From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:49726) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UDIrn-0001Vp-39 for qemu-devel@nongnu.org; Wed, 06 Mar 2013 13:15:06 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1UDIrj-0004rf-R0 for qemu-devel@nongnu.org; Wed, 06 Mar 2013 13:15:03 -0500 Received: from mx1.redhat.com ([209.132.183.28]:15750) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UDIrj-0004r8-Ig for qemu-devel@nongnu.org; Wed, 06 Mar 2013 13:14:59 -0500 Date: Wed, 6 Mar 2013 13:14:54 -0500 From: Jeff Cody Message-ID: <20130306181454.GB3743@localhost.localdomain> References: <51378200.5010705@dlhnet.de> <513785A0.1010001@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <513785A0.1010001@redhat.com> Subject: Re: [Qemu-devel] [PATCH 1/7] block: only force IO completion in .bdrv_truncate if we are shrinking List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Paolo Bonzini Cc: kwolf@redhat.com, sw@weilnetz.de, Peter Lieven , qemu-devel@nongnu.org, stefanha@redhat.com On Wed, Mar 06, 2013 at 07:06:24PM +0100, Paolo Bonzini wrote: > Il 06/03/2013 18:50, Peter Lieven ha scritto: > >> > Commit 9a665b2b made bdrv_truncate() call bdrv_drain_all(), but this breaks > >> > QCOW images, as well other future image formats (such as VHDX) that may call > >> > bdrv_truncate(bs->file) from within a read/write operation. For example, QCOW > >> > will cause an assert, due to tracked_requests not being empty (since the > >> > read/write that called bdrv_truncate() is still in progress). > > I'm not sure such bdrv_truncate calls are necessary. QCOW2 doesn't have > them (almost; there is one in qcow2_write_compressed, I'm not even sure > that one is necessary though), and I think QCOW's breaks using it with a > block device as a backing file. > > Paolo QCOW breaks with it using a normal raw posix file as a device. As a test: qemu-img create -f qcow test.qcow 5G. Now run qemu with that drive mounted, and try to partition and format it. QEMU now asserts. The nicety of being able to using truncate during a write call, especially for VHDX (which can have relatively large block/cluster sizes), so to grow the file sparsely in a dynamically allocated file.