From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:45983) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Uf5dS-0004Pe-3p for qemu-devel@nongnu.org; Wed, 22 May 2013 05:47:07 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Uf5dQ-0000R4-L0 for qemu-devel@nongnu.org; Wed, 22 May 2013 05:47:06 -0400 Received: from mail-wg0-x230.google.com ([2a00:1450:400c:c00::230]:60138) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Uf5dQ-0000QQ-F2 for qemu-devel@nongnu.org; Wed, 22 May 2013 05:47:04 -0400 Received: by mail-wg0-f48.google.com with SMTP id f12so594449wgh.27 for ; Wed, 22 May 2013 02:47:03 -0700 (PDT) Date: Wed, 22 May 2013 11:47:00 +0200 From: Stefan Hajnoczi Message-ID: <20130522094700.GC30148@stefanha-thinkpad.redhat.com> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Subject: Re: [Qemu-devel] 'qemu-nbd' explicit flush List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Mark Trumpold Cc: qemu-devel@nongnu.org, markt@tachyon.net On Tue, May 21, 2013 at 08:01:10PM +0000, Mark Trumpold wrote: > Linux kernel 3.3.1 with Qemu patch to enable kernel flushing: > http://thread.gmane.org/gmane.linux.drivers.nbd.general/1108 Did you check that the kernel is sending NBD_FLUSH commands? You can use tcpdump and then check the captured network traffic. > Usage example: > 'qemu-nbd --cache=writeback -c /dev/nbd0 /images/my-qcow.img' > 'mount /dev/nbd0 /my-mount-point' > > Everything does flush correctly when I first unmount and then disconnect the device; however, in my case I am not able to unmount things before snapshotting. > > I tried several approaches externally to flush the device. For example: > 'mount -o remount,ro /dev/nbd0' > 'blockdev --flushbufs /dev/nbd0' Did you try plain old sync(1)? > I have been looking at the Qemu source code and in user space 'nbd.c' in routine 'nbd_trip' I see the case 'NBD_CMD_FLUSH' which looks to be called from the NBD socket interface. Here I see 'bdrv_co_flush(exp->bs)' which looks promising; however, I don't know how to setup the 'bs' pointer for the call. bs is the block device which was exported using: exp = nbd_export_new(bs, dev_offset, fd_size, nbdflags, nbd_export_closed); in qemu-nbd.c:main(). > Ideally, I would like to add a command line parm to 'qemu-nbd.c' to explicitely do the flush, but so far no luck. Doing that is a little tricky, I think there are two options: 1. Add a signal handler (like SIGHUP or SIGUSR1) to qemu-nbd which flushes all exports. 2. Instantiate a block/nbd.c client that connects to the running qemu-nbd server (make sure format=raw). Then call bdrv_flush() on the NBD client. You must use the qemu-nbd --shared=2 option. Stefan