LX-SSD: Enhancing the Lifespan of NAND Flash- based Memory via Recycling Invalid Pages

NAND flash-based SSDs utilize out-of-place update to prevent erase operations from blocking regular requests, in the meanwhile, out-of-place update creates a huge amount of invalid pages. Traditional FTLs perceive these invalid pages as useless data and discard them during garbage collection. But given value locality, certain values are likely to appear afterward, resulting in abundant invalid pages being still useful. In this paper, we propose a content-aware FTL to explore such locality and recycle invalid pages to achieve Lifetime eXtension SSD (LX-SSD), which is hereafter named as Recyclable SSD. Such recycling invalid pages enables an SSD to recycle invalid pages at the page granularity and without involving block erase operation. Since block erasures serve as a criterion for quantifying the lifespan of SSDs, the reduced block erasures could result in lifespan extension. In order to retain high performance, we design a set of acceleration techniques to reduce the overhead of extra computational costs. We also modify the GC algorithm to delay certain useful invalid pages from being erased. We evaluate LX-SSD on real-world workloads, with content information, and the results show that our Recyclable SSD can achieve 10.5%-18.4% write reduction and 40.6%-58.9% reduction in block erasures, which results in 10.5%18.4% overall reduction in I/O response time, and significant lifespan extension.