Abstract:
A method and system for storing files in a computer readable format using a file allocation table wherein a storage device is formatted with a single file allocation table when accessed by a system which includes means for reconstituting the file allocation table in the event of a failure. Also provided is a method for determining how many file allocation tables to write to a storage device based on a number of characteristics of the storage device such as type, data structure previously provided on the device etc, and/or on user input.

Description:
TECHNICAL FIELD 
       [0001]    This invention relates to the storage of files in a computer readable format and, in particular, to the provision of file allocation tables used for storing computer readable files. 
       BACKGROUND TO THE INVENTION 
       [0002]    Computing devices use various storage devices to store data. For each storage device, a system is needed so that the computer system is able to find and access the data stored on the device. This system is referred to as a “file system”. 
         [0003]    Various kinds of file system are known. For example, the Windows operating systems generally use the FAT (FAT12, FAT16 and FAT32) and NTFS file systems whereas Linux uses the Extended File Systems (ext2, ext3, ext4). The FAT file systems utilise file allocation tables. In this respect, a degree of confusion exists in that the acronym FAT is used in the art to refer to the specific, proprietary file system used by the Windows operating systems as well as to refer to a generic file allocation table. The term “FAT file system” as used herein is used to refer to the Windows specific file system whereas the term “file allocation table” is used to refer to the data structure. 
         [0004]    The FAT file systems are described, for example, in the document “Microsoft Extensible Firmware Initiative FAT32 File System Specification. FAT: General Overview of On-Disk Format”, Version 1.03, Dec. 6, 2000; Microsoft Corporation. 
         [0005]    The FAT file systems utilise file allocation tables as a directory of the contents of the storage device. The file allocation table defines a singly linked list of the “extents” (clusters) of a file mapped to an identification of a file. Therefore, the operating system of a computing device is able to locate the information of a file on a storage device by referring to a file allocation table. 
         [0006]    In the past, each storage device has been provided with two file allocation tables. This has been done to provide redundancy which, it was intended, would improve the reliability of the device. 
         [0007]    However, it has been found that the intended reliability is belied by the problem of determining which of the file allocation tables is corrupt if the two tables do not match. Furthermore, maintaining two file allocation tables significantly increases the time used to write data to a storage device and the additional file allocation table utilizes additional storage space. 
       SUMMARY OF THE INVENTION 
       [0008]    According to a first aspect, the invention provides a system for storing computer accessible files, said system comprising:
       a storage device for a computing device, said storage device comprising a file system having one and only one file allocation table and a plurality of files which are accessible by means of the file allocation table; and   restoring means for restoring the file allocation table in the event of a failure.       
 
         [0011]    A single file allocation table significantly reduces the time taken to write information to the storage device, thereby improving the performance of a computing device incorporating a storage device with a single file allocation table. However, to maintain the robustness previously provided by two file allocation tables, the system includes restoring means for restoring the file allocation table in the event of a failure. 
         [0012]    The space on the storage device previously used to store the second file allocation table may now be used for data storage, therefore increasing the capacity of the device. 
         [0013]    The system may further comprise write means for ensuring safe write operations to the storage device. 
         [0014]    The write means may be adapted to ensure sector atomic writes to said storage device. 
         [0015]    The storage device may be a NAND flash drive and said write means may then include a device driver and a flash translation layer. 
         [0016]    The restoring means may include means for reconstructing the file allocation table from a contents of said files. 
         [0017]    The restoring means may include power detection means for indicating an unexpected power interruption to said storage device. 
         [0018]    The restoring means may be adapted to restore the file allocation table as a result of the power detection means indicating an unexpected power interruption to said storage device. 
         [0019]    The failure may be a failure of the file allocation table. The failure may be detected by detecting a power failure, or by detecting an error in the data stored on the device. In a preferred embodiment; the failure corresponds to a mismatch between the file allocation table and a content of said plurality of files. 
         [0020]    The file system is preferably the FAT file system. 
         [0021]    According to a further aspect, the invention provides for a method of formatting a storage device with a file system, said file system comprising at least one file allocation table, said method comprising the steps of:
       (a) determining one or more characteristics of said storage device;   (b) on the basis of said determined characteristics, determining a number of file allocation tables to be included with said file system; and   (c) constructing a file system on said device, said file system comprising said determined number of file allocation tables.       
