Stochastic block allocation for improved wear leveling

• US 8,762,625 B2
• Filed: 04/14/2011
• Issued: 06/24/2014
• Est. Priority Date: 04/14/2011
• Status: Expired due to Fees

• Current Assignee: Apple Inc.
• Original Assignee: Apple Inc.
• Inventors: Post, Daniel J., Wakrat, Nir J.
• Primary Examiner(s): Bradley, Matthew
• Assistant Examiner(s): Perez-Velez, Rocio Del Mar
• Application Number:
  US13/086,963
• Publication Number:
  US 20120265922A1
• Time in Patent Office: 1,167 Days
• Field of Search: 711/103
• US Class Current: 711/103
• CPC Class Codes: G06F 12/0246 in block erasable memory, e.g. flash memory
• CPC Class Codes: G06F 2212/7211 Wear leveling
• CPC Class Codes: G06F 3/0616 in relation to life time, e.g. increasing Mean Time Between Failures [MTBF]
• CPC Class Codes: G06F 3/064 Management of blocks
• CPC Class Codes: G06F 3/0679 Non-volatile semiconductor memory device, e.g. flash memory, one time programmable memory [OTP]
• CPC Class Codes: G06F 7/588 Random number generators, i.e. based on natural stochastic processes
• CPC Class Codes: G11C 16/349 Arrangements for evaluating degradation, retention or wearout, e.g. by counting erase cycles

Abstract

Systems and methods are disclosed for stochastic block allocation for improved wear leveling for a system having non-volatile memory (“NVM”). The system can probabilistically allocate a block or super block for wear leveling based on statistics associated with the block or super block. In some embodiments, the system can select a set of blocks or super blocks based on a pre-determined threshold of a number of cycles (e.g., erase cycles and/or write cycles). The block or super block can then be selected from the set of super blocks. In other embodiments, the system can use a fully stochastic approach by selecting a block or super block based on a biased random variable. The biased random variable may be generated based in part on the number of cycles associated with each block or super block of the NVM.

Claims

1. A memory interface for accessing a nonvolatile memory (“

NVM”

) having a plurality of blocks, wherein each block is associated with a number of cycles, the memory interface comprising;

a bus controller operative to communicate with the NVM; and

control circuitry operative to;

determine that a block needs to be allocated;

generate a biased random variable based at least in part on the number of cycles associated with each block of the plurality of blocks of the NVM;

allocate a block from the plurality of blocks based at least on the biased random variable;

assign a plurality of weights to the plurality of blocks, wherein each weight of the plurality of weights is assigned to a block based on the number of cycles associated with the block;

generate a sum of the plurality of weights; and

select the biased random variable based on the sum of the plurality of weights.

2. The memory interface of claim 1, wherein the number of cycles is at least one of a number of erase cycles or a number of write cycles.

3. The memory interface of claim 1, wherein the control circuitry is further operative to direct the bus controller to perform wear leveling on the NVM using the allocated block.

4. The memory interface of claim 1, wherein the plurality of weights have lower or higher values as the number of cycles corresponding to the plurality of blocks increases.

5. The memory interface of claim 1, wherein the control circuitry is further operative to:

subtract the sum of the plurality of weights by one to generate an upper range value;

select the biased random variable with a range between zero and the upper range value.

6. The memory interface of claim 1, wherein the control circuitry is further operative to:

for a block of the plurality of blocks, compare the weight assigned to the block to the biased random variable;

determine whether the biased random variable is less than the weight assigned to the block;

in response to determining that the biased random variable is less than the weight assigned to the block, allocate the block for wear leveling.

7. The memory interface of claim 6, wherein in response to determining that the biased random variable is greater than or equal to the weight assigned to the block, the control circuitry is further operative to:

update the biased random variable by subtracting the weight assigned to the block from the biased random variable; and

select another block of the plurality of blocks for comparison to the updated random variable.