I have a working configuration using the signal approach suggested by Stefan.
'qemu-nbd.c' is patched as follows:
do {
main_loop_wait(false);
+ if (sighup_reported) {
+ sighup_reported = false;
+ bdrv_drain_all();
+ bdrv_flush_all();
}
} while (!sigterm_reported && (persistent || !nbd_started || nb_fds > 0));
The driving script was patched as follows:
mount -o remount,ro /dev/nbd0
blockdev --flushbufs /dev/nbd0
+ kill -HUP <qemu-nbd process id>
I needed to retain 'blockdev --flushbufs' for things to work. Seems the
'bdrv_flush_all' is flushing what is being missed by the blockdev flush. I did
not go back an retest with 'fsync' or other approaches I had tried before.
Thanks again Paolo and Stefan for your help!!
Regards,
Mark T.
-----Original Message-----
From: Paolo Bonzini [mailto:pbonz...@redhat.com]
Sent: Wednesday, May 22, 2013 04:07 AM
To: 'Stefan Hajnoczi'
Cc: 'Mark Trumpold', qemu-devel@nongnu.org, ma...@tachyon.net
Subject: Re: 'qemu-nbd' explicit flush
Il 22/05/2013 11:47, Stefan Hajnoczi ha scritto:
> 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)?
This could also work:
dd if=/dev/zero of=dummy oflag=sync bs=512 count=1
> 1. Add a signal handler (like SIGHUP or SIGUSR1) to qemu-nbd which
> flushes all exports.
That would be a useful addition anyway.
Paolo
