From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:43613) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1anptY-0007cA-Q0 for qemu-devel@nongnu.org; Wed, 06 Apr 2016 12:01:34 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1anptU-0006Uo-VW for qemu-devel@nongnu.org; Wed, 06 Apr 2016 12:01:28 -0400 References: <1459783562-28859-1-git-send-email-kwolf@redhat.com> <20160406095947.GB5098@noname.redhat.com> From: Paolo Bonzini Message-ID: <570532CC.5090209@redhat.com> Date: Wed, 6 Apr 2016 18:01:16 +0200 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH for-2.6] block: Forbid I/O throttling on nodes with multiple parents for 2.6 List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Alberto Garcia , Kevin Wolf Cc: stefanha@redhat.com, qemu-block@nongnu.org, qemu-devel@nongnu.org On 06/04/2016 17:55, Alberto Garcia wrote: >> > [BB] overlay >> > | >> > | >> > [BDS] qcow2 [BB] backing >> > | | >> > +--------------------+ >> > . | >> > . I/O throttling >> > . | >> > [BDS] raw >> > . >> > . >> > . > I see, thanks for the explanation! > > But what's the use of having a BlockBackend on a node that is used as a > backing image? Is it because "we allow this so let's prevent the user > from doing weird things", or is there anything useful we can do with it? The backing image can be the one that is visible to the guest, while the overlay can be a block-backup target (with 'sync':'none) whose BlockBackend is created by the NBD server. Paolo