Improving Read Performance of SSDs via Balanced Redirected Read

Modern SSDs have been a competitive alternative to traditional hard disks because of higher random access performance, lower power consumption and less noise. Although SSDs usually have multiple channels with each channel connected to multiple chips to improve their performance with parallel channels and chips, some recent studies show that contentions among I/O requests make SSDs read performance degrade notoriously, even worse than random write performance. Meanwhile, MLC/TLC flash memory technology increases modern SSDs' capacity while sacrificing their reliability. So fault tolerance, such as chip-level RAID, is also a necessity for SSDs. We propose a Balanced Redirected Read (BRR), which redirects some read requests from busy chips to relatively idle chips by decoding target data chunks with the data/parity chunks in a parity group. We also design a new layout of RAID-5 in SSDs, which distributes adjacent chips to different parity groups such that the degrees of loads on different chips in a group are more likely different. So BRR has more chances to redirect read requests from busy chips to idle chips. Compared with the recently proposed Parallel Issue Queuing (PIQ) using reordering I/O requests technique, BRR schedules the requests among chips more balanced. We implement BRR atop a trace driven simulator, and extensive experiments with real-world workloads show that BRR reduces the waiting time of read requests over PIQ (FIFO) by as much as 38.4% (77.1%) and averagely 14.2% (23.8%), and BRR can also slightly improve write performance due to the improvement of read performance.