Short (Q)-Ary Fixed-Rate WOM Codes for Guaranteed Rewrites and With Hot/Cold Write Differentiation

Abstract

To the body of works on rewrite codes for constrained memories, we add a comprehensive study in a direction that is especially relevant to practical storage. The subject of this paper is codes for the q-ary extension of the write-once memories model, with input sizes that are fixed throughout the write sequence. Seven code constructions are given with guarantees on the number of writes they can support. For the parameters addressed by the constructions, we also prove upper bounds on the number of writes, which prove the optimality of three of the constructions. We concentrate on codes with short block lengths to keep the complexity of decoding and updates within the feasibility of practical implementation. Even with these short blocks the constructed codes are shown to be within a small additive constant from capacity for an arbitrarily large number of input bits. Part of the study addresses a new rewrite model where some of the input bits can be updated multiple times in a write sequence (hot bits), while other are updated at most once (cold bits). We refer to this new model as hot/cold rewrite codes. It is shown that adding cold bits to a rewrite code has a negligible effect on the total number of writes, while adding an important feature of leveling the physical wear of memory cells between hot and cold input data.

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