Abstract:
A garbage collection method for a flash memory is provided. The flash memory includes a spare block pool and a data block pool, wherein the spare block pool includes spare blocks and the data block pool includes data blocks. The method includes the steps of: receiving target data from a host and writing the target data to a current data block of the data blocks; sorting an erase count of each data block when performing a wear-leveling process to write the target data; sorting a valid page number of each first block when it is determined that at least two first blocks in the data blocks have the smallest erase count; and selecting a second block having a smallest valid page number from the first blocks and writing valid pages of the second block to one of the spare blocks to perform a data cleaning process.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This Application claims priority of Taiwan Patent Application No. 103108601, filed on Mar. 12, 2014, the entirety of which is incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a flash memory, and in particular, to a data storage device having a flash memory and a garbage collection method for a flash memory. 
     2. Description of the Related Art 
     Flash memory is a common non-volatile data storage device, which can be erased and programmed electrically. For example, an NAND flash memory is usually for use in a memory card, a USB flash device, a solid-state disk (SSD), an embedded multimedia card, etc. 
     A storage array of a flash memory may comprise a plurality of blocks, and each block comprises a plurality of pages. When a used block is to be released as a spare block, all pages of the used block have to be erased in the operation. The technique of garbage collection is used to move the valid data located in various blocks to a spare space, so that the blocks only contain invalid data and can be erased and released as spare blocks. 
     However, a lot of resources and computations are required when using conventional garbage collection methods, resulting in incapability of performing a wear-leveling process on flash memory. 
     BRIEF SUMMARY OF THE INVENTION 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
     In an exemplary embodiment, a data storage device is provided. The data storage device is coupled to a host. The data storage device comprises: a flash memory, comprising a spare block pool, and a data block pool, wherein the spare block pool comprises a plurality of spare blocks, and the data block pool comprises a plurality of data blocks; and a controller, receiving target data from the host and writing the target data to a current data block of the plurality of data blocks, wherein when the controller determines that a wear-leveling process needs to be performed to write the target data, the controller sorts an erase count of each data block, wherein when the controller determines that no two blocks in the data blocks simultaneously have the same smallest erase count, the controller further determines whether the difference of the erase count corresponding to at least one first block in the data blocks and the smallest erase count is less than a first threshold value, wherein when the difference between the erase count corresponding to at least one first block in the data blocks and the smallest erase count is less than a first threshold value, the controller selects a second block having a smallest valid page number from the at least one first block to perform a data cleaning process, thereby writing valid pages of the second block to one of the spare blocks. 
     In another exemplary embodiment, a garbage collection method for a flash memory is provided. The flash memory is coupled to a host, and the flash memory comprises a spare block pool and a data block pool, wherein the spare block pool comprises a plurality of spare blocks and the data block pool comprises a plurality of data blocks. The garbage collection method comprises the steps of: receiving target data from the host and writing the target data to a current data block of the data blocks; sorting an erase count of each data block when performing a wear-leveling process to write the target data; determining whether the difference of the erase count corresponding to at least one first block in the data blocks and the smallest erase count is less than a first threshold value when it is determined that none of any two blocks in the data blocks have the smallest erase count simultaneously; and selecting a second block having a smallest valid page number from the at least one first block to perform a data cleaning process, thereby writing valid pages of the second block to one of the spare blocks when the difference between the erase count corresponding to at least one first block in the data blocks and the smallest erase count is less than a first threshold value. 
     In yet another exemplary embodiment, a data storage device is provided. The data storage device is coupled to a host. The data storage device comprises: a flash memory, comprising a spare block pool, and a data block pool, wherein the spare block pool comprises a plurality of spare blocks, and the data block pool comprises a plurality of data blocks; and a controller, receiving target data from the host and writing the target data to a current data block of the plurality of data blocks, wherein when the controller determines that a wear-leveling process is to be performed to write the target data, the controller sorts an erase count of each data block, wherein when the controller determines that at least two blocks in the data blocks have the smallest erase count simultaneously, the controller further sorts a valid page number of each first block, selects a second block having the smallest valid page number from the first blocks, and writes valid pages of the second block to one of the spare blocks to perform a data cleaning process. 
