Re: [RFC patch 0/1] block: vhost-blk backend

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jul 25, 2022 at 11:55:26PM +0300, Andrey Zhadchenko wrote:
> 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 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

That one seems stalled. Do you plan to work on that too?

> Also by using kernel modules we can bypass iothread limitation and finaly scale
> block requests with cpus for high-performance devices. This is planned to be
> implemented in next version.
> 
> Linux kernel module part:
> https://lore.kernel.org/kvm/20220725202753.298725-1-andrey.zhadchenko@virtuozzo.com/
> 
> test setups 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       |
> 
> Andrey Zhadchenko (1):
>   block: add vhost-blk backend


From vhost/virtio side the patchset looks ok. But let's see what do
block devs think about it.


>  configure                     |  13 ++
>  hw/block/Kconfig              |   5 +
>  hw/block/meson.build          |   1 +
>  hw/block/vhost-blk.c          | 395 ++++++++++++++++++++++++++++++++++
>  hw/virtio/meson.build         |   1 +
>  hw/virtio/vhost-blk-pci.c     | 102 +++++++++
>  include/hw/virtio/vhost-blk.h |  44 ++++
>  linux-headers/linux/vhost.h   |   3 +
>  8 files changed, 564 insertions(+)
>  create mode 100644 hw/block/vhost-blk.c
>  create mode 100644 hw/virtio/vhost-blk-pci.c
>  create mode 100644 include/hw/virtio/vhost-blk.h
> 
> -- 
> 2.31.1