NON-VOLATILE STORAGE SYSTEM COMPENSATING PRIOR PROBABILITY FOR LOW-DENSITY PARITY CHECK CODES

• US 20130111299A1
• Filed: 11/01/2011
• Published: 05/02/2013
• Est. Priority Date: 11/01/2011
• Status: Active Grant

• Current Assignee: Hitachi, Ltd.
• Original Assignee: Hitachi, Ltd.
• Inventors: Hashimoto, Akiyoshi, Suzuki, Akifumi, Tsunehiro, Takashi
• Granted Patent: US 8,719,668 B2
• Time in Patent Office: Days
• US Class Current: 714/758
• CPC Class Codes: G06F 11/1068 in sector programmable memories, e.g. flash disk G06F11/1072 takes precedence
• CPC Class Codes: G06F 11/108 Parity data distribution in semiconductor storages, e.g. in SSD

Abstract

A nonvolatile storage system has multiple nonvolatile storage media and a controller coupled to the multiple nonvolatile storage media. The controller has a storage area for storing management information including probability management information denoting error probability information of a unit physical area in a nonvolatile storage medium, and an error correcting circuit for carrying out coding and decoding by a low density parity check code. An error probability is a probability that data within the unit physical area is incorrect, and is a prior probability, which is the probability of prior to this data having been read. The controller, in a data read process, identifies based on management information an error probability, which is the error probability of a unit physical area including the physical area of the read source and is better suited to the state of the read source in the current read process than the error probability identified for the same unit physical area in a past read process, and uses the identified error probability to correct the read data using the error correcting circuit.

Claims

1. A nonvolatile storage system, comprising:

multiple nonvolatile storage media; and

a controller, which is coupled to the multiple nonvolatile storage media, wherein the controller includes;

a storage area for storing management information, which includes probability management information denoting error probability information of a unit physical area of the nonvolatile storage medium; and

an error correcting circuit for performing coding and decoding by a low density parity check code,

the error probability is a probability that data within the unit physical area is incorrect, and is a prior probability, which is the probability of prior to this data having been read, and

the controller, in a data read process;

(A) reads data from a read-source physical area in a read-source non-volatile storage medium;

(B) identifies from the management information an error probability, which is the error probability of a unit physical area including the read-source physical area and is better suited to the state of the read source in the current read process than the error probability identified for the same unit physical area in a past read process; and

(C) uses the error probability identified in the (B) to correct the data read in the (A) using the error correcting circuit.

2. A nonvolatile storage system according to claim 1, comprising:

multiple nonvolatile storage devices configuring a RAID group; and

a storage controller, which is coupled to the multiple nonvolatile storage devices,

wherein each of the nonvolatile storage devices has the nonvolatile storage medium and the controller, and

(a) the read-source nonvolatile storage device detects that the (C) has failed and there is an error that cannot be corrected,

(b) the read-source nonvolatile storage device sends failure occurrence information denoting that a failure has occurred to the storage controller,

(c) the read-source nonvolatile storage device sends free information denoting whether or not there is a free physical area in the read-source nonvolatile storage device to the storage controller,

(d) the storage controller receives the free information,

(e) the storage controller determines whether or not the free information denotes the existence of a free physical area, and

(f) in a case where the result of the determination in the (e) is affirmative, the storage controller reads data from nonvolatile storage devices in the RAID group other than the read-source nonvolatile storage device, constructs data stored in the read-source nonvolatile storage device based on the data that has been read, and sends the constructed data to the read-source nonvolatile storage device, in accordance with which this data is stored in the free physical area of the read-source nonvolatile storage device.

3. A nonvolatile storage system according to claim 2, wherein the storage controller, upon receiving the failure occurrence information, sends a failure information request to the read-source nonvolatile storage device,

the read-source nonvolatile storage device, in response to this request, sends failure information and the free information in the (d) to the storage controller, and

the failure information includes information denoting the read-source nonvolatile storage medium of the data with the uncorrectable error.

