Don't Stack Your Log On My Log

Log-structured applications and file systems have been used to achieve high write throughput by sequentializing writes. Flash-based storage systems, due to flash memory’s out-of-place update characteristic, have also relied on log-structured approaches. Our work investigates the impacts to performance and endurance in flash when multiple layers of log-structured applications and file systems are layered on top of a log-structured flash device. We show that multiple log layers affects sequentiality and increases write pressure to flash devices through randomization of workloads, unaligned segment sizes, and uncoordinated multi-log garbage collection. All of these effects can combine to negate the intended positive affects of using a log. In this paper we characterize the interactions between multiple levels of independent logs, identify issues that must be considered, and describe design choices to mitigate negative behaviors in multi-log configurations.