiently small that the ECC can still recover the lost segment using erasure decoding principles known in the art. That is, an entire segment  42  may be lost but recovered by the ECC using the remaining segments  42 . Furthermore, the segment size may be chosen to be small enough that some errors can be corrected in addition to one erased segment.  
         [0023]     Additionally, the segment size may be constant in a given radial zone of the disk  18  but may be different as between different radial zones of the disk  18 . The segment size in each zone, for example, can be chosen to avoid split segments. Moreover, segments  42  from different sectors  40  can be interleaved with each other on the disk  18  to further reduce vulnerability to large defects.  
         [0024]      FIG. 3  shows a dual sync sector layout in which a sector  50 , e.g., a 4 kB sector, has differently-sized first and second data segments  52 ,  54 , each with its own respective preamble  56 ,  58 . Servo segments  60  may be provided. In the embodiment shown in  FIG. 3 , the first preamble  56  includes a sync field and at least one sync byte in accordance with sync principles known in the art, but is then followed by a relatively short first segment  52  of the data that is smaller than the burst erasure correction power of the ECC, which in turn is followed by the second preamble  58 , which is then followed by the remainder of the recorded data (second data segment  54 ). In other words, the distance in data elements between preambles  56 ,  58  is keyed to the burst erasure correction capability of the ECC, and more specifically is less than the burst erasure correction capability of the ECC. Accordingly, if the first preamble  56  is compromised the second preamble  58  may be used for its sync information to recover the lost data using the ECC. Any burst long enough to compromise both sync fields would also be long enough to overwhelm the ECC in any case. Thus, the minimum number of segments depends on the ECC burst erasure correction power, with more segments being used to fit an even number of segments between servo sectors which occur in fixed locations around the track.  
         [0025]     While the particular HDD SECTOR FORMAT WITH REDUCED VULNERABILITY TO DEFECTS AND BURST ERRORS as herein shown and described in detail is fully capable of attaining the above-described objects of the invention, it is to be understood that it is the presently preferred embodiment of the present invention and is thus representative of the subject matter which is broadly contemplated by the present invention, that the scope of the present invention fully encompasses other embodiments which may become obvious to those skilled in the art, and that the scope of the present invention is accordingly to be limited by nothing other than the appended claims, in which reference to an element in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather “one or more”. It is not necessary for a device or method to address each and every problem sought to be solved by the present invention, for it to be encompassed by the present claims. Furthermore, no element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims. Absent express definitions herein, claim terms are to be given all ordinary and accustomed meanings that are not irreconcilable with the present specification and file history.