From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([140.186.70.92]:38514) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1QTaE9-0002nV-Ra for qemu-devel@nongnu.org; Mon, 06 Jun 2011 09:52:25 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1QTaE7-0003Ff-Qc for qemu-devel@nongnu.org; Mon, 06 Jun 2011 09:52:21 -0400 Received: from mx1.redhat.com ([209.132.183.28]:37115) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1QTaE7-0003FV-9X for qemu-devel@nongnu.org; Mon, 06 Jun 2011 09:52:19 -0400 From: Markus Armbruster References: <1305808412-16994-1-git-send-email-kwolf@redhat.com> <1305808412-16994-6-git-send-email-kwolf@redhat.com> <20110526181208.46aba2ae@doriath> <20110601104456.5469329b@doriath> <4DE64690.8020500@redhat.com> <4DEC98FF.5010605@redhat.com> <4DECCE6C.9010103@redhat.com> Date: Mon, 06 Jun 2011 15:52:15 +0200 In-Reply-To: <4DECCE6C.9010103@redhat.com> (Kevin Wolf's message of "Mon, 06 Jun 2011 14:56:12 +0200") Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Subject: Re: [Qemu-devel] [PATCH 05/18] ide: Turn debug messages into assertions List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Kevin Wolf Cc: Stefan Hajnoczi , qemu-devel@nongnu.org, Luiz Capitulino Kevin Wolf writes: > Am 06.06.2011 13:57, schrieb Markus Armbruster: >>>>> Not sure what's the best way of fixing this. Maybe just ignoring >>>>> -snapshot for read-only block devices? >>>> >>>> Why not, the combination is pointless. >>> >>> It could start making a difference in some obscure combinations. Imagine >>> a read-only image with a backing file, -snapshot and the 'commit' >>> monitor command. >>> >>> Sounds pretty insane, but I wouldn't bet that people aren't using it... >> >> People try all kinds of insane things. The question is whether we can >> change it anyway. > > We have a backing file chain like base <- cow [<- tmp], and the drive is > read-only. > > Currently, 'commit' means that tmp is committed to cow (i.e. nothing > happens because it's read-only). After changing it, we would commit the > content of cow to base and possibly corrupt other images that are based > on base. > > We can hope that nobody would be hit by it in practice, but it's not a > change I'd feel very comfortable about. So the one effect -snapshot has on a read-only drive is to neuter the commit command. Hmm. Naive question: shouldn't commit require the drive to be read/write? It writes both the backing image and the COW... [...]