An aggressive worn-out flash block management scheme to alleviate SSD performance degradation

Abstract

Since NAND flash cannot be updated in place, SSDs must perform all writes in pre-erased pages. Consequently, pages containing superseded data must be invalidated and garbage collected. This garbage collection adds significant cost in terms of the extra writes necessary to relocate valid pages from erasure candidates to clean blocks, causing the well-known write amplification problem. SSDs reserve a certain amount of flash space which is invisible to users, called over-provisioning space, to alleviate the write amplification problem. However, NAND blocks can support only a limited number of program/erase cycles. As blocks are retired due to exceeding the limit, the reduced size of the over-provisioning pool leads to degraded SSD performance. In this work, we propose a novel system design that we call the Smart Retirement FTL (SR-FTL) to reuse the flash blocks which have been cycled to the maximum specified P/E endurance. We take advantage of the fact that the specified P/E limit guarantees data retention time of at least one year while most active data becomes stale in a period much shorter than one year, as observed in a variety of disk workloads. Our approach aggressively manages worn blocks to store data that requires only short retention time. In the meantime, the data reliability on worn blocks is carefully guaranteed. We evaluate the SR-FTL by both simulation on an SSD simulator and prototype implementation on an OpenSSD platform. Experimental results show that the SR-FTL successfully maintains consistent over-provisioning space levels as blocks wear and thus the degree of SSD performance degradation near end-of-life. In addition, we show that our scheme reduces block wear near end-of-life by as much as 84% in some scenarios.