CG-Resync: Conversion-guided resynchronization for a SSD-based RAID array

SSD-based RAID arrays have been widely adopted in large-scale systems. One requirement on a RAID is to provide data consistency, which can be an issue during serving write requests. While using NVRAM or on-storage logging can ensure the consistency, the approaches can either be very expensive or substantially compromise performance. For SSD-based RAID, scanning the entire storage space during rebooting after a crash can recover the consistency. However, it takes a long resychronization time. To address the issue efficiently and cost-effectively, we propose CG-Resync, a scheme providing consistency assurance for SSD-based RAIDs by leveraging logging mechanism readily available in almost all SSDs for accommodating flash's out-of-place-write requirement. To identify uncompleted writes resulting in inconsistent stripes, we use guided conversion in managing SSD's internal logs. In particular, only when a stripe becomes consistent does CG-Resync allow the updated data on the stripe to be removed from the log. We evaluate CG-Resync and experiments show that it provides improved RAID reliability and availability upon a crash with little performance loss during regular I/O operations.