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
