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Spdk/doc/vhost.md
Pawel Wodkowski c0dce4a709 doc/vhost: vhost block documentation 
Add descritpion of vhost block.

Change-Id: I20c36d6b249aec09db0b5665c7ea28aea5a61358
Signed-off-by: Pawel Wodkowski <pawelx.wodkowski@intel.com>
Signed-off-by: Dariusz Stojaczyk <dariuszx.stojaczyk@intel.com>
Reviewed-on: https://review.gerrithub.io/364646
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Reviewed-by: Paweł Niedźwiecki <pawelx.niedzwiecki@intel.com>
Tested-by: Daniel Verkamp <daniel.verkamp@intel.com>
Reviewed-by: Daniel Verkamp <daniel.verkamp@intel.com>
2017-07-05 13:21:32 -04:00

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vhost

vhost Getting Started Guide

The Storage Performance Development Kit vhost application is named "vhost". This application extends SPDK to present virtio storage controllers to QEMU-based VMs and process I/O submitted to devices attached to those controllers.

Prerequisites

The base SPDK build instructions are located README.md in SPDK main directory. This guide assumes familiarity with building SPDK using the default options.

Supported Guest Operating Systems

The guest OS must contain virtio drivers. The SPDK vhost target has been tested with Ubuntu 16.04, Fedora 25, Windows 2012 R2.

Building

SPDK

The vhost target is built by default. To enable/disable building the vhost target, either modify the following line in the CONFIG file in the root directory:

    CONFIG_VHOST?=y

Or specify on the command line:

    make CONFIG_VHOST=y

Once built, the binary will be at app/vhost/vhost.

QEMU

Vhost functionality is dependent on QEMU patches to enable virtio-scsi and virtio-block in userspace - those patches are currently working their way through the QEMU mailing list, but temporary patches to enable this functionality are available in the spdk branch at https://github.com/spdk/qemu.

Configuration

SPDK

A vhost specific configuration file is used to configure the SPDK vhost target. A fully documented example configuration file is located at etc/spdk/vhost.conf.in. This file defines the following:

Storage Backends

Storage backends are devices which will be exposed to the guest OS. In case of vhost block controller they are exposed as block devices, in case of vhost scsi as SCSI LUNs on devices attached to the vhost-scsi controller. SPDK supports several different types of storage backends, including NVMe, Linux AIO, malloc ramdisk and CephRBD. Refer to @ref bdev_getting_started for additional information on specifying storage backends in the configuration file.

Mappings Block Controllers and Storage Backends

The vhost target is exposing block controllers to the virtual machines. The device in the vhost controller is associated with an SPDK block device and configuration file defines those associations. The block device to Dev mapping is specified in the configuration file as:

[VhostBlkX]
  Name vhost.X          # Name of vhost socket
  Dev BackendX          # "BackendX" is block device name from previous
                        # sections in config file
  #Cpumask 0x1          # Optional parameter defining which core controller uses

Mappings Between SCSI Controllers and Storage Backends

The vhost target is exposing SCSI controllers to the virtual machines. Each device in the vhost controller is associated with an SPDK block device and configuration file defines those associations. The block device to Dev mappings are specified in the configuration file as:

[VhostScsiX]
  Name vhost.X          # Name of vhost socket
  Dev 0 BackendX        # "BackendX" is block device name from previous
                        # sections in config file
  Dev 1 BackendY
  ...
  Dev n BackendN
  #Cpumask 0x1          # Optional parameter defining which core controller uses

Vhost Sockets

Userspace vhost uses UNIX domain sockets for communication between QEMU and the vhost target. Each vhost controller is associated with a UNIX domain socket file with filename equal to the Name argument in configuration file. Sockets are created at current working directory when starting the SPDK vhost target.

Core Affinity Configuration

Vhost target can be restricted to run on certain cores by specifying a ReactorMask. Default is to allow vhost target work on core 0. For NUMA systems it is essential to run vhost with cores on each socket to achieve optimal performance.

To specify which core each controller should use, it can be defined by optional Cpumask parameter in configuration file. For NUMA systems the Cpumask should specify cores on the same CPU socket as its associated VM. Application will pick one core from ReactorMask masked by Cpumask. Cpumask must be subset of ReactorMask.

QEMU

Userspace vhost-scsi adds the following command line option for QEMU:

       -device vhost-user-scsi-pci,id=scsi0,chardev=char0

Userspace vhost-block adds the following command line option for QEMU:

       -device vhost-user-blk-pci,logical_block_size=4096,size=512M,chardev=char0

In order to start qemu with vhost you need to specify following options:

  • Socket, which QEMU will use for vhost communication with SPDK:
       -chardev socket,id=char0,path=/path/to/vhost/socket
  • Hugepages to share memory between vm and vhost target
       -object memory-backend-file,id=mem,size=1G,mem-path=/dev/hugepages,share=on

Running Vhost Target

To get started, the following example is usually sufficient:

app/vhost/vhost -c /path/to/vhost.conf

A full list of command line arguments to vhost can be obtained by:

app/vhost/vhost -h

Example

Assume that qemu and spdk are in respectively qemu and spdk directories.

     ./qemu/build/x86_64-softmmu/qemu-system-x86_64 \
             -m 1024 \
             -object memory-backend-file,id=mem,size=1G,mem-path=/dev/hugepages,share=on \
             -numa node,memdev=mem \
             -drive file=$PROJECTS/os.qcow2,if=none,id=disk \
             -device ide-hd,drive=disk,bootindex=0 \
             -chardev socket,id=char0,path=./spdk/vhost.0 \
             -device vhost-user-scsi-pci,id=scsi0,chardev=char0 \
             -chardev socket,id=char1,path=./spdk/vhost.1 \
             -device vhost-user-blk-pci,logical_block_size=4096,size=512M,chardev=char1 \
             --enable-kvm

Experimental features

Multi-Queue Block Layer (blk_mq)

It is possible to use multiqueue feature in vhost. To enable it on linux it is required to modify kernel options inside virtual machine.

Instructions below for Ubuntu OS:

  1. vi /etc/default/grub
  2. Make sure mq is enabled: GRUB_CMDLINE_LINUX="scsi_mod.use_blk_mq=1"
  3. sudo update-grub
  4. Reboot virtual machine

To achieve better performance make sure to increase number of cores assigned to vm.

Known bugs and limitations

Hot plug is not supported

Hot plug is not supported in vhost yet. Event queue path doesn't handle that case yet.