How to improve the performance of the d-choices garbage collection algorithm in flash-based SSDs

Abstract

The performance of flash-based solid state drives is greatly impacted by the garbage collection algorithm. The d-choices garbage collection algorithm, which selects a victim block with the fewest number of valid pages among d randomly selected blocks, is known to perform well in terms of the write amplification. However, the number of erasures performed on a block may be quite unbalanced, which reduces the lifespan of SSDs that can only tolerate a limited number of erasures per block. This unequal wear is caused by the hot/cold data separation used to achieve a low write amplification, as blocks holding hot data tend to endure more erasures. Methods to reduce this unequal wear often cause a significant increase in the write amplification (which slows down the device). In this paper we propose a new mechanism that allows us to severely reduce the unequal wear and thus improve the lifespan of the drive, without a significant increase in the write amplification. In fact, in many cases this mechanism even reduces the write amplification (eliminating the trade-off between low write amplification and a large lifespan altogether). To assess the performance of this new mechanism we rely both on a mean field model and simulation experiments.