From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755941Ab2BAINi (ORCPT ); Wed, 1 Feb 2012 03:13:38 -0500 Received: from mx1.redhat.com ([209.132.183.28]:1092 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753793Ab2BAINh (ORCPT ); Wed, 1 Feb 2012 03:13:37 -0500 Message-ID: <4F28F42A.1050103@redhat.com> Date: Wed, 01 Feb 2012 09:13:30 +0100 From: Paolo Bonzini User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:9.0) Gecko/20111222 Thunderbird/9.0 MIME-Version: 1.0 To: Pekka Enberg CC: Linux Kernel Mailing List , linux-scsi , "kvm@vger.kernel.org" , Rusty Russell , Sasha Levin Subject: Re: [PATCH v4 0/3] virtio-scsi driver References: <1327077939-7084-1-git-send-email-pbonzini@redhat.com> <4F26596B.6030703@redhat.com> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@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