 
         [0025]    The file system may be the FAT file system. 
         [0026]    The step of determining one or more characteristics of said storage device may include the step of determining a type of the storage device. 
         [0027]    The step of determining one or more characteristics of said storage device may include the step of any one of:
       determining whether said storage device is removable;   determining whether said storage device is remotely accessible;   determining whether said storage device is adapted to guarantee safe data writes; or   determining whether said storage device is adapted to avoid corrupted sectors.       
 
         [0032]    The step of determining one or more characteristics of said storage device may be based on a data structure with which said storage device is provided. 
         [0033]    The storage device may include management software and the step of determining one or more characteristics of said storage device may then be based on whether the management software accesses more than one file allocation table. 
         [0034]    The step of determining one or more characteristics of said storage device may be based on whether or not said storage device is shared by more than one operating system. 
         [0035]    According to a preferred embodiment, the step of determining one or more characteristics of said storage device comprises the steps of determining whether said device is a fixed flash drive accessed by means of a flash translation layer; and wherein a system utilising said flash drive comprises means for reconstituting said file allocation table in the event of a failure. 
         [0036]    Alternatively, the number of file allocation tables may be specified by a user or by a manufacturer of the storage device. 
         [0037]    According to a further aspect, the invention provides an operating system arranged to cause a computing device to operate as herein described. 
         [0038]    According to a further aspect, the invention provides a computer program or a suite of computer programs suitable for causing a computing device to operate as herein described. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0039]    Embodiments of the invention will now be described with reference to the accompanying drawings in which: 
           [0040]      FIG. 1  is an illustration of a mobile computing device operating in accordance with a preferred embodiment of the invention; 
           [0041]      FIG. 2  is a schematic illustration of various components of the mobile computing device of  FIG. 1 ; 
           [0042]      FIG. 3  is a schematic illustration of a storage device according to a preferred embodiment of the invention; and 
           [0043]      FIG. 4  is a flow chart of a preferred embodiment of the invention. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0044]      FIG. 1  illustrates a mobile computing device  10  which includes a casing  12 , a keypad  14 , a screen  16 , a speaker  18 , a microphone  20  and an antennae  22 . The mobile computing device  10  may be used as a mobile phone in a manner known in the art. The keypad  14 , screen  16 , speaker  18  and microphone  20  are devices and their interactions with one another and with a central processing unit (not shown in the diagrams) is controlled by an operating system. In the embodiment shown, the operating system used is the Symbian operating system produced by Symbian Software Ltd., UK although it will be realised that the invention is not limited in this respect. 
         [0045]      FIG. 2  is a schematic illustration of some components of the mobile computing device  10 . The operating system is represented by kernel  22 . A user application  24  communicates with the kernel  22  which is connected to system memory  26 . Likewise device drivers  28  and  30  are connected to kernel  22  and control the operation of the respective devices: keyboard  34  and monitor  36 . Keyboard  34  and monitor  36  are provided by way of example; the device  10  includes a number of additional devices not illustrated in  FIG. 2 . The kernel  22  controls the operation of the devices  34  and  36  by means of the device drivers  28  and  30  according to the operation of the user application  24 , in a known manner. 
         [0046]    Device  10  further includes removable non-volatile memory  38  controlled by device driver  32 . In the embodiment shown, the non-volatile memory  38  is a NAND flash drive and is controlled by the device driver  32  by means of a flash translation layer  40 . Flash translation layer  40  is an application which is loaded by the kernel  12  when the non-volatile flash drive  38  is installed and may, for example, be the Unistore II application. The flash translation layer  40  provides a mapping between the file allocation system used by the operating system and the flash memory  38 . 
