Fully Automatic Stream Management for Multi-Streamed SSDs Using Program Contexts

Multi-streamed SSDs can significantly improve both the performance and lifetime of flash-based SSDs when their streams are properly managed. However, existing stream management solutions do not adequately support the multi-streamed SSDs for their wide adoption. No existing stream management technique works in a fully automatic fashion for general I/O workloads. Furthermore, the limited number of available streams makes it difficult to effectively manage streams when a large number of streams are required. In this paper, we propose a fully automatic stream management technique, PCStream, which can work efficiently for general I/O workloads with heterogeneous write characteristics. PCStream is based on the key insight that stream allocation decisions should be made on dominant I/O activities. By identifying dominant I/O activities using program contexts, PCStream fully automates the whole process of stream allocation within the kernel with no manual work. In order to overcome the limited number of supported streams, we propose a new type of streams, internal streams, which can be implemented at low cost. PCStream can effectively double the number of available streams using internal streams. Our evaluations on real multi-streamed SSDs show that PCStream achieves the same efficiency as highly-optimized manual allocations by experienced programmers. PCStream improves IOPS by up to 56% over the existing automatic technique by reducing the garbage collection overhead by up to 69%.