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For a two-variance model of the Flash read channel that degrades as a function of the number of program/erase cycles, this paper demonstrates that selecting write voltages to maximize the minimum page mutual information (MI) can increase device lifetime. In multi-level cell (MLC) Flash memory, one of four voltage levels is written to each cell, according to the values of the most-significant bit (MSB) page and the least-significant bit (LSB) page. In our model, each voltage level is then distorted by signal-dependent additive Gaussian noise that approximates the Flash read channel. When performing an initial read of a page in MLC flash, one (for LSB) or two (for MSB) bits of information are read for each cell of the page. If LDPC decoding fails after the initial read, then an enhanced-precision read is performed. This paper shows that jointly designing write voltage levels and read thresholds to maximize the minimum MI between a page and its associated initial or enhanced-precision read bits can improve LDPC decoding performance.
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DREAM: Data Representation Aware of Damage to Extend the Lifetime of MLC NAND Flash Memory
MLC NAND flash memory uses the voltages of the memory cells to represent bits. High voltages cause much more damage on the cells than low voltages. The free space that need not store bits is leveraged to reduce the usage of those high voltages and thus extend the lifetime of the MLC memory. However, limited by the conventional data representation rule that represents bits by the voltage of one single cell, the high voltages are still used in a high probability. To fully explore the potential of the free space on reducing the usage of high voltages, we propose a novel data representation aware of damage, named DREAM. DREAM uses the voltage combinations of multiple cells instead of the voltage of one single cell to represent bits. It enables to represent the same bits through flexibly replacing the high voltages in some cells with the low voltages in other cells when free space is available. Hence, high voltages which cause more damage are less used and the lifetime of the MLC memory is extended. Theoretical analysis results demonstrate the effectiveness and efficiency of DREAM.
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- PAR ID:
- 10065108
- Date Published:
- Journal Name:
- the 10th USENIX Workshop on Hot Topics in Storage and File Systems (HotStorage '18)
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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