METHOD FOR PERFORMING BLOCK MANAGEMENT/FLASH MEMORY MANAGEMENT, AND ASSOCIATED MEMORY DEVICE AND CONTROLLER THEREOF

[0001] 1. Field of the Invention

[0002] The present invention relates to access to a Flash memory having multiple channels, and more particularly, to a method for performing block management/Flash memory management, and to an associated memory device and a controller thereof

[0003] 2. Description of the Prior Art

[0004] As technologies of Flash memories progress in recent years, many kinds of portable memory devices (e.g. memory cards respectively complying with SD/MMC, CF, MS, and XD standards) or solid state drives (SSDs) equipped with Flash memories are widely implemented in various applications. Therefore, the control of access to Flash memories in these memory devices has become an important issue.

[0005] Taking NAND Flash memories as an example, they can mainly be divided into two types, i.e. Single Level Cell (SLC) Flash memories and Multiple Level Cell (MLC) Flash memories. Each transistor that is considered a memory cell in SLC Flash memories only has two charge levels that respectively represent a logical value 0 and a logical value 1. In addition, the storage capability of each transistor that is considered a memory cell in MLC Flash memories can be fully utilized. More specifically, the voltage for driving memory cells in the MLC Flash memories is typically higher than that in the SLC Flash memories, and different voltage levels can be applied to the memory cells in the MLC Flash memories in order to record information of at least two bits (e.g. binary values 00, 01, 11, or 10) in a transistor that is considered a memory cell. Theoretically, the storage density of the MLC Flash memories may reach twice the storage density of the SLC Flash memories, which is considered good news for NAND Flash memory manufacturers who encountered a bottleneck of NAND Flash technologies.

[0006] As MLC Flash memories are cheaper than SLC Flash memories, and are capable of providing higher capacity than SLC Flash memories while the space is limited, MLC Flash memories have been a main stream for implementation of most portable memory devices on the market. However, various problems of the MLC Flash memories have arisen due to their unstable characteristics. In order to ensure that the access control of a memory device over the Flash memory therein can comply with related standards, the controller of the Flash memory should have some handling mechanisms in order to properly handle its data access operations.

[0007] According to the related art, the memory device having the aforementioned handling mechanisms may still suffer from some deficiencies. For example, the handling mechanisms of the related art cannot ensure that the best processing performance of a Flash memory adapted to have multiple channel configurations can be achieved during a cleaning operation. More particularly, during a cleaning operation with regard to a meta block, if there is no valid page in a certain channel within the meta block, the channel is in an idle state when operations of moving/copying valid data of the meta block are performed. Therefore, a novel method is required for performing block management in order to achieve the best processing performance during a cleaning operation.

[0008] It is therefore an objective of the claimed invention to provide a method for performing block management/Flash memory management, and to provide an associated memory device and a controller thereof, in order to solve the above-mentioned problems.

[0009] It is another objective of the claimed invention to provide a method for performing block management/Flash memory management, and to provide an associated memory device and a controller thereof, in order to achieve the best processing performance during a cleaning operation.

[0010] According to a preferred embodiment of the present invention, a method for performing block management is provided. The method is applied to a controller of a Flash memory having multiple channels, wherein the Flash memory comprises a plurality of blocks respectively corresponding to the channels. The method comprises: selecting at least one meta block having at least one valid page as at least one candidate meta block for being cleaned, and accumulating respective valid page counts of blocks respectively corresponding to the channels within the at least one candidate meta block, in order to generate a plurality of accumulated values respectively corresponding to the channels, wherein blocks of each meta block respectively correspond to the channels, and data of each valid page comprises valid data; and when it is detected that all of the accumulated values reach a first threshold value, triggering a cleaning operation with regard to all candidate meta blocks, in order to simultaneously move/copy valid data respectively corresponding to the channels during the cleaning operation.

