PA-SSD: A Page-Type Aware TLC SSD for Improved Write/Read Performance and Storage Efficiency

TLC flash has three types of pages to accommodate the three bits in each TLC physical cell exhibiting very different program latencies, LSB (fast), CSB (medium), and MSB (slow). Conventional TLC SSD designs on page allocation to write requests do not take page types and their latency difference into consideration, missing on an important opportunity to exploit the potentials of fast writes. This paper proposes PA-SSD, a page-type aware TLC SSD design, to effectively improve the overall performance by judiciously and coordinately utilizing the three types of pages on TLC flash when serving user write requests. The main idea behind PA-SSD is to coordinately allocate the same type of pages for sub-requests of any given user write request, to mitigate the potential program latency imbalance among the sub-requests. We achieve the PA-SSD design goal by addressing two key research problems: (1) how to properly determine page-type for each user write request and (2) how to allocate a physical page for each sub-request with an assigned page type from (1). For the first problem, seven page-type specifying schemes are proposed to investigate their effects under different workloads. On the other hand, we approach the second problem by redesigning the page allocation strategy in TLC SSD to uniformly and sequentially determine pages for allocation following the programming process of TLC flash. Under a wide range of workloads, our experiments show that PA-SSD can accelerate both the write and read performance without any sacrifice to storage capacity. Particularly, our proposed queue-depth based page-type specifying scheme improves write performance by 2.4 times and read performance by 1.5 times over the conventional TLC SSD.