         [0047]      FIG. 3  is a symbolic illustration of the non-volatile memory  38  which includes a single file allocation table  52  and two files. The data on memory  38  has been stored according to the FAT file system. The constituents of each file may, or may not be, stored in a physically contiguous manner. For example, memory  38  includes a first file comprising clusters  58   a ,  58   b ,  58   c  and  58   d . As illustrated in  FIG. 3 , clusters  58   b ,  58   c  and  58   d  are physically contiguous, but cluster  58   a  is separated from the other clusters. A further file comprises clusters  60   a ,  60   b ,  60   c  and  60   d . The file allocation table  52  includes pointers  54   a ,  54   b ,  54   c  and  54   d  which indicate where in the memory  38  the corresponding data clusters  58   a ,  58   b ,  58   c  and  58   d  of the first file may be found. Similarly, the file allocation table  52  includes pointers  56   a ,  56   b ,  56   c  and  56   d  which indicate where in the memory  38  the corresponding data clusters  60   a ,  60   b ,  60   c  and  60   d  of the second file may be found. Only two files are illustrated in the memory  38  of  FIG. 3 , but it will be realised that the memory may include many more files, the location for the data of each of which will be specified by the file allocation table. 
         [0048]    The physical arrangement of the data on the memory  38  may differ from the structure illustrated in  FIG. 3 . The flash translation layer  40  acts to ensure that the data structure of the memory  38  appears to the kernel  22  in the manner illustrated in  FIG. 3 . Furthermore, as the memory  38  is a NAND flash drive, the flash translation layer  40  and the device driver  32  ensure that only sector atomic writes are permitted to the memory  38  by ensuring that no partial sectors are written (for example, an unexpected power interruption will not result in a partial sector being written to the memory  38 ). 
         [0049]    The kernel  22  of the embodiment illustrated uses an application capable of restoring the file allocation table  52  from the contents of the data portions of the memory  38 . In the embodiment shown, this application is the ScanDisk application which ships with the Symbian operating system, although other suitable applications exist. Furthermore, the kernel  22  includes a flag (not shown) which is set if the power to the memory  38  has been unexpectedly interrupted. If the kernel boots up and the flag is set, ScanDisk will be automatically run. This will verify that the file allocation table  52  is correct and, if not, fix it. This flag and the ScanDisk application form part of the “Rugged FAT” implementation of Symbian OS. Alternative restoring software for reconstructing the file allocation tables are known, for example, the R-Studio software is capable of restoring the file allocation table and operates on the Windows operating systems, but the same methods may be used with operating systems such as the Symbian OS to restore the file allocation table. 
         [0050]    A power fault in the form of a power interruption during a write operation is the most common cause of a corrupted file allocation table. However, other causes of a failure in the file allocation table are possible and may be used to initiate the launch of ScanDisk or the appropriate recovery software. For example, the kernel  22  ( FIG. 2 ) may employ error detection software which verifies the data retrieved from the memory  38 . In this instance, if an error is detected, the ScanDisk application will be launched to verify and, if necessary, repair the file allocation table of memory  38 . Generally, the failure will arise where there is a mismatch between the file allocation table of memory  38  and the data stored on the memory. 
         [0051]    The file allocation table will be restored by analysing the contents of the data portion of the storage device. With reference to  FIG. 3 , the data portion is the clusters  58   a  to  58   d  and  60   a  to  60   d . By analysing the data portions, the recovery software will be able to determine which file(s) each cluster belongs to, and by maintaining a record of the physical position of each cluster, the recovery software is able to reconstitute the file allocation table. 
         [0052]    As previously mentioned, the memory  38  differs from data storage devices known in the art in that it comprises a single file allocation table  52 . However, the flash translation layer  40 , together with the unexpected power interrupt flag and the ScanDisk utility ensure that a backup version of the file allocation table is not required in this embodiment. 
         [0053]    A flash drive constructed according to the invention and using a single file allocation table with a Unistore II™ NAND layer has been found to have a write speed of more than 2 Mbytes/second whereas the same flash drive formatted with two file allocation tables displayed write speeds of about 500 Kbytes/second. Similar results have been found on Scandisk™ SD products. 
         [0054]    According to a further aspect, the invention relates to formatting a storage device. The process of formatting involves writing the basic data structure which is to be used to store data on that device. Formatting is carried out by an application interacting with the kernel  22  of  FIG. 2 . The formatting application is able to set the number of file allocation tables in the storage device in a known manner. However, in certain circumstances, a single file allocation table is required, whereas in other circumstances, two file allocation tables will be required. Therefore the invention further relates to deciding whether one or two file allocation tables are required for a particular storage device. 
