From: Stefan Hajnoczi <stefanha@gmail.com>
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>
Subject: Re: [Qemu-devel] 'qemu-