[0011] While the method mentioned above is disclosed, an associated memory device is further provided. The memory device comprises: a Flash memory having multiple channels, wherein the Flash memory comprises a plurality of blocks respectively corresponding to the channels; and a controller arranged to access the Flash memory and manage the plurality of blocks. In addition, the controller selects at least one meta block having at least one valid page as at least one candidate meta block for being cleaned, and accumulates respective valid page counts of blocks respectively corresponding to the channels within the at least one candidate meta block, in order to generate a plurality of accumulated values respectively corresponding to the channels, wherein blocks of each meta block respectively correspond to the channels, and data of each valid page comprises valid data. Additionally, when it is detected that all of the accumulated values reach a first threshold value, the controller triggers a cleaning operation with regard to all candidate meta blocks, in order to simultaneously move/copy valid data respectively corresponding to the channels during the cleaning operation.

[0012] While the method mentioned above is disclosed, a controller of a memory device is further provided. The controller is utilized for accessing a Flash memory having multiple channels, wherein the Flash memory comprises a plurality of blocks respectively corresponding to the channels. The controller comprises: a read only memory (ROM) arranged to store a program code; and a microprocessor arranged to execute the program code to control the access to the Flash memory and manage the plurality of blocks. In addition, the controller that executes the program code by utilizing the microprocessor selects at least one meta block having at least one valid page as at least one candidate meta block for being cleaned, and accumulates respective valid page counts of blocks respectively corresponding to the channels within the at least one candidate meta block, in order to generate a plurality of accumulated values respectively corresponding to the channels, wherein blocks of each meta block respectively correspond to the channels, and data of each valid page comprises valid data. Additionally, when it is detected that all of the accumulated values reach a first threshold value, the controller that executes the program code by utilizing the microprocessor triggers a cleaning operation with regard to all candidate meta blocks, in order to simultaneously move/copy valid data respectively corresponding to the channels during the cleaning operation.

[0013] These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.

[0020] Please refer to FIG. 1A, which illustrates a diagram of a memory device 100 according to a first embodiment of the present invention. In particular, the memory device 100 of this embodiment can be a portable memory device, examples of which may include, but not limited to, memory cards complying with SD/MMC, CF, MS, or XD standards. Here, the portable memory device is taken as an example of the memory device 100 in this embodiment. This is for illustrative purposes only, and is not meant to be a limitation of the present invention. According to a variation of this embodiment, the memory device 100 can be a solid state drive (SSD). Referring to FIG. 1A, the memory device 100 comprises a Flash memory 120 comprising at least one information block 120B, and further comprises a controller arranged to access the Flash memory 120, where the aforementioned controller of this embodiment is a memory controller 110. According to this embodiment, the memory controller 110 comprises a microprocessor 112, a read only memory (ROM) 112M, a control logic 114, at least one buffer memory 116, and an interface logic 118. In addition, the ROM 112M is arranged to store a program code 112C, and the microprocessor 112 is arranged to execute the program code 112C to control the access to the Flash memory 120. Please note that the program code 112C can be stored in the buffer memory 116 or any of various kinds of memories according to variations of this embodiment.

[0021] As shown in FIG. 1A, the aforementioned at least one information block 120B is utilized for storing information regarding block management, and more particularly, a valid page count table 120V and a clean list 120L. The valid page count table 120V can be utilized for recording valid page counts (in which a valid page count is the number of some valid pages) and valid page distribution (i.e. the distribution of valid pages) within the Flash memory 120. In practice, the valid page count table 120V can be recorded according to various kinds of methods. According to the valid page count table 120V, the aforementioned controller (e.g. the memory controller 110 that executes the program code 112C by utilizing the microprocessor 112) can rapidly obtain the respective valid page counts of respective blocks belonging to the same meta block. In addition, the clean list 120L can be utilized for dynamically recording the indexes of some meta blocks to be cleaned. When some condition(s) are satisfied, the controller can rapidly clean the meta blocks indicated by the indexes in the clean list 120L. For example, the controller can move/copy the valid data of these meta blocks to some other location in the Flash memory 120, and erase these meta blocks. Please note that any information stored in the information block 120B can be stored into the memory controller 110 (e.g. into the buffer memory 116), or into any memory that can be accessed by the memory controller 110. Here, storing the valid page count table 120V and the clean list 120L in the Flash memory 120 is taken as an example in this embodiment, and is not meant to be a limitation of the present invention. As long as the implementation of the present invention is not hindered, the valid page count table 120V and the clean list 120L can be stored in any memory that can be accessed by the memory controller 110, no matter whether this memory is within or outside the memory controller 110.

