Commit 36506448 authored by Dariusz Stojaczyk's avatar Dariusz Stojaczyk Committed by Jim Harris
Browse files

doc/virtio: minor updates 



Reflect three changes:
 * Virtio now works with Linux and QEMU vhost targets
 * The Virtio library was separated from bdev modules
 * Virtio Blk bdev module has been added

Change-Id: I7d3075ca863a1142b3ddcc4d18e982f3b7c72202
Signed-off-by: default avatarDariusz Stojaczyk <dariuszx.stojaczyk@intel.com>
Reviewed-on: https://review.gerrithub.io/405288


Tested-by: default avatarSPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: default avatarPawel Wodkowski <pawelx.wodkowski@intel.com>
Reviewed-by: default avatarDaniel Verkamp <daniel.verkamp@intel.com>
Reviewed-by: default avatarJim Harris <james.r.harris@intel.com>
parent 07526d85

doc/virtio.md

+10 −16
Original line number Diff line number Diff line 
# Virtio SCSI driver {#virtio}

# Virtio driver {#virtio}



# Introduction {#virtio_intro}



Virtio SCSI driver is an initiator for SPDK @ref vhost application. The

driver allows any SPDK app to connect to another SPDK instance exposing

a vhost-scsi device. The driver will enumerate targets on the device (which acts

as a SCSI controller) and create *virtual* bdevs usable by any SPDK application.

Sending an I/O request to the Virtio SCSI bdev will put the request data into

a Virtio queue that is processed by the host SPDK app exposing the

controller. The host, after sending I/O to the real drive, will put the response

back into the Virtio queue. Then, the response is received by the Virtio SCSI

driver.

SPDK Virtio driver is a C library that allows communicating with Virtio devices.

It allows any SPDK application to become an initiator for (SPDK) vhost targets.



Virtio SCSI driver supports two different usage models:

The driver supports two different usage models:

* PCI - This is the standard mode of operation when used in a guest virtual

machine, where QEMU has presented the virtio-scsi controller as a virtual

PCI device.

* User vhost - Can be used to connect to a vhost-scsi socket directly on the

same host.

machine, where QEMU has presented the virtio controller as a virtual PCI device.

* vhost-user - Can be used to connect to a vhost socket directly on the same host.



The driver, just like the SPDK @ref vhost, is using pollers instead of standard

interrupts to check for an I/O response. If used inside a VM, it bypasses interrupt

and context switching overhead of QEMU and guest kernel, significantly boosting

the overall I/O performance.



This Virtio library is currently used to implement two bdev modules:

@ref bdev_config_virtio_scsi and Virtio Blk. These modules will export generic

SPDK block devices usable by any other SPDK application.



# Limitations {#virtio_limitations}



Current Virtio-SCSI implementation has a couple of limitations:

 * supports only up to 8 hugepages (implies only 1GB sized pages are practical)

 * single LUN per target
 
 * only SPDK vhost-scsi controllers supported