Minimizing overhead of out-of-channel data exchanges to balance wear-outs and I/Os in RAID-enabled SSDs

The channel-level RAID implementation has been introduced to NAND flash-based solid-state drives (SSDs), to fight against channel failures. But, it suffers from unbalanced wear-outs and I/O workloads across channels, due to the nature of in-channel updates on data/parity chunks of data stripes, leading to a decline in I/O performance and a negative impact on the lifespan of RAID-enabled SSDs. This paper introduces an approach to yield wear-out and I/O balances, with the minimal overhead caused by location exchanges of data/parity chunks belonging to the same stripe, when fulfilling write requests or garbage collections (GCs). To this end, we build an assessment model for measuring the balance level of all SSD channels, and then trigger a location exchange of data/parity chunks in the same data stripe, if the exchange operation can be beneficial to wear-out and I/O balances. Besides, we introduce a scheme of pairGC to further minimize the overhead of data location exchanges. Specifically, it pairs the GC operations on different channels and conducts out-of-channel page moves, to achieve the goal of data location exchanges without additional cost. Through a series of emulation experiments on 8 disk traces of real-world applications, we show that our proposal can greatly improve I/O performance by 40.0 % on average, as well as noticeably balance I/O workloads over SSD channels and prolong the endurance of SSDs, in contrast to the state-of-the-art RAID optimization schemes inside SSDs.