SLC-to-TLC Copy-Back Read Verification for Flash Reliability
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Solution Overview
Problem
The internal copy-back process in flash memory systems from single-level cell (SLC) to triple-level cell (TLC) pages lacks error correction, leading to reliability issues due to propagated errors, which can affect the performance and accuracy of error correction codes, especially with the increasing complexity of multi-level cell technologies.
Innovation Solution
Implementing a read-verify operation before transferring SLC pages to TLC pages, where SLC pages are decoded and errors are verified against an error threshold; if errors exceed the threshold, they are corrected externally before programming the TLC page, ensuring reliable data transfer and reducing the burden on the storage controller.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If internal copy-back is performed without error correction, then write performance is improved and system resource costs are reduced, but data reliability deteriorates due to propagated errors
Solution Approach 1:
The patent performs error verification on SLC pages before transferring them to TLC pages. The controller reads each SLC page, verifies errors using ECC decoding, and determines whether error correction is needed before the copy-back operation. This preliminary error checking prevents unreliable data from being propagated to TLC pages while maintaining efficient internal copy-back for clean pages.
Solution Approach 2:
The patent applies different error handling strategies to different SLC pages based on their individual error conditions. Pages with zero or few errors undergo direct internal copy-back without correction, while pages exceeding the error threshold are corrected using ECC before transfer. This selective approach optimizes overall system performance by treating each page according to its specific quality.
2Reliability
If error correction is performed on all SLC pages before copy-back, then data reliability is improved, but write performance deteriorates due to increased processing time and system resource usage
Solution Approach 1:
The patent performs error correction only on the subset of SLC pages that exceed the error threshold, rather than correcting all pages. The controller verifies errors on all SLC pages during copy-back but applies correction only when necessary. This partial correction approach maintains data reliability for problematic pages while avoiding the performance penalty of correcting already-clean pages.
3Reliability
If SLC pages with errors below threshold are corrected, then data reliability is improved, but system resource costs and processing time increase
Solution Approach 1:
The patent introduces an error threshold parameter that determines when correction is necessary. SLC pages are verified using ECC decoding, and only pages exceeding the threshold (e.g., more than a certain number of bit errors) undergo correction. This parameter-based filtering optimizes the balance between reliability and processing efficiency by ignoring pages with acceptable error levels.
Data Source
AI summary
Systems and methods for internal copy-back with read-verify are described. In one embodiment, a storage device includes a controller to select a first single level cell (SLC) page of a plurality of SLC pages on the storage device to transfer to a triple level cell (TLC) page. The controller, in conjunction with an error correcting code (ECC) decoder, read-verifies the first SLC page. Read-verifying the first SLC page includes reading the first SLC page to an internal page buffer, decoding the first SLC page read into the internal page buffer, determining a number of errors contained in the first SLC page based at least in part on the decoding, and verifying whether the number of errors contained in the first SLC page satisfies an error threshold. The controller transfers the first SLC page to the TLC page according to a result of read-verifying the first SLC page.