[0022] According to this embodiment, the buffer memory 116 comprises buffers Buf(0) and Buf(1). For example, the buffers Buf(0) and Buf(1) can be a plurality of buffering regions within the buffer memory 116, respectively. In another example, the aforementioned at least one buffer memory 116 comprises a plurality of buffer memories, which can be utilized for implementing the buffers Buf(0) and Buf(1), respectively. In this embodiment, the aforementioned controller (e.g. the memory controller 110 that executes the program code 112C by utilizing the microprocessor 112) can store a previous version of (a portion or all of) the valid page count table 120V into the Flash memory 120, and store a copy version of the valid page count table 120V into the buffer Buf(0), for use of dynamically updating the valid page count table 120V. In addition, the controller stores the latest version of the valid page count table 120V within the buffer Buf(0) into the Flash memory 120, in order to replace the previous version of the valid page count table 120V. Similarly, the controller can store a previous version of (a portion or all of) the clean list 120L into the Flash memory 120, and store a copy version of the clean list 120L into the buffer Buf(1), for use of dynamically updating the clean list 120L. In addition, the controller stores the latest version of the clean list 120L within the buffer Buf(1) into the Flash memory 120, in order to replace the previous version of the clean list 120L. This is for illustrative purposes only, and is not meant to be a limitation of the present invention. As long as the implementation of the present invention is not hindered, the aforementioned updating/replacing operations can be performed with regard to a portion or all of any of the valid page count table 120V and the clean list 120L.

[0023] FIG. 1B illustrates some implementation details of the memory device 100 shown in FIG. 1A according to an embodiment of the present invention. As shown in FIG. 1B, the aforementioned Flash memory 120 comprises a plurality of Flash memory chips such as the Flash memory chips 120-0, 120-1, . . . , and 120-(N - 1), where the Flash memory 120 communicates with the memory controller 110 through multiple channels such as the channels Ch(0), Ch(1), . . . , and Ch(N-1), and the Flash memory chips such as the Flash memory chips 120-0, 120-1, . . . , and 120-(N-1) respectively correspond to the channels. Typically, in addition to the aforementioned at least one information block 120B, the Flash memory 120 further comprises a plurality of blocks respectively corresponding to the channels, and when the controller (e.g. the memory controller 110 that executes the program code 112C by utilizing the microprocessor 112) performs data erasure operations on the Flash memory chips, the minimum erasure unit can be a block. In this embodiment, the controller performs data erasure operations in units of meta blocks, where a meta block may comprise some blocks respectively corresponding to the channels, and the controller can erase the Flash memory 120 with the meta block being a minimum erasure unit. For example, a meta block may comprise a block of the Flash memory chip 120-0, a block of the Flash memory chip 120-1, . . . , and a block of the Flash memory chip 120-(N-1). In addition, a block can be utilized for recording a specific amount of pages, and when the controller performs data writing operations on any of the Flash memory chips, the minimum writing unit can be a page.

[0024] In practice, the memory controller 110 that executes the program code 112C by utilizing the microprocessor 112 is capable of performing various control operations by utilizing the internal components within the memory controller 110. For example, the memory controller 110 utilizes the control logic 114 to control access to each Flash memory chip (e.g. operations of accessing at least one block or at least one page), utilizes the buffer memory 116 to perform buffering operations for the memory controller 110, and utilizes the interface logic 118 to communicate with a host device.

