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

From: Paolo Bonzini
Subject: Re: [Qemu-devel] 'qemu-nbd' explicit flush
Date: Wed, 22 May 2013 13:07:30 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130514 Thunderbird/17.0.6

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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