A real-time hot swapping technique for SSD RAID systems

NAND flash memory solid state devices are widely used in many platforms including consumer electronics and safety-critical embedded systems because they offer high performance and reliability. In previous work, we have developed a novel RAID architecture for NAND flash that protects a system from data loss in the case of failure, or wear-out, of individual flash chips. These mechanisms permit the recovery of data onto a new replacement chip when a particular element in the array reaches its endurance limit. However the use of this architecture in a hard real-time system is limited as the memory needs to be taken off-line while the replacement is actioned and so memory access times become non-deterministic with respect to time. In this paper we present a hot swapping technique for replacing elements in the array that are approaching their wear-out level that does not involve taking the array off-line - thereby increasing the systems availability and providing guaranteed response time. The result is that our hot-swapping architecture is more suited to applications with hard real-time constraints. We have implemented this technique in our FPGA-based SSD-aware RAID controller, complete with a metadata management framework to increase throughput and efficiency.