Re: [PATCH v4 0/3] virtio-scsi driver

From: Paolo Bonzini
Date: Wed Feb 01 2012 - 03:13:40 EST

On 02/01/2012 08:31 AM, Pekka Enberg wrote:
What's the benefit of virtio-scsi over virtio-blk?

Most of this is in the spec or the KVM Forum 2011 presentation.

1) scalability limitations: virtio-blk-over-PCI puts a strong upper
limit on the number of devices that can be added to a guest. Common
configurations have a limit of ~30 devices. While this can be worked
around by implementing a PCI-to-PCI bridge, or by using multifunction
virtio-blk devices, these solutions either have not been implemented
yet, or introduce management restrictions.

2) limited flexibility: virtio-blk does not support all possible storage
scenarios. For example, persistent reservations require you to pass a
whole LUN to the guest, they do not work with images. In principle,
virtio-scsi provides anything that the underlying SCSI target supports.
The SCSI target can also be the in-kernel LIO target, which can
talk to virio-scsi via vhost.

3) limited extensibility: over the time, many features have been added
to virtio-blk. Each such change requires modifications to the virtio
specification, to the guest drivers, and to the device model in the
host. The virtio-scsi spec has been written to follow SAM conventions,
and exposing new features to the guest will only require changes to the
host's SCSI target implementation.

Are we going to support both or eventually phase out virtio-blk?

Certainly older guests will have no virtio-scsi support, so it's going
to stay with us for a long time.

Have the virtio specification changes been reviewed? Can we guarantee
stable ABI for the virtio-scsi driver?

Of course. I would have proposed it for staging otherwise.

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