36. Vhost_scsi Sample Application

The vhost_scsi sample application implemented a simple SCSI block device, which used as the backend of Qemu vhost-user-scsi device. Users can extend the exist example to use other type of block device(e.g. AIO) besides memory based block device. Similar with vhost-user-net device, the sample application used domain socket to communicate with Qemu, and the virtio ring was processed by vhost_scsi sample application.

The sample application reuse lots codes from SPDK(Storage Performance Development Kit, https://github.com/spdk/spdk) vhost-user-scsi target, for DPDK vhost library used in storage area, user can take SPDK as reference as well.

36.1. Testing steps

This section shows the steps how to start a VM with the block device as fast data path for critical application.

36.2. Compiling the Application

To compile the sample application see Compiling the Sample Applications.

The application is located in the examples sub-directory.

You will also need to build DPDK both on the host and inside the guest

36.2.1. Start the vhost_scsi example

./vhost_scsi -m 1024

36.2.2. Start the VM

qemu-system-x86_64 -machine accel=kvm \
    -m $mem -object memory-backend-file,id=mem,size=$mem,\
    mem-path=/dev/hugepages,share=on -numa node,memdev=mem \
    -drive file=os.img,if=none,id=disk \
    -device ide-hd,drive=disk,bootindex=0 \
    -chardev socket,id=char0,path=/tmp/vhost.socket \
    -device vhost-user-scsi-pci,chardev=char0,bootindex=2 \


You must check whether your Qemu can support “vhost-user-scsi” or not, Qemu v2.10 or newer version is required.

36.3. Vhost_scsi Common Issues

  • vhost_scsi can not start with block size 512 Bytes:

    Currently DPDK vhost library was designed for NET device(althrough the APIs are generic now), for 512 Bytes block device, Qemu BIOS(x86 BIOS Enhanced Disk Device) will enumerate all block device and do some IOs to those block devices with 512 Bytes sector size. DPDK vhost library can not process such scenarios(both BIOS and OS will enumerate the block device), so as a workaround, the vhost_scsi example application hardcoded the block size with 4096 Bytes.

  • vhost_scsi can only support the block device as fast data disk(non OS image):

    Make sure bootindex=2 Qemu option is given to vhost-user-scsi-pci device.