[0025] According to this embodiment, in addition to accessing the Flash memory 120, the memory controller 110 is capable of properly managing the plurality of blocks mentioned above. More specifically, the controller can dynamically select some meta block(s) as candidate meta block(s) for being cleaned, and dynamically update the clean list 120L, for reference of cleaning meta blocks. In addition, the controller can dynamically update the valid page count table 120V, for use of block management, and for reference of cleaning meta blocks. Referring to FIG. 2A, related details are further described as follows.

[0026] FIG. 2A is a flowchart of a method 910 for performing block management according to an embodiment of the present invention. The method can be applied to the memory device 100 in any of the respective embodiments shown in FIGS. 1A-1B (and associated variations), and more particularly, the controller of the Flash memory 120 adapted to have multiple channel configurations, such as the aforementioned controller (e.g. the memory controller 110 that executes the program code 112C by utilizing the microprocessor 112). The method is described as follows.

[0027] In Step 912, the controller selects at least one meta block M(m) having at least one valid page (e.g. a meta block M(m) or multiple meta blocks {M(m)}, where each meta block M(m) has at least one valid page) as at least one candidate meta block for being cleaned, and accumulates respective valid page counts {VPC_m(0), VPC_m(1), . . . , VPC_m(N-1)} of blocks respectively corresponding to the channels {Ch(0), Ch(1), . . . , Ch(N-1)} within the aforementioned at least one candidate meta block, in order to generate a plurality of accumulated values {VPC_A(0), VPC_A(1), . . . , VPC_A(N-1)} respectively corresponding to the channels {Ch(0), Ch(1), . . . , Ch(N-1)} , where the respective blocks of each meta block respectively correspond to the channels, and the data of each valid page comprises valid data. More particularly, within a plurality of meta blocks (e.g. some meta blocks continuously checked by the controller), the controller can select a meta block having the least number of valid pages as a candidate meta block, where the requirement that the meta block has at least one valid page should be satisfied. For example, in a situation where the aforementioned at least one meta block is merely one meta block M(m), the controller can select a meta block having the least number of valid pages from a batch of meta blocks that are continuously checked by the controller itself as a candidate meta block. In another example, in a situation where the aforementioned at least one meta block comprises multiple meta blocks {M(m)}, the controller can select a meta block having the least number of valid pages from a first batch of meta blocks that are continuously checked by the controller itself as a first candidate meta block, and further select a meta block having the least number of valid pages from a second batch of meta blocks that are continuously checked by the controller itself as a second candidate meta block, and the rest may be deduced by analogy.

[0028] According to this embodiment, the controller temporarily stores at least one index m of the aforementioned at least one meta block M(m) (e.g. an index m or multiple indexes {m}) in the clean list 120L, in order to label the aforementioned at least one meta block M(m) as the aforementioned at least one candidate meta block. For example, the aforementioned at least one index m corresponds to at least one set of physical block addresses. More particularly, the aforementioned at least one set of physical block addresses are physical block addresses of the respective blocks within the meta block M(m), where the relationship between the index m and the set of physical block addresses can be recorded according to various kinds of methods. This is for illustrative purposes only, and is not meant to be a limitation of the present invention. According to different variations of this embodiment, the aforementioned at least one index m may represent at least one set of physical block addresses and/or the aforementioned at least one index m may correspond to at least one set of physical block addresses. Please note that a set of physical block addresses disclosed above can be replaced by a set of logical block addresses, such as the logical block addresses that the respective blocks within the meta block M(m) correspond.