4. A nonvolatile storage system according to claim 3, wherein the non-volatile storage device is a nonvolatile memory device,

the nonvolatile storage medium is an m-value multi level cell flash memory,

each flash memory is configured from multiple blocks,

each of the blocks is configured from multiple pages,

the physical area and the unit physical area are pages,

the probability management information denotes error probability information for each set of pages for each flash memory, and

the set of pages is a set of pages for which the remainders obtained by dividing the page numbers by m are identical.

5. A nonvolatile storage system according to claim 4, wherein the error probability information is a value denoting an error probability, the controller, in a data read process:

(D) computes an error probability corresponding to a set of pages, which includes the read-source page, by dividing the number of bits corrected in the (C) by the size of the data that has been read in the (A); and

(E) writes the error probability computed in the (D) to the probability management information as the error probability corresponding to the set of pages that includes the read-source page, and

in accordance with this, the error probability identified in the (B) in the next data read process, for which the same set of pages as the read-source set of pages is the read source, constitutes the error probability that is suited to the state of the read-source set of pages in the next data read process.

6. A nonvolatile storage system according to claim 4, wherein the probability management information includes a last data writing time, which is the time at which the data has been last written, and a time-dependent parameter, which is a parameter that depends on time, for each set of pages,

the error probability information is the time-dependent parameter,

the controller, in a case where data has been written to a page, writes the time at which this data has been written to the page in the probability management information as a last data writing time corresponding to the set of pages that includes the programming-destination page,

the controller, in the (B) of the data read process, identifies on the basis of the management information the last data writing time and the time-dependent parameter corresponding to the set of pages that includes the read-source page, computes an elapsed time from the identified last data writing time to the read time, which is the time of the data read process, and computes the error probability corresponding to the set of pages that includes the read-source page by using the identified time-dependent parameter and the computed elapsed time, and

the controller, in the data read process:

(D) computes the error probability corresponding to the set of pages that includes the read-source page by dividing the number of bits corrected in the (C) by the size of the data that has been read in the (A); and

(E) uses the error probability computed in the (B), the error probability computed in the (D), the last data writing time used in the (B), and the read time used in the (B) to compute the time-dependent parameter corresponding to the set of pages that includes the read-source page, and writes the computed time-dependent parameter to the probability management information as the time-dependent parameter corresponding to the set of pages that includes the read-source page, and in accordance with this, the error probability identified in the (B) in the next data read process, for which the same set of pages as the read-source set of pages is the read source, constitutes the error probability that is suited to the state of the read-source set of pages in the next data read process.

7. A nonvolatile storage system according to claim 3, wherein the probability management information includes an error probability for each combination of one or more types of parameters, which have an impact on the error probability,

the controller, in the (B), identifies the error probability corresponding to a value of the one or more types of parameters in the data read process, and

in accordance with this, the error probability identified in the (B) of this data read process constitutes the error probability that is better suited to the read-source page than the error probability identified in the (B) of a past data read process in which the same page as the read source page is the read source.

8. A nonvolatile storage system according to claim 7, wherein the one or more types of parameters is one or more types from among the number of data writing and/or the number of data erases for each block, the number of data reads for each block, the last data writing time for each block, and the temperature of the flash memory.

9. A nonvolatile storage system according to claim 1, wherein the error probability information is a value denoting an error probability,

the controller, in a data read process:

(D) computes the error probability corresponding to the unit physical area including the read-source physical area by dividing the number of bits corrected in the (C) by the size of the data read in the (A); and

(E) writes the error probability computed in the (D) to the probability management information as the error probability corresponding to the unit physical area including the read-source physical area, and

in accordance with this, the error probability identified in the (B) in the next data read process for which the same unit physical area as the read-source unit physical area is the read source, constitutes the error probability that is suited to state of the read-source physical area of the next data read process.

