GD-FTL: Improving the Performance and Lifetime of TLC SSD by Downgrading Worn-out Blocks

Abstract

Driven by the demand for high performance and large capacity storage system of modern diverse data-intensive latency sensitive applications, Solid State Drives (SSD) built upon Triple Level Cell (TLC) flash memory have been playing an indispensable role in the storage systems of almost all computing platforms. While providing the benefit of huge storage capacity increase, TLC flash memory in the meantime has suffered from significantly degraded raw reliability and thus drastically decreased lifetime entailed as Program/Erase (P/E) cycles. In an SSD, once a flash block has endured a rated number of P/E cycles, it becomes worn out. Worn out blocks are locked in the grown bad block list and then excluded from any further usage till the end of the drive, decreasing the over-provisioning space of an SSD and resultantly slowing down performance. In this work, we suggest a novel Flash Translation Layer (FTL) called Graceful Degradation FTL (GD-FTL), which smartly salvages worn-out TLC blocks by using them as Single Level Cell (SLC) blocks. Doing so brings us two-fold advantages: (1) we can always maintain a good level of over-provisioning space, preventing sharp SSD performance degradation; (2) the lifetime of SSD can be significantly extended due to the huge difference in the P/E cycles between TLC and SLC blocks. Moreover, GD-FTL causes very minimal changes to the FTL design and requires negligible implementation overheads.