[0029] In Step 914, when it is detected that all of the accumulated values {VPC_A(0), VPC_A(1), . . . , VPC_A(N-1)} reach a first threshold value such as the threshold value TH_A, the controller triggers a cleaning operation with regard to all candidate meta blocks, in order to simultaneously move/copy the valid data respectively corresponding to the channels {Ch(0), Ch(1), . . . , Ch(N-1)} during the cleaning operation, and more particularly, to simultaneously move/copy the valid data to at least one other meta block during the cleaning operation. According to this embodiment, with regard to the utilization states of the respective meta blocks, the controller can utilize a data region and a spare region within the Flash memory 120 to classify the meta blocks, where the data region and the spare region represent (or correspond to) logical region division, rather than physical region division. Please note that any meta block that is classified into the data region is not a blank meta block, and any meta block that is classified into the spare region is a blank meta block. When a new blank meta block is needed (e.g. new blank meta block is utilized for being written/programmed), the controller can pop a meta block from the spare region to perform a writing operation. In order to prevent a problem that the number of meta blocks in the spare region is not enough, the controller can erase a portion of meta blocks in the data region in advance to re-classify these meta blocks into the spare region, where by performing the cleaning operation mentioned above, the controller can prevent any valid data (if exists) within these meta blocks from being lost, and more particularly, prevent the valid data within the candidate meta blocks mentioned above from being lost.

[0030] For example, the indexes {m} of the candidate meta blocks are temporarily stored in the clean list 120L, respectively. When it is detected that all of the accumulated values {VPC_A(0), VPC_A(1), . . . , VPC_A(N-1)} reach the threshold value TH_A, the controller triggers the cleaning operation with regard to the candidate meta blocks indicated by the indexes {m}, in order to simultaneously move/copy the valid data respectively corresponding to the channels {Ch(0), Ch(1), . . . , Ch(N-1)} during the cleaning operation. As a result, by simultaneously performing the moving/copying operations of the respective channels {Ch(0), Ch(1), . . . , Ch(N-1)} , the present invention can efficiently move/copy all of the valid data involved with the cleaning operation, so the ratio R_ALL—CH of the time T_ALL—CH that the moving/copying operations of the respective channels are performed at the same time to the total time T_TOTAL of all of the moving/copying operations involved with the cleaning operation in this embodiment can be much greater than that of the related art, and more particularly, can even approach 1 in an ideal situation. Therefore, the present invention can greatly enhance the overall processing performance of the memory device.

[0031] In practice, after triggering the cleaning operation, the controller can remove the aforementioned at least one index m from the clean list 120L, and more particularly, remove the index {m} of the candidate meta blocks mentioned in Step 914 from the clean list 120L. After moving/copying the valid data to other location(s) (e.g. other meta block(s)), the controller can erase the candidate meta blocks to generate new blank meta blocks, where each block within the new blank meta blocks is a blank block. In addition, the threshold value TH_A can be a fixed value. This is for illustrative purposes only, and is not meant to be a limitation of the present invention. According to different variations of this embodiment, the controller can set the threshold value TH_A in advance and/or dynamically adjust the threshold value TH_A, in order to control the aforementioned ratio R_ALL—CH.

[0032] According to a variation of this embodiment, the controller can perform a partially cleaning operation, i.e. the cleaning operation performed in a situation where the threshold value TH_A is less than the number of pages in a block. For example, the threshold value TH_A can be a fixed value that is less than the number of pages in a block. In another example, the controller can dynamically adjust the threshold value TH_A, and more particularly, dynamically adjust the threshold value TH_A to happen to be a certain value at present, where the value is less than the number of pages in a block. Typically, during moving/copying the valid data to other location(s) (e.g. other meta block(s)), the controller can merely wait, and the host device is typically forced to wait and cannot access the memory device 100 right away, so if the threshold value TH_A is too high, the performance of the controller may be degraded and accessing the memory device 100 by the controller may be hindered.

[0033] Please note that the valid page count of the meta block M(m) represented by the index m stored in the clean list 120L may probably be decreased as some subsequent writing operations are performed. More particularly, in a situation where the threshold value TH_A is less than the number of pages in a block, the probability that the valid page count of the meta block M(m) represented by the index m stored in the clean list 120L is decreased may be increased. Although the meta block M(m) represented by the index m indeed has at least one valid page when the controller temporarily stores the index m in the clean list 120L, this by no means implies that the aforementioned at least one valid page will never become at least one invalid page. For example, after the subsequent writing operations mentioned above are performed, the data of the aforementioned at least one valid page will probably become invalid data due to the corresponding update data being written, which may cause the meta block M(m) to become a meta block that has no valid page.

