Avoiding Costly Read Latency Variations in SSDs Through I/O Determinism

Transcription

1 Avoiding Costly Read Latency Variations in SSDs Through I/O Determinism Steven Wells Toshiba America Electronic Components, Inc. August 2017

2 Excellent Latency SSD designs have been about high average bandwidth for the last 30 years At best we spoke about bandwidth consistency A new paradigm focusing on latency is emerging from the needs of hyperscale data centers IO Determinism August

3 Latency Tail Impact Legacy Mindset Hyperscale Mindset 1 worker n iterations Latency O(n) partition among workers n workers 1 lookup each Latency O(1) aggregate results Execution time Average lookup latency Execution time Longest lookup latency August

4 Real Implementations are... In practice, a single user request may result in thousands of subqueries, with a critical path that is dozens of subqueries long. The fork/join structure of subqueries causes latency outliers to have a disproportionate effect on total latency, and the large number of subqueries would cause slowdowns or unavailability to quickly propagate Challenges to Adopting Stronger Consistency at Scale - Ajoux et. Al., (Facebook & USC) August

5 Latency is Affected by Maintenance August

6 The Maintenance of an SSD Ready Latency Mixed Read/Write Workload (Simulation, us) % 99% Program Collisions 99.9% 99.99% % Erase Collisions % August

7 28 foot wire (aspect ratio of BiCS 3) Suspends is Not a Solution for Deterministic Reads Memory Cell 8 foot wire (aspect ratio of BiCS 3) Read Please! Cell Array Toshiba BiCS FLASH 64-Layer 3D Memory Can t hear during program or erase August 2017 It s Level 6 7

8 A New Working Model and Prior Work Analogy: Fire department has multiple engines. Each is taken offline periodically for maintenance. But not all at the same time! Prior System Work: Flash on Rails - Consistent Flash Performance through Redundancy (USENIX 2014) With SSD size scaling into 16TB and beyond, SSD level isn t the right solution Reads Writes Cache Writes of current period Writes of previous period SSD 1 Read Mode SSD 2 Write Mode At any given time each of the 2 drives is either performing reads or writes. While one drive is reading the other drive is performing writes of the previous period. 8

9 die die NVM Sets Architecture channel channel Classic SSD architecture uses bands of devices on every channel to maximize bandwidth. Maintenance is also on every channel and every device New SSD array architecture creates independent NVM Sets ~1TB/set 9

10 Proof of Concept CH0 CH1 CH2 CH3 CH4 CH5 CH6 CH7 CH8 CH9 CH10 CH11 CH12 CH13 CH14 CH15 Set 0 Set 1 Set 2 Set 3 August

11 Proof of Concept Results QD1 4K Random Read Latency vs. Write Disturbances 90% % 99.9% 99.99% % % % QD1 4KRR Latency (usec) No Sets (4KRW) Horizontal Sets Vertical Sets August

12 performance Managing Pressure Fatigue Producer pressure well beyond the capability of the consumer can degrade performance Pressure can be both host and internal pressure Managing pressure allows reads to immediately take a priority position with a minimum of latency August

13 Proof of Concept Final Result QD1 4K Random Read Latency vs. Write Disturbances 90% % 99.9% 99.99% % ~50x Latency Improvement % % QD1 4KRR Latency (usec) No Sets (4KRW) Horizontal Sets Vertical Sets Final FW Optimizations August

14 Data from our floor demo For a fixed QD workload: 50x latency improvement 20x increase in read IOPS 4x reduction in write IOPS August

15 Closing Thoughts ~2 order of magnitude single SSD read latency improvement New SSD array architecture creating isolated NVM Sets A well behaved host is responsible for ping-ponging and other activities such as set to set wear leveling FW optimizations are required to support the new paradigm August

16 Stop by the Toshiba Booth! Booth #407 August