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doc/virtio: minor updates

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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: Dariusz Stojaczyk <dariuszx.stojaczyk@intel.com>
Reviewed-on: https://review.gerrithub.io/405288
Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Pawel Wodkowski <pawelx.wodkowski@intel.com>
Reviewed-by: Daniel Verkamp <daniel.verkamp@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
This commit is contained in:
Dariusz Stojaczyk 2018-03-26 08:10:07 +02:00 committed by Jim Harris
parent 07526d85ef
commit 36506448d4
1 changed files with 10 additions and 16 deletions
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doc/virtio.md
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# 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