Features/VirtioSCSI

From QEMU

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

Note: you must use either Paolo's trees or Stefan's trees, you cannot mix QEMU from Paolo with Linux from Stefan and vice versa.

Paolo's code with QEMU SCSI emulation

QEMU (virtio-scsi): git://github.com/bonzini/qemu.git virtio-scsi
Linux (virtio-scsi): git://github.com/bonzini/virtio-scsi.git

Stefan's code with tcm_vhost

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

Zhi Yong Wu's code

 Note: His git tree has mainly done two works:
 1.) integrated vhost-scsi with qemu.git/master;
 2.) updated tcm_vhost based on latest virio-scsi spec