Although QEMU virtio-blk is quite fast, there is still some room for improvements. Disk latency can be reduced if we handle virito-blk requests in host kernel so we avoid a lot of syscalls and context switches. The idea is quite simple - QEMU gives us block device and we translate any incoming virtio requests into bio and push them into bdev. The biggest disadvantage of this vhost-blk flavor is raw format. Luckily Kirill Thai proposed device mapper driver for QCOW2 format to attach files as block devices: https://www.spinics.net/lists/kernel/msg4292965.html Also by using kernel modules we can bypass iothread limitation and finaly scale block requests with cpus for high-performance devices. There have already been several attempts to write vhost-blk: Asias' version: https://lkml.org/lkml/2012/12/1/174 Badari's version: https://lwn.net/Articles/379864/ Vitaly's https://lwn.net/Articles/770965/ The main difference between them is API to access backend file. The fastest one is Asias's version with bio flavor. It is also the most reviewed and have the most features. So his module is partially based on it. Multiple virtqueue support was addded, some places reworked. test setup and results: fio --direct=1 --rw=randread --bs=4k --ioengine=libaio --iodepth=128 QEMU drive options: cache=none filesystem: xfs SSD: | randread, IOPS | randwrite, IOPS | Host | 95.8k | 85.3k | QEMU virtio | 57.5k | 79.4k | QEMU vhost-blk | 95.6k | 84.3k | RAMDISK (vq == vcpu): | randread, IOPS | randwrite, IOPS | virtio, 1vcpu | 123k | 129k | virtio, 2vcpu | 253k (??) | 250k (??) | virtio, 4vcpu | 158k | 154k | vhost-blk, 1vcpu | 110k | 113k | vhost-blk, 2vcpu | 247k | 252k | vhost-blk, 4vcpu | 576k | 567k | Major features planned for the next versions: - DISCARD\WRITE_ZEROES support - multiple vhost workers Andrey Zhadchenko (1): drivers/vhost: vhost-blk accelerator for virtio-blk guests drivers/vhost/Kconfig | 12 + drivers/vhost/Makefile | 3 + drivers/vhost/blk.c | 831 +++++++++++++++++++++++++++++++++++++ include/uapi/linux/vhost.h | 5 + 4 files changed, 851 insertions(+) create mode 100644 drivers/vhost/blk.c -- 2.31.1
