From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paolo Bonzini Subject: Re: [PATCH v4 0/3] virtio-scsi driver Date: Wed, 01 Feb 2012 09:13:30 +0100 Message-ID: <4F28F42A.1050103@redhat.com> References: <1327077939-7084-1-git-send-email-pbonzini@redhat.com> <4F26596B.6030703@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org To: Pekka Enberg Cc: Linux Kernel Mailing List , linux-scsi , "kvm@vger.kernel.org" , Rusty Russell , Sasha Levin List-Id: linux-scsi@vger.kernel.org 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. Paolo