10. A nonvolatile storage system according to claim 9, wherein the non-volatile storage device is a nonvolatile memory device,

the nonvolatile storage medium is an m-value multilevel cell flash memory,

each flash memory is configured from multiple blocks,

each of the blocks is configured from multiple pages,

the physical area and the unit physical area are pages,

the probability management information denotes error probability information for each set of pages for each flash memory, and

the set of pages is a set of pages for which the remainders obtained by dividing the page numbers by m are identical.

11. A nonvolatile storage system according to claim 1, wherein the probability management information includes a last data writing time, which is the time at which the data has been last written, and a time-dependent parameter, which is a parameter that depends on time, for unit physical area,

the error probability information is the time-dependent parameter,

the controller, in a case where data has been written to a physical area, writes the time at which this data has been written to the physical area in the probability management information as the last data writing time corresponding to the unit physical area that includes the writing-destination physical area,

the controller, in the (B) of the data read process, identifies on the basis of the management information the last data writing time and the time-dependent parameter corresponding to the unit physical area that includes the read-source physical area, computes the elapsed time from the identified last data writing time to the read time, which is the time of the data read process, and computes the error probability corresponding to the read-source unit physical area by using the identified time-dependent parameter and the computed elapsed time, and

the controller, in the data read process:

(D) computes the error probability corresponding to the read-source unit physical area by dividing the number of bits corrected in the (C) by the size of the data that has been read in the (A); and

(E) uses the error probability computed in the (B), the error probability computed in the (D), the last data writing time used in the (B), and the read time used in the (B) to compute the time-dependent parameter corresponding to the set of pages that includes the read-source page, and writes the computed time-dependent parameter to the probability management information as the time-dependent parameter corresponding to the read-source unit physical area, and

in accordance with this, the error probability computed in the (B) in the next data read process, for which the same unit physical area as the read-source unit physical area is the read source, constitutes the error probability that is suited to the state of the read-source unit physical area in the next data read process.

12. A nonvolatile storage system according to claim 11, wherein the non-volatile storage device is a nonvolatile memory device,

the nonvolatile storage medium is an m-value multilevel cell flash memory,

each flash memory is configured from multiple blocks,

each of the blocks is configured from multiple pages,

the physical area and the unit physical area are pages,

the probability management information denotes error probability information for each set of pages for each flash memory, and

the set of pages is a set of pages for which the remainders obtained by dividing the page numbers by m are identical.

13. A nonvolatile storage system according to claim 1, wherein the probability management information includes an error probability for each combination of one or more types of parameters, which have an impact on the error probability,

the controller, in the (B), identifies the error probability corresponding to a value of the one or more types of parameters in the data read process, and

in accordance with this, the error probability identified in the (B) of this data read process constitutes the error probability that is better suited to the read-source unit physical area than the error probability identified in the (B) of a past data read process in which the same unit physical area as the read source unit physical area is the read source.

14. A nonvolatile storage system according to claim 1, wherein the controller detects that the (C) has failed and there is an error that cannot be corrected, and outputs failure information, and

the failure information includes information denoting the read-source nonvolatile storage medium with the uncorrectable-error data.

15. A storage control method performed in a nonvolatile storage system, which comprises multiple nonvolatile storage media and a controller coupled to these media, wherein in a data read process:

(A) the controller reads data from a read-source physical area in a read-source nonvolatile storage medium;

(B) the controller identifies from the management information an error probability, which is the error probability of a unit physical area including the read-source physical area and is better suited to the state of the read source in the current read process than the error probability identified for the same unit physical area in a past read process; and

(C) the controller uses the error probability identified in the (B) to correct the data read in the (A) in accordance with a low density parity check code,

wherein the management information is information including probability management information denoting error probability information of a unit physical area of the nonvolatile storage medium, and

the error probability is a probability that data within the unit physical area is incorrect, and is a prior probability, which is the probability of prior to this data having been read.