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From: "Mark Trumpold" <markt@netqa.com>
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Date: Thu, 23 May 2013 23:35:24 +0000
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Subject: Re: [Qemu-devel] 'qemu-nbd' explicit flush
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To: Mark Trumpold <markt@netqa.com>, Paolo Bonzini <pbonzini@redhat.com>, Stefan Hajnoczi <stefanha@gmail.com>
Cc: qemu-devel@nongnu.org, markt@tachyon.net

I had one question I forgot to ask..

Is it possible to switch from '--cache=3Dwriteback' functionality 
to '--cache=3Dwritethrough' (and visa versa) while qemu-nbd is 
connected to the '/dev/nbd<x>' device?

Thank you,
Mark T.


-----Original Message-----
From: Mark Trumpold [mailto:markt@netqa.com]
Sent: Thursday, May 23, 2013 02:58 PM
To: 'Paolo Bonzini', 'Stefan Hajnoczi'
Cc: 'Mark Trumpold', qemu-devel@nongnu.org, markt@tachyon.net
Subject: Re:  'qemu-nbd' explicit flush

I have a working configuration using the signal approach suggested by Ste=
fan.

'qemu-nbd.c' is patched as follows:

    do {
        main_loop_wait(false);
+       if (sighup_reported) {
+           sighup_reported =3D 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: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=3Dwriteback -c /dev/nbd0 /images/my-qcow.img'
>>     'mount /dev/nbd0 /my-mount-point'
>>
>> Everything does flush correctly when I first unmount and then disconne=
ct 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 exampl=
e:
>>     'mount -o remount,ro /dev/nbd0'
>>     'blockdev --flushbufs /dev/nbd0'
> 
> Did you try plain old sync(1)?

This could also work:

  dd if=3D/dev/zero of=3Ddummy oflag=3Dsync bs=3D512 count=3D1

> 1. Add a signal handler (like SIGHUP or SIGUSR1) to qemu-nbd which
>    flushes all exports.

That would be a useful addition anyway.

Paolo