GCaR: Garbage Collection aware Cache Management with Improved Performance for Flash-based SSDs

Abstract

Garbage Collection (GC) is an important performance concern for flash-based SSDs, because it tends to disrupt the normal operations of an SSD. This problem continues to plague flash-based storage systems, particularly in the high performance computing and enterprise environment. An important root cause for this problem, as revealed by previous studies, is the serious contention for the flash resources and the severe mutually adversary interference between the user I/O requests and GC-induced I/O requests. The on-board buffer cache within SSDs serves to play an essential role in smoothing the gap between the upper-level applications and the lower-level flash chips and alleviating this problem to some extend. Nevertheless, the existing cache replacement algorithms are well optimized to reduce the miss rate of the buffer cache by reducing the I/O traffic to the flash chips as much as possible, but without considering the GC operations within the flash chips. Consequently, they fail to address the root cause of the problem and thus are far from being sufficient and effective in reducing the expensive I/O traffic to the flash chips that are in the GC state. To address this important performance issue in flash-based storage systems, particularly in the HPC and enterprise environment, we propose a Garbage Collection aware Replacement policy, called GCaR, to improve the performance of flash-based SSDs. The basic idea is to give higher priority to caching the data blocks belonging to the flash chips that are in the GC state. This substantially lessens the contentions between the user I/O operations and the GC-induced I/O operations. To verify the effectiveness of GCaR, we have integrated it into the SSD extended Disksim simulator. The simulation results show that GCaR can significantly improve the storage performance by reducing the average response time by up to 40.7%.