Retired-page utilization in write-once memory &amp;#x2014; A coding perspective

Berman, Amit; Birk, Yitzhak

doi:10.1109/isit.2013.6620389

Cited by 4 publications

(3 citation statements)

References 10 publications

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“…Indeed once a flash page is written, its unwritten cells (only) can accept a second write cycle. Several studies propose WOM codes for lifetime extension by reduction in SSD block erasures [52], [4], [21], [22]. Table I shows a WOM code example that allows twice the encoding of different configurations of 2 information bits using only 3 physical storage cells/bits.…”

Section: B Write-once Memory Codementioning

confidence: 99%

PEARL: Plausibly Deniable Flash Translation Layer using WOM coding

Chen,

Chakraborti,

Sion

2020

Preprint

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When adversaries are powerful enough to coerce users to reveal encryption keys, encryption alone becomes insufficient for data protection. Plausible deniability (PD) mechanisms resolve this by enabling users to hide the mere existence of sensitive data, often by providing plausible "cover texts" or "public data volumes" hosted on the same device.Unfortunately, with the increasing prevalence of (NAND) flash as a high-performance cost-effective storage medium, PD becomes even more challenging in the presence of realistic adversaries who can usually access a device at multiple points in time ("multi-snapshot"). This is because read/write operations to flash do not result in intuitive corresponding changes to the underlying device state.The problem is further compounded by the fact that this behavior is mostly proprietary. For example, in a majority of commercially-available flash devices, an issued delete or overwrite operation from the upper layers almost certainly won't result in an actual immediate erase of the underlying flash cells.To address these challenges, we designed a new class of writeonce memory (WOM) codes to store hidden bits in the same physical locations as other public bits. This is made possible by the inherent nature of NAND flash and the possibility of issuing multiple writes to target cells that have not previous been written to in existing pages.We designed PEARL, a general-purpose Flash Translation Layer (FTL) that allows users to plausibly deniably store hidden data in NAND flash devices. We implemented and evaluated PEARL on a widely used simulator FlashSim [32]. PEARL performs well on real-world workloads, comparably to non-PD baselines. PEARL is the first system that achieves strong plausible deniability for NAND flash devices, secure against realistic multisnapshot adversaries.

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Section: B Write-once Memory Codementioning

confidence: 99%

PEARL: Plausibly Deniable Flash Translation Layer using WOM coding

Chen,

Chakraborti,

Sion

2020

Preprint

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“…The main goal of this work is to study the connection between the over-provisioning ρ (or storage rate α) and the number of block erasures. This connection depends upon the overprovisioning value, GC algorithm, and the probability distribution of the page write requests 1 . We assume in this work that requests are uniformly distributed over the U logical pages.…”

Section: Problem Setupmentioning

confidence: 99%

“…WOM codes in flash memories were investigated both theoretically with respect to the number of block erasures, and practically by simulations; see e.g. [1], [8], [11]- [13], [19].…”

Section: Introductionmentioning

confidence: 99%

When do WOM codes improve the erasure factor in flash memories?

Yaakobi

Yucovich

Maor

et al. 2015

2015 IEEE International Symposium on Information Theory (ISIT)

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Flash memory is a write-once medium in which reprogramming cells requires first erasing the block that contains them. The lifetime of the flash is a function of the number of block erasures and can be as small as several thousands. To reduce the number of block erasures, pages, which are the smallest write unit, are rewritten out-of-place in the memory. A Write-once memory (WOM) code is a coding scheme which enables to write multiple times to the block before an erasure. However, these codes come with significant rate loss. For example, the rate for writing twice (with the same rate) is at most 0.77.In this paper, we study WOM codes and their tradeoff between rate loss and reduction in the number of block erasures, when pages are written uniformly at random. First, we introduce a new measure, called erasure factor, that reflects both the number of block erasures and the amount of data that can be written on each block. A key point in our analysis is that this tradeoff depends upon the specific implementation of WOM codes in the memory. We consider two systems that use WOM codes; a conventional scheme that was commonly used, and a new recent design that preserves the overall storage capacity. While the first system can improve the erasure factor only when the storage rate is at most 0.6442, we show that the second scheme always improves this figure of merit.

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