[0034] FIG. 2B illustrates some implementation details of the method shown in FIG. 2A according to an embodiment of the present invention. The working flow shown in FIG. 2B is described as follows.

[0035] In Step 932, the controller selects a meta block having at least one valid page, such as the meta block M(m), as a candidate meta block for being cleaned, and accumulates the respective valid page counts {VPC_m(0), VPC_m(1), . . . , VPC_m(N-1)} of blocks respectively corresponding to the channels {Ch(0), Ch(1), . . . , Ch(N-1)} within the candidate meta block to the accumulated values {VPC_A(0), VPC_A(1), . . . , VPC_A(N-1)}, respectively.

[0036] In Step 933, the controller checks whether all of the accumulated values {VPC_A(0), VPC_A(1), . . . , VPC_A(N-1)} reach the first threshold value such as the threshold value TH_A. When it is detected that all of the accumulated values {VPC_A(0), VPC_A(1), . . . , VPC_A(N-1)} reach the first threshold value, Step 934 is entered; otherwise, Step 932 is re-entered.

[0037] In Step 934, the controller triggers a cleaning operation with regard to all candidate meta blocks, in order to simultaneously move/copy the valid data respectively corresponding to the channels {Ch(0), Ch(1), . . . , Ch(N-1)} during the cleaning operation.

[0038] According to this embodiment, the controller can decrease the ratio of the idle time of at least one idle channel (i.e., the time that one or more channels are in their idle states) to the overall time of the cleaning operation as best it can. More particularly, the controller can simultaneously move/copy the valid data respectively corresponding to the channels {Ch(0), Ch(1), . . . , Ch(N-1)} during the cleaning operation, so the aforementioned ratio R_ALL—CH in this embodiment can be much greater than that of the related art. Therefore, the present invention can greatly enhance the overall processing performance of the memory device.

[0039] According to some embodiments, such as the embodiment shown in FIGS. 3A-3D and the embodiment shown in FIGS. 4A-4B, in a situation where the number N of the channels Ch(0), Ch(1), . . . , and Ch(N-1) is equal to four, the aforementioned valid page counts {VPC_m(0), VPC_m(1), . . . , VPC_A(N-1)} and the aforementioned accumulated values {VPC_A(0), VPC_A(1), . . . , VPC_A(N-1)} can be written as {VPC_m(0), VPC_m(1), VPC_m(2), VPC_m(3)} and {VPC_A(0), VPC_A(1), VPC_A(2), VPC_A(3)}, respectively.

[0040] FIGS. 3A-3D respectively illustrate some candidate meta blocks M(m₁₁), M(m₁₂), M(m₁₃), and M(m₁₄) involved with the method 910 shown in FIG. 2A according to an embodiment of the present invention, where the shaded portions in the respective candidate meta blocks represent valid pages. For better comprehension, this embodiment is described according to the working flow shown in FIG. 2B.

[0041] First of all, after Step 932 is first entered, the controller selects the candidate meta block M(m₁₁) shown in FIG. 3A, with the respective valid page counts {VPC_m11(0), VPC_m11(1), VPC_m11(2), VPC_m11(3)} thereof being (40, 0, 0, 0), so the controller accumulates the accumulated values {VPC_A(0), VPC_A(1), VPC_A(2), VPC_A(3)} by changing them from a set of initial values (0, 0, 0, 0) to (40, 0, 0, 0). In this embodiment, the threshold value TH_A is equal to 20 (i.e. TH_A=20). As the accumulated values {VPC_A(0), VPC_A(1), VPC_A(2), VPC_A(3)} do not entirely reach the threshold value TH_A, Step 932 is re-entered.

