1.  The emerged flash-memory based solid state drives (SSDs) have rapidly replaced the traditional hard disk drives (HDDs) in many applications.  Characteristics of the SSDs Read/write in pages (typically 2KB), erase in blocks (typically containing 64 or 128 pages), no overwrite Erase latency 2 to 10 ms; read/write 0.1 ms or less Out-of-place update [1] scheme to prevent excessive erasure Flash Translation Layer (FTL) is used to manage the mapping from logical to physical address SSDs requires garbage collection because Out-of- place update uses up space fast and generates invalid pages Flash-based SSD Basics  Hide the garbage collection overhead using write buffer  Reserve certain buffer space so that it can handle user write requests at the same time of garbage collecting  Need to consider how to flush buffered data to flash memory Need to be preemptive GC interrupts flushing but activates GC-Cache Reserve Cache/buffer space to handle write during GC period Solution 1.F. Chen, D. A. Koufaty, and X. Zhang, “Understanding Intrinsic Characteristics and System Implications of Flash Memory Based Solid State Drives”, SIGMETRICS 09. 2.W. Bux, X.-Y. Hu, I. Iliadis, and R. Haas, “Scheduling in Flash- Based Solid-State Drives - Performance Modeling and Optimization,” MASCOTS 12. References  On-device RAM as buffer/cache SSD usually equipped with RAM as write buffer or read cache to bridge the gap between SSD and IO system Abundant size from 128MB to 2GB Write buffer usually for write acceleration, no reuse Read cache for reusing frequently/recently accessed data  Existing Cache/Buffer Schemes not consider the characteristics of SSDs Assume underlying device is hard disk drive that do not need block erasure or garbage collection Write buffer usually periodically flushed to disk may encounter long latency caused by garbage collection  Write buffer becomes less effective when garbage collection overhead is high High Response Time Spikes Show Even with Write Buffer Implemented Problems in Existing Cache/Buffer Schemes for SSDs SSD Cache Management  Steps of operation 1.Free block space insufficiency detected 2.Choose a victim block 3.Copy valid pages in the victim block to a free block 4.Update the mapping information of the pages migrated 5.Erase the victim block  Overhead caused by garbage collection  Technique to minimize the overhead Victim selection algorithm Data separation based on access pattern Hide the latency with parallelism Garbage Collection Basics  Use modeling method based on Maximum Throughput [2] to evaluate our design No cache v.s. normal cache v.s. GC-Cache  Initial modeling result shows a maximum of 40% reduction in response time  Planning Trace-driven simulation Evaluation Plan and Initial Result  First known research to use on-device cache to hide the garbage collection overhead in SSDs  Simple design: reserves cache/buffer space that only activated during GC  Potential issue: need to be implemented in SSD’s controller  Initial modeling shows great potential of reducing response time of SSDs Conclusion