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numam-spdk/doc/virtio.md
Dariusz Stojaczyk 71ea826507 bdev_virtio: added doc page 
Change-Id: Ia88ae52117068ac395dad9ad3d7ac818e41077fb
Signed-off-by: Dariusz Stojaczyk <dariuszx.stojaczyk@intel.com>
Reviewed-on: https://review.gerrithub.io/380956
Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Paul Luse <paul.e.luse@intel.com>
Reviewed-by: Daniel Verkamp <daniel.verkamp@intel.com>
2017-10-20 19:44:23 -04:00

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Virtio SCSI driver

Introduction

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.

The driver, just like the SPDK @ref vhost, is using pollers instead of standard interrupts to check for an I/O response. It bypasses kernel interrupt and context switching overhead of QEMU and guest kernel, significantly boosting the overall I/O performance.

Virtio SCSI 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.

Multiqueue

The Virtio SCSI controller will automatically manage virtio queue distribution. Currently each thread doing an I/O on a single bdev will get an exclusive queue. Multi-threaded I/O on bdevs from a single Virtio-SCSI controller is not supported.

Limitations

The Virtio SCSI driver is still experimental. Current implementation has many limitations:

  • supports only up to 8 hugepages (implies only 1GB sized pages are practical)
  • single LUN per target
  • only SPDK vhost-scsi controllers supported
  • no RPC
  • no multi-threaded I/O for single-queue virtio devices