On the Application Level Impact of SSD Performance Anomalies

Hardware-induced performance variability has long been an undesirable fact of life in the storage stack. SSDs have not managed to break the trend. Despite continuous evolution in their internal design, SSD performance variability has remained a concern. This unfortunate trend is also bound to continue for SSDs for the foreseeable future due to increasingly complex controller design and responsibilities. Nevertheless, applications demand high, predictable and stable performance. It is therefore important to measure and understand the performance implications of this hardware-induced variability at the application layer. It is equally vital to assess to what extent the mechanisms available at the software level are able to alleviate or mask the variability. In this paper we uncover and analyze three novel and surprising performance anomalies induced by SSDs. We focus on reads. At the application layer, each anomaly leads to significant read throughput slowdown. The first anomaly, intrinsic slowdown, slows down reads for a variable amount of time when reading from new file system extents. Second, temporal slowdown slows down reads periodically, even in the absence of any writes. Third, in permanent slowdown, reads from some files eventually become consistently slow and never recover. Individually, each of the slowdowns can cause a read throughput loss of 10%-15%, but when they occur concurrently the cumulative throughput loss can reach 30%. We further analyze to what extent available software mechanisms can mask these performance anomalies. We find that only two of the three slowdowns can be masked via increased I/O request parallelism.