     In yet another embodiment, a garbage collection method for a flash memory is provided. The flash memory is coupled to a host, and the flash memory comprises a spare block pool and a data block pool, wherein the spare block pool comprises a plurality of spare blocks and the data block pool comprises a plurality of data blocks. The garbage collection method comprises the steps of: receiving target data from the host and writing the target data to a current data block of the data blocks; sorting an erase count of each data block when performing a wear-leveling process to write the target data; sorting a valid page number of each first block when it is determined that at least two first blocks in the data blocks have the smallest erase count simultaneously; and selecting a second block having a smallest valid page number from the at least two first blocks and writing valid pages of the second block to one of the spare blocks to perform a data cleaning process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG. 1  is a block diagram of an electronic device in accordance with an embodiment of the invention; and 
         FIGS. 2A and 2B  are portions of a flow chart illustrating a garbage collection method of a flash memory in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
       FIG. 1  is a block diagram of an electronic device in accordance with an embodiment of the invention. The electronic device  100  may comprise a host  110  and a data storage device  120 . The data storage device  120  may comprise a flash memory  130  and a controller  140 , wherein the controller  140  may control access to the flash memory  130  according to the commands from the host  110 . The controller  140  may comprise a computation unit  142  and a memory unit  144  (e.g. a read-only memory (ROM)). The program codes and data stored in the memory unit  144  may be firmware executed by the computation unit  142 , so that the controller  140  may control the flash memory  130  based on the firmware. The flash memory  130  may comprise a plurality of blocks, and each block comprises a plurality of pages. 
     In an embodiment, the flash memory  130  may comprise a spare block pool  150 , and a data block pool  160 . The spare block pool  150  comprises a plurality of spare blocks  151 ˜ 15   n  for storing invalid data. The data block pool  160  comprise a plurality of data blocks  161 ˜ 16   m  for storing data. In an embodiment, the controller  140  may manage the blocks of the flash memory  130  according to the commands from the host  110 . A block in the flash memory  130  is assigned based on a physical address, and the host  110  may assign a block based on a logical address. Accordingly, the controller  140  has to convert the logical address from the host  110  to a physical address. In an embodiment, the controller  140  may record the relationship between the logical addresses and physical addresses into an address link table. 
     In an embodiment, each of the data blocks  161 ˜ 16   m  may comprise a plurality of pages. When data is stored in a page of the data blocks, the page can be regarded as a data page. When the page has a corresponding logical address, the page can be regarded as a valid page. In an embodiment, the controller  140  may calculate the number of total valid pages in each of the data blocks  161 ˜ 16   m  to obtain a valid page number, and record the valid page number of the data blocks  161 ˜ 16   m  to a valid number table. In addition, the erase count of each block may indicate the number of erasing operations performed on the block. In an embodiment, the controller  140  may record the erase count of each block in the flash memory  130  in an erase count table. In an embodiment, the flash memory  130  may further comprise the valid number table and the erase count table (not shown in FIG.  1 ). 
     In an embodiment, when the host  110  writes data to the data storage device  120  consecutively, the number of valid spare blocks in the spare block pool  150  of the flash memory  130  may be small. Afterwards, the host  110  may further read data of several pages from the data storage device  120  repeatedly, and re-supply power to the data storage device  120  after interrupting power to the data storage device  120  (i.e. power cycling). When a power cycling operation occurs, the controller  140  may determine whether the number of spare blocks is lower than a threshold value. When the number of spare blocks is lower than the threshold value, the controller  140  may start performing garbage collection. 
