BypassD: Enabling fast userspace access to shared SSDs

Abstract

Modern storage devices, such as Optane NVMe SSDs, offer ultra-low latency of a few microseconds and high bandwidth of multiple gigabytes per second. At these speeds, the kernel software I/O stack is a substantial source of overhead. Userspace approaches avoid kernel software overheads but face challenges in supporting shared storage without major changes to file systems, the OS or the hardware.We propose a new I/O architecture, BypassD, for fast, userspace access to shared storage devices. BypassD takes inspiration from virtual memory: it uses virtual addresses to access a device and relies on hardware for translation and protection. Like memory-mapping a file, the OS kernel constructs a mapping for file contents in the page table. Userspace I/O requests then use virtual addresses from these mappings to specify which file and file offset to access. BypassD extends the IOMMU hardware to translate file offsets into device Logical Block Addresses. Existing applications require no modifications to use BypassD. Our evaluation shows that BypassD reduces latency for 4KB accesses by 42% compared to standard Linux kernel and performs close to userspace techniques like SPDK that do not support device sharing. By eliminating software overheads, BypassD improves performance of real workloads, such as the WiredTiger storage engine, by ∼20%.