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Date: Mon, 27 May 2013 14:36:19 +0200
From: Stefan Hajnoczi <stefanha@gmail.com>
Message-ID: <20130527123619.GD23204@stefanha-thinkpad.redhat.com>
References: <20130524090516.GE21639@stefanha-thinkpad.redhat.com>
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Subject: Re: [Qemu-devel] 'qemu-nbd' explicit flush
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To: Mark Trumpold <markt@netqa.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, "markt@tachyon.net" <markt@tachyon.net>

On Sat, May 25, 2013 at 09:42:08AM -0800, Mark Trumpold wrote:
> On 5/24/13 1:05 AM, "Stefan Hajnoczi" <stefanha@gmail.com> wrote:
> >On Thu, May 23, 2013 at 09:58:31PM +0000, Mark Trumpold wrote:
> >One thing to be careful of is whether these operations are asynchronous.
> >The signal is asynchronous, you have no way of knowing when qemu-nbd is
> >finished flushing to the physical disk.
> 
> Right, of course.  I missed the obvious.

I missed something too.  Paolo may have already hinted at this when he
posted a dd oflag=sync command-line option:

blockdev --flushbufs is the wrong tool because ioctl(BLKFLSBUF) only
writes out dirty pages to the block device.  It does *not* guarantee to
send a flush request to the device.

Therefore, the underlying image file may not be put into an up-to-date
state by qemu-nbd.


I suggest trying the following instead of blockdev --flushbufs:

  python -c 'import os; os.fsync(open("/dev/loopX", "r+b"))'

This should do the same as blockdev --flushbufs *plus* it sends and
waits for the NBD FLUSH command.

You may have to play with this command-line a little but the main idea
is to open the block device and fsync it.

Stefan