Features/VirtioSCSI

Overview

A virtio storage interface for efficient I/O that overcomes virtio-blk limitations and supports advanced SCSI hardware.

Features

• /dev/sda inside guest
• Multiple LUNs per PCI adapter, solves virtio-blk 1:1 scalability problem
• SCSI passthrough (tape, CD burning, ...)

Status

• Virtio device specification
  • Committed
  • http://ozlabs.org/~rusty/virtio-spec/virtio-0.9.4.pdf
• Linux guest driver
  • Patch on mailing list
  • https://lkml.org/lkml/2012/2/5/75
• QEMU hardware emulation using SCSI emulation
  • Patches on mailing list
  • http://lists.gnu.org/archive/html/qemu-devel/2012-02/msg01642.html
• QEMU hardware emulation using vhost-scsi
  • (Stalled) work-in-progress, contact Stefan
  • QEMU: http://repo.or.cz/w/qemu/stefanha.git/shortlog/refs/heads/virtio-scsi
  • Linux: https://github.com/stefanha/linux/tree/tcm_vhost
• Libvirt support
  • Patches on mailing list
  • http://www.redhat.com/archives/libvir-list/2012-January/msg00537.html
• SeaBIOS support
  • Patches on mailing list

Future tasks

• Pick up vhost-scsi work again:
  • Port QEMU hw/virtio-scsi.c vhost-scsi support onto latest code
  • add QEMU Object Model (QOM) support to the vhost-scsi device
  • Port LIO vhost-scsi code onto latest lio.git code
  • Update vhost-scsi to implement latest virtio-scsi device specification
  • Ensure vhost-scsi I/O still works
  • Design libvirt integration for LIO
• QEMU SCSI target:
  • Add support for WRITE SAME commands
  • Add support for rerror/werror to scsi-generic
• Complete virtio-scsi spec implementation:
  • Add support for hotplug events
  • Add support for asynchronous media change notifications
  • Multi-queue (with fine-grained locking in the Linux driver?)
• Use cases to test:
  • CD-ROM/DVD burning passthrough
  • Tape passthrough
  • 100s of LUNs (scalability)
• Use cases to flesh out:
  • LUN hotplug (known to work with manual hotplug)
  • Multipath inside guest
• Performance analysis
  • Compare against virtio-blk I/O (rough numbers: 6% slower on iozone with a tmpfs-backed disk)
  • Compare passthrough performance against bare metal host SCSI
  • Measure scalability and where the bottlenecks are (full virtqueues? locks?)

Specifications

• Virtio specification
• SCSI Standards - especially SAM, SPC, SBC
• PCI Local Bus 3.0 - useful for understanding virtio-pci

Repositories

* QEMU (virtio-scsi): git://github.com/bonzini/qemu.git virtio-scsi
* Linux (virtio-scsi): git://github.com/bonzini/virtio-scsi.git
* QEMU (vhost-scsi): http://repo.or.cz/w/qemu/stefanha.git/shortlog/refs/heads/virtio-scsi
* tcm_vhost: https://github.com/stefanha/linux/tree/tcm_vhost

Links

• KVM Forum 2011 presentation Virtio SCSI: An alternative virtualized storage stack for KVM