From mboxrd@z Thu Jan 1 00:00:00 1970 From: Markus Armbruster Subject: Re: [Qemu-devel] KVM call agenda for October 25 Date: Wed, 26 Oct 2011 13:12:51 +0200 Message-ID: References: <4EA6ACFE.6090109@redhat.com> <4EA6B41B.3000903@codemonkey.ws> <4EA6C00B.3030701@redhat.com> <4EA6C25C.8000502@codemonkey.ws> <4EA7C1B8.9000903@redhat.com> <4EA7D5E0.7070101@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Kevin Wolf , qemu-devel@nongnu.org, kvm@vger.kernel.org To: Paolo Bonzini Return-path: Received: from mx1.redhat.com ([209.132.183.28]:58651 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932673Ab1JZLMz (ORCPT ); Wed, 26 Oct 2011 07:12:55 -0400 In-Reply-To: <4EA7D5E0.7070101@redhat.com> (Paolo Bonzini's message of "Wed, 26 Oct 2011 11:41:52 +0200") Sender: kvm-owner@vger.kernel.org List-ID: Paolo Bonzini writes: > On 10/26/2011 10:48 AM, Markus Armbruster wrote: >> Sector size is a device property. >> >> If the user asks for a 4K sector disk, and the backend can't support >> that, we need to reject the configuration. Just like we reject >> read-only backends for read/write disks. > > Isn't it the other way round, i.e. the user asks for a 512-byte sector > disk (i.e. the default) with cache=none but the disk has 4k sectors? Let me rephrase: If the user asks for a FOO disk, and the backend can't support that, we need to reject the configuration. Just like we reject read-only backends for read/write disks. > We're basically saying "choose between NFS and migration if you have > 4k sector disks but your guest doesn't support them". Understandable > perhaps, but not exactly kind, and virtualization is also about > shielding from this kind of hardware dependency even at the cost of > performance. QEMU should just warn about performance degradations, > erroring out would be a policy decision that should be up to > management. I don't have strong opinions on that. >> It's okay to default device properties to some backend-dependent value, >> if that improves usability. > > On the other hand, not all guests support 4k-sectors properly. You can't pick perfect defaults for all conceivable guests. Life's tough.