         [0055]    An embodiment of the invention is illustrated in  FIG. 4  which illustrates a process for formatting a storage device. In step  70 , the formatting application is started. This may occur as a result of a user input, or as a result of a system event such as the installation of the storage device and the detection that the storage device is unformatted. 
         [0056]    Steps  72  and  74  represent alternatives. A user may, in step  74 , specify the number of file allocation tables with which the storage device is to be formatted. Alternatively, the formatting application will determine a number of characteristics of the storage device in step  72 , as described below. 
         [0057]    On the basis of either the user input of step  74  or the characteristics determined in step  72 , the number of file allocation tables for that storage device are set in step  76 . In step  78 , the formatting application will format the storage device with the specified number of file allocation tables and with a predetermined file system. In the embodiments discussed herein the file system used is the FAT file system although it is to be realised that this aspect of the invention is equally applicable to other file systems using variable numbers of file allocation tables. 
         [0058]    A number of characteristics of the storage device concerned may be determined in step  72  and then used in step  76  to determine the number of file allocation tables with which the storage device is to be formatted. In one embodiment, where the formatting application is operating on a mobile computing device running the Symbian operating system, the formatting application will determine whether or not the storage device is removable, whether the device is a flash drive and whether the system on which the formatting application includes the ScanDisk application. If it is determined that the device drive is a fixed flash drive which is accessed by a flash translation layer and that the system includes the ScanDisk application, the formatting application will format the storage device with a single file allocation table. 
         [0059]    In further embodiments the determination of the number of file allocations in step  76  will be based on other characteristics determined in step  72 . A number of examples of determined characteristics for a storage device are discussed below. It is to be realised that the formatting application may use each characteristic on its own, or in combination with other characteristics, to determine the number of file allocation tables to write to the storage device. If the characteristics are used in combination, the formatting application could weigh certain determined characteristics in preference to others. 
         [0060]    Generally, a determination that the storage device is removable will tend to support the use of two file allocation tables with the device as it is assumed that both are required for the device to be compatible with applications and operating systems expecting two file allocation tables. 
         [0061]    If the storage device is shared by more than one operating system running on the same computing device, this would support the use of two file allocation tables on the assumption that the other operating system may require the presence of two file allocation tables. 
         [0062]    Furthermore, if the storage device is remotely accessible by a computing device other than the computing device to which the storage device is attached, and the remote operating system is capable of accessing the storage device in a raw format (i.e. without communicating with the operating system of the computing device to which the computing device is connected), it will support the assumption that two file allocation tables will be required. 
         [0063]    Further, if the storage device or the system on which the device is installed has no way of avoiding corrupted sectors (which functionality may, depending on device and system, be provided by device access software, or by the device itself) it will support the decision in step  78  to write two file allocation tables to the device. 
         [0064]    Alternatively, the decision of how many file allocation tables to include on the storage device is based on a data structure with which said storage device is provided. In other words if the storage device prior to formatting had a single file allocation table, this may be maintained in the formatting. Alternatively, if the storage device was provided with two file allocation tables, this too may be maintained in the subsequent formatting. 
         [0065]    Certain storage devices such as flash drives include management software which, for example, ensure write load-balancing across the device. Alternatively, this management software may be incorporated as part of a flash translation layer, or otherwise be provided as part of the device  10 , for example. The decision of how many file allocation tables to include may be based on how the management software operates. If this management software accesses two file allocation tables, the storage device will be formatted with two file allocation tables. 
         [0066]    However, in a further embodiment, each of the aforementioned considerations may be overridden by a user. The formatting application takes input from a user specifying the number of file allocation tables to use and the storage device is then formatted with the number of file allocation tables. 
         [0067]    It is to be realised that the storage device may be formatted at the place of manufacture and in this case, the manufacturer will determine the number of file allocation tables. 
         [0068]    In an alternative embodiment, the user&#39;s input of step  74  may be combined with the determined characteristics of step  72  so that the user&#39;s preference is overridden if it is determined that the storage device has certain characteristics. For example, if the user specifies in step  72  that the storage device is to have a single file allocation table, but the formatting application determines at step  72  that the device is not a flash drive, the formatting application will format the device with two file allocation tables.