Alleviating garbage collection interference through spatial separation in all flash arrays^∗

We present SWAN, a novel All Flash Array (AFA) management scheme. Recent flash SSDs provide high I/O bandwidth (e.g., 3-10GB/s) so the storage bandwidth can easily surpass the network bandwidth by aggregating a few SSDs. However, it is still challenging to unlock the full performance of SSDs. The main source of performance degradation is garbage collection (GC). We find that existing AFA designs are susceptible to GC at SSD-level and AFA software-level. In designing SWAN, we aim to alleviate the performance interference caused by GC at both levels. Unlike the commonly-used temporal separation approach that performs GC at idle time, we take a spatial separation approach that partitions SSDs into the front-end SSDs dedicated to serve write requests and the back-end SSDs where GC is performed. Compared to temporal separation of GC and application I/O, which is hard to be controlled by AFA software, our approach guarantees that the storage bandwidth always matches the full network performance without being interfered by AFA-level GC. Our analytical model confirms this if the size of front-end SSDs and the back-end SSDs are properly configured. We provide extensive evaluations that show SWAN is effective for a variety of workloads.