Re: [Qemu-devel] 'qemu-nbd' explicit flush

["Mark Trumpold" <markt@netqa.com>]

Thank you guys for responding!!

> 
> > 1. Add a signal handler (like SIGHUP or SIGUSR1) to qemu-nbd which
> >    flushes all exports.
> 
> That would be a useful addition anyway.
> 
> Paolo

This is exactly what I was going to try today.  I'm just getting familiar with Qemu source.
I'll let you know how it goes..

Thanks again Paolo and Stefan.
Regards,
Mark Trumpold


-----Original Message-----
From: Paolo Bonzini [mailto:pbonzini@redhat.com]
Sent: Wednesday, May 22, 2013 04:07 AM
To: 'Stefan Hajnoczi'
Cc: 'Mark Trumpold', qemu-devel@nongnu.org, markt@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