대용량 SSD를 위한 요구 기반 FTL 캐시 분리 기법

Demand-based FTL Cache Partitioning for Large Capacity SSDs

  • 투고 : 2019.02.24
  • 심사 : 2019.04.03
  • 발행 : 2019.04.30


As the capacity of SSDs rapidly increases, the amount of DRAM to keep a mapping table size in SSDs becomes very huge. To address a Demand-based FTL (DFTL) scheme that caches part of mapping entries in DRAM is considered to be a feasible alternative. However, owing to its unpredictable behaviors, DFTL fails to provide consistent I/O response times. In this paper, we a) analyze a root cause that results in fluctuation on read latency and b) propose a new demand-based FTL scheme that ensures guaranteed read response time with low write amplification. By preventing mapping evictions while serving reads, the proposed technique guarantees every host read requests to be done in 2 NAND read operations. Moreover, only with 25% of a cache ratio, the proposed scheme improves random write performance and random mixed performance by 1.65x and 1.15x, respectively, over the traditional DFTL.

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그림 1. 요구 기반 FTL 읽기 응답시간 분석 Fig. 1 DFTL read latency analysis

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그림 2. 요구 기반 FTL의 두 가지 캐시 형태 (a) 엔트리 단위 저장 방식, (b) 페이지 단위 저장 방식 Fig. 2 Two types of mapping cache on DFTL (a) Fine-grained mapping cache, (b) Coarse-grained mapping cache

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그림 3. DFTL 캐시 분리 기법의 구조 Fig. 3 DFTL cache partitioning architecture

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그림 4. Batch-update 동작 과정 Fig. 4 Batch-update process

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그림 5. 읽기/쓰기 성능 Fig. 5 Read/Write throughput

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그림 6. 쓰기 증폭 인자 Fig. 6 Write amplification factor

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그림 7. 읽기 응답시간 누적분포 Fig. 7 Read latency CDF


연구 과제 주관 기관 : 한국연구재단


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