     Specifically, before starting to perform garbage collection, the controller  140  has to determine a clean source block from the data blocks, and determine a clean destination block from the spare blocks. When determining the clean source block, the controller  140  may sort the erase count of each block, and select the block having the smallest erase count as the clean source block. If there are several blocks having the smallest erase count simultaneously, the controller  140  may select the first block having the smallest erase count in the order. However, if the selected clean source block has too many valid pages, the spare blocks obtained in the garbage collection operation may be fewer. In the worst case, the controller  140  has to perform many garbage collection operations to obtain a complete spare block, resulting in low performance of the data storage device  120 . 
     In another embodiment, when determining the clean source block, the controller  140  may sort the erase count of each block, and select the block having the smallest erase count as the clean source block. If there are several blocks having the smallest erase count simultaneously, the controller  140  may further sort the blocks having the smallest erase count based on their valid page number, and select the block having both the smallest erase count and smallest valid page number as the clean source block. Accordingly, the block having a larger number of valid pages will be less likely selected, and the garbage collection operation may become more effective, resulting in better performance of the wear leveling process. 
     In yet another embodiment, when determining the clean source block, the controller  140  may sort the erase count of each block, and select the block having the smallest erase count as the clean source block. If there are several blocks having the smallest erase count simultaneously, the controller  140  may further sort the blocks having the smallest erase count based on their valid page number. The difference between the embodiment and the aforementioned embodiment is that the controller  140  may firstly select several first blocks, wherein the difference of the erase count of the selected first blocks and the smallest erase count of the blocks is less than a threshold value (e.g. 3). Afterwards, the controller  140  may sort the valid page number of the selected first blocks, and select a second block having the smallest valid page number from the selected first blocks as the clean destination block, resulting in more effective garbage collection. 
       FIGS. 2A and 2B  are portions of a flow chart illustrating a garbage collection method of a flash memory in accordance with an embodiment of the invention. As illustrated in  FIG. 2A  and  FIG. 2B , in step S 202 , the controller  140  may retrieve at least one spare block from a spare block pool. In step S 204 , the controller  140  may receive the data sent from the host  110 , and write the data into a current data block. In step S 206 , the controller may perform a clean check operation to check the data storage status of each block in the flash memory. In step S 208 , the controller  140  may determine whether the number of spare blocks is less than a first threshold value. If so, step S 210  is performed. Otherwise, the flow ends. In step S 210 , the controller  140  determines whether to perform a wear-leveling process. If so, step S 214  is performed. Otherwise, step S 212  is performed. In step S 212 , the controller  140  may set data moving information for the wear-leveling process, wherein the data moving information is determined by selecting the block having the smallest valid page number as the clean source block, and the clean destination block is determined by using the block index. In step S 214 , the controller  140  may sort the erase count of each block of the flash memory  130  in an ascending order. In step S 216 , the controller  140  may determine whether multiple first blocks have the same smallest erase count. If so, step S 222  is performed. Otherwise, step S 218  is performed. 
     In step S 218 , the controller may determine whether the difference of the erase count of at least one first block and the smallest erase count of the blocks is less than a second threshold value. If so, step S 220  is performed. Otherwise, step S 224  is performed. In step S 220 , the controller  140  may set the data moving information for the wear-leveling process, wherein the data moving information comprises the clean source block having the smallest valid page number, and the difference of the erase count of the clean source block and the smallest erase count is less than the second threshold value, and the clean destination block is determined by the index of the current spare block. In step S 222 , the controller  222  may order the valid page number of each first block in an ascending order. In step S 224 , the controller  140  may set the data moving information for the wear-leveling process, wherein the data moving information comprises a clean source block having the smallest erase count, and the clean destination block is determined by the index of the current spare block. 
     In step S 226 , the controller  140  may set the data moving information for the wear-leveling process, wherein the data moving information comprises a clean source block having the smallest valid page number and the smallest erase count, and the block having the largest erase count is determined as the clean destination block. In step S 230 , the controller  140  may perform a data cleaning process to move data in the flash memory  130  based on the data moving information. 
     While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.