From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753474Ab2BMH6F (ORCPT ); Mon, 13 Feb 2012 02:58:05 -0500 Received: from mx1.redhat.com ([209.132.183.28]:14582 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750698Ab2BMH6C (ORCPT ); Mon, 13 Feb 2012 02:58:02 -0500 Message-ID: <4F38C27E.7030300@redhat.com> Date: Mon, 13 Feb 2012 09:57:50 +0200 From: Dor Laor Reply-To: dlaor@redhat.com User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:10.0) Gecko/20120131 Thunderbird/10.0 MIME-Version: 1.0 To: Christian Borntraeger CC: James Bottomley , Paolo Bonzini , Christian Hoff , borntrae@linux.vnet.ibm.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, linux-scsi@vger.kernel.org, mst@redhat.com, rusty@rustcorp.com.au Subject: Re: Pe: [PATCH v5 1/3] virtio-scsi: first version References: <4F30F4EE.4080607@redhat.com> <4F311154.9080407@de.ibm.com> <4F3119A0.7080005@redhat.com> <4F312492.1040002@de.ibm.com> <4F313526.2050907@redhat.com> <4F3390FB.80107@redhat.com> <1329077777.21613.60.camel@dabdike.int.hansenpartnership.com> <4F38B657.7060405@de.ibm.com> In-Reply-To: <4F38B657.7060405@de.ibm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 02/13/2012 09:05 AM, Christian Borntraeger wrote: > On 12/02/12 21:16, James Bottomley wrote: >> Well, no-one's yet answered the question I had about why. > > Just to give one example from a different angle: > In the big datacenters tape libraries are still very important, and lots > of them have a scsi attachement. virtio-blk certainly is not the right > way to handle those. Furthermore it seems even pretty hard to craft > a virtio-tape since most of those libraries have vendor specific library > controls (via sg). We would need to duplicate scsi generic (hint, hint :-) > >> virtio-scsi seems to be a basic duplication of virtio-blk except that it seems to >> fix some problems virtio-blk has. Namely queue parameter discover, >> which virtio-blk doesn't seem to do. There may also be a reason to cut >> the stack lower down. Error handling is most often cited for this, but >> no-one's satisfactorily explaned why it's better to do error handling in >> the guest instead of the host. >> >> Could someone please explain to me why you can't simply fix virtio-blk? > > I dont think that virtio-scsi will replace virtio-blk everywhere. For non-scsi > block devices, image files or logical volumes virtio-blk seems to be the right > approach, I think. +1 virtio-scsi is superior w.r.t: - Device support: tapes, cdroms, other - Does guest-host mapped multipath - Supports plenty of virtual disks mapped to the guest w/o need for a pci slot per each virtio-blk - offload fancy/new/sophisticated scsi commands from the guest to the storage array w/o need for qemu implementation. Example XCOPY. There are some more goodies like ability to support windows guest clustering w/o hacky versions of scsi pass through over virtio-blk. virtio-blk is also a candidate to change the request based towards bio based implementation, so sticking to it does not buy us too much. > >> Or would virtio-blk maintainers give a reason why they're unwilling to >> have it fixed? > > I dont consider virtio-blk broken. It just doesnt cover everything. > > Christian > > -- > To unsubscribe from this list: send the line "unsubscribe kvm" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html