Okeanos: Wasteless Journaling for Fast and Reliable Multistream Storage

Synchronous small writes play a critical role in the reliability and availability of file systems and applications that use them to safely log recent state modifications and quickly recover from failures. However, storage stacks usually enforce page-sized granularity in their data transfers from memory to disk. We experimentally show that subpage writes may lead to storage bandwidth waste and high disk latencies. To address the issue in a journaled file system, we propose wasteless journaling as a mount mode that coalesces synchronous concurrent small writes of data into full page-sized blocks before transferring them to the journal. Additionally, we propose selective journaling that automatically applies wasteless journaling on data writes whose size lies below a fixed preconfigured threshold. In the Okeanos prototype implementation that we developed, we use microbenchmarks and application-level workloads to show substantial improvements in write latency, transaction throughput and storage bandwidth requirements.