[0042] Next, the controller selects the candidate meta block M(m₁₂) shown in FIG. 3B, with the respective valid page counts {VPC_m12(0), VPC_m12(1), VPC_m12(2), VPC_m12(3)} thereof being (0, 28, 0, 0), so the controller accumulates the accumulated values {VPC_A(0), VPC_A(1), VPC_A(2), VPC_A(3)} by changing them from (40, 0, 0, 0) to (40, 28, 0, 0). As the accumulated values {VPC_A(0), VPC_A(1), VPC_A(2), VPC_A(3)} do not entirely reach the threshold value TH_A, Step 932 is re-entered.

[0043] Afterward, the controller selects the candidate meta block M(m₁₃) shown in FIG. 3C, with the respective valid page counts {VPC_m13(0), VPC_m13(1), VPC_m13(2), VPC_m13(3)} thereof being (0, 0, 39, 0), so the controller accumulates the accumulated values {VPC_A(0), VPC_A(1), VPC_A(2), VPC_A(3)} by changing them from (40, 28, 0, 0) to (40, 28, 39, 0). As the accumulated values {VPC_A(0), VPC_A(l), VPC_A(2), VPC_A(3)} do not entirely reach the threshold value TH_A, Step 932 is re-entered.

[0044] In addition, the controller selects the candidate meta block M(m₁₄) shown in FIG. 3D, with the respective valid page counts {VPC_m14(0), VPC_m14(1), VPC_m14(2), VPC_m14(3)} thereof being (0, 0, 0, 57), so the controller accumulates the accumulated values {VPC_A(0), VPC_A(l), VPC_A(2), VPC_A(3)} by changing them from (40, 28, 39, 0) to (40, 28, 39, 57). As the accumulated values {VPC_A(0), VPC_A(1), VPC_A(2), VPC_A(3)} do entirely reach the threshold value TH_A, Step 934 is entered.

[0045] Please note that, as the controller can simultaneously move/copy the valid data respectively corresponding to the channels {Ch(0), Ch(1), Ch(2), Ch(3)} during the cleaning operation, the aforementioned ratio R_{ALL CH} in this embodiment can be much greater than that of the related art. Therefore, the present invention can greatly enhance the overall processing performance of the memory device.

[0046] FIGS. 4A-4B respectively illustrate some candidate meta blocks M(m₂₁) and M(m₂₂) involved with the method 910 shown in FIG. 2A according to another embodiment of the present invention, where the shaded portions in the respective candidate meta blocks represent valid pages. For better comprehension, this embodiment is described according to the working flow shown in FIG. 2B.

[0047] First of all, after Step 932 is first entered, the controller selects the candidate meta block M(m₂₁) shown in FIG. 4A, with the respective valid page counts {VPC_m21(0), VPC_m21(1), VPC_m21(2), VPC_m21(3)} thereof being (10, 60, 28, 0), so the controller accumulates the accumulated values {VPC_A(0), VPC_A(1), VPC_A(2), VPC_A(3)} by changing them from a set of initial values (0, 0, 0, 0) to (10, 60, 28, 0). Please note that, when selecting a meta block as a candidate meta block such as the candidate meta block M(m₂₁), the controller can determine (more particularly, determine which meta block should be selected) according to the valid page counts. For example, the controller can refer to the valid page count table 120V, in order to select a meta block having the least number of valid pages as the initial candidate meta block M(m₂₁). In this embodiment, the threshold value TH_A is equal to 20 (i.e. TH_A=20). As the accumulated values {VPC_A(0), VPC_A(1), VPC_A(2), VPC_A(3)} do not entirely reach the threshold value TH_A, Step 932 is re-entered.

