From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:48575) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1S72iF-0002VJ-2q for qemu-devel@nongnu.org; Mon, 12 Mar 2012 06:42:48 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1S72i8-0003yd-TT for qemu-devel@nongnu.org; Mon, 12 Mar 2012 06:42:46 -0400 Received: from mx1.redhat.com ([209.132.183.28]:26957) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1S72i8-0003yW-Lm for qemu-devel@nongnu.org; Mon, 12 Mar 2012 06:42:40 -0400 Received: from int-mx02.intmail.prod.int.phx2.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) by mx1.redhat.com (8.14.4/8.14.4) with ESMTP id q2CAgcX3027074 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=OK) for ; Mon, 12 Mar 2012 06:42:39 -0400 Message-ID: <4F5DD31C.20608@redhat.com> Date: Mon, 12 Mar 2012 12:42:36 +0200 From: Avi Kivity MIME-Version: 1.0 References: <1331226917-6658-1-git-send-email-pbonzini@redhat.com> <1331226917-6658-8-git-send-email-pbonzini@redhat.com> <4F58F286.6060209@redhat.com> <4F5A3306.3010006@redhat.com> In-Reply-To: <4F5A3306.3010006@redhat.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [RFC PATCH 07/17] block: make high level discard operation always zero List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Kevin Wolf Cc: Paolo Bonzini , qemu-devel@nongnu.org On 03/09/2012 06:42 PM, Kevin Wolf wrote: > >> > >> While (3) can be worked around, the only way around the other two, > >> unfortunately, is support in the formats and protocols. We will still > >> provide device options to opt out of this, but with raw and qed covered > >> (+ qcow2 without backing file, and qcow3 in the future) it should not > >> be too bad. > > > > Can't qcow2 with a backing file also be supported? Zero out the first > > cluster, and remember it. The following discards can reuse this zero > > cluster, as long as it hasn't been overwritten. > > qcow2 can't handle clusters that are referenced twice from the same L1 > table. This would require a reverse lookup to adjust the QCOW_O_COPIED > flags in the L2 tables containing the other references. Don't follow, sorry. What adjustment are you talking about? If it's a 1->0 transition, is it mandatory to adjust the flag? That is, it it legal to have a refcount of exactly one, but have the flag clear? -- error compiling committee.c: too many arguments to function