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

From: Mark Trumpold
Subject: Re: [Qemu-devel] 'qemu-nbd' explicit flush
Date: Thu, 23 May 2013 23:35:24 +0000

I had one question I forgot to ask..

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

Thank you,
Mark T.

-----Original Message-----
From: Mark Trumpold [mailto:address@hidden
Sent: Thursday, May 23, 2013 02:58 PM
To: 'Paolo Bonzini', 'Stefan Hajnoczi'
Cc: 'Mark Trumpold', address@hidden, address@hidden
Subject: Re:  'qemu-nbd' explicit flush

I have a working configuration using the signal approach suggested by Stefan.

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

    do {
+       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!!
Mark T.

-----Original Message-----
From: Paolo Bonzini [mailto:address@hidden
Sent: Wednesday, May 22, 2013 04:07 AM
To: 'Stefan Hajnoczi'
Cc: 'Mark Trumpold', address@hidden, address@hidden
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.


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