[0048] According to this embodiment, when it is detected that a difference VPC_{A DIFF} between a maximum accumulated value VPC_A—MAXand a minimum accumulated value VPC_A—MIN within the accumulated values {VPC_A(0), VPC_A(1), VPC_A(2), VPC_A(3)} reaches a second threshold value such as the threshold value TH_D, the controller temporarily selects a meta block having compensation characteristics as a candidate meta block, where the maximum accumulated value VPC_{A MAX} and the minimum accumulated value VPC_{A MIN} respectively correspond to two channels of the channels {Ch(0), Ch(1), Ch(2), Ch(3)}, with the two channels being temporarily referred to as a maximum channel Ch_MAX and a minimum channel Ch_MIN at this moment, respectively. On the contrary, within the meta block having the compensation characteristics, the valid page count of the block corresponding to the maximum channel Ch_MAX is less than the valid page count of the block corresponding to the minimum channel Ch_MIN. More particularly, when it is detected that the difference VPC_{A DIFF} between the maximum accumulated value VPC_{A MAX} and the minimum accumulated value VPC_{A MIN} within the accumulated values {VPC_A(0), VPC_A(1), VPC_A(2), VPC_A(3)} reach the second threshold value such as the threshold value TH_D, the controller temporarily prevents selecting any meta block having no compensation characteristic as a candidate meta block, where within the meta block having no compensation characteristic, the valid page count of the block corresponding to the maximum channel Ch_MAX is greater than or equal to the valid page count of the block corresponding to the minimum channel Ch_MIN.

[0049] In this embodiment, the threshold value TH_D is equal to 40 (i.e. TH_D=40). After the controller selects the candidate meta block M(m₂₁), the maximum accumulated value VPC_A—MAX and the minimum accumulated value VPC_{A MIN} are respectively the values 60 and 0 within (10, 60, 28, 0), so the maximum channel Ch_MAX and the minimum channel Ch_MIN are respectively the channels Ch(1) and Ch(3), and the difference VPC_A—DIFF is equal to 60. In this situation, VPC_A—DIFF>TH_D, which means the next meta block to be selected (e.g. a meta block M(m) under consideration) should have compensation characteristics. In practice, within the meta block M(m) under consideration, when the valid page count VPC_m(1) of a block corresponding to the channel Ch(1) is less than the valid page count VPC_m(3) of a block corresponding to the channel Ch(3) (i.e. VPC_m(1) <VPC_m(3)), the meta block M(m) to be selected can be regarded as a meta block having compensation characteristics.

[0050] As a result, the controller selects the candidate meta block M(m₂₂) shown in FIG. 4B, with the respective valid page counts {VPC_m22(0), VPC_m22(1), VPC_m22(2), VPC_m22(3)} thereof being (50, 11, 20, 35), so the controller accumulates the accumulated values {VPC_A(0), VPC_A(l), VPC_A(2), VPC_A(3)} by changing them from (10, 60, 28, 0) to (60, 71, 48, 35). As the accumulated values {VPC_A(0), VPC_A(1), VPC_A(2), VPC_A(3)} do entirely reach the threshold value TH_A, Step 934 is entered.

[0051] Please note that, as the controller can simultaneously move/copy the valid data respectively corresponding to the channels {Ch(0), Ch(1), Ch(2), Ch(3)} during the cleaning operation, the aforementioned ratio R_{ALL CH} in this embodiment can be much greater than that of the related art. In addition, the controller can compensate the accumulated values by utilizing the meta blocks having compensation characteristics, in order to control the differences between the accumulated values {VPC_A(0), VPC_A(1), VPC_A(2), VPC_A(3)} to be within a predetermined range (e.g. the range defined by the second threshold value such as the threshold value TH_D), so the aforementioned ratio R_{ALL CH} in this embodiment can be much greater than that of the related art. Therefore, the present invention can greatly enhance the overall processing performance of the memory device.

[0052] In the embodiment shown in FIGS. 4A-4B, the threshold value TH_D can be a fixed value. This is for illustrative purposes only, and is not meant to be a limitation of the present invention. According to different variations of this embodiment, the controller can set the threshold value TH_D in advance and/or dynamically adjust the threshold value TH_D, in order to control the aforementioned ratio R_{ALL CH}.

[0053] It is an advantage of the present invention that the block management implemented according to the present invention can ensure that the best processing performance of a Flash memory adapted to have multiple channel configurations can be achieved during a cleaning operation. In contrast to the related art, in a situation where the chip area and the associated costs are not greatly increased, the present invention can achieve the goal of giving consideration to both operation performance and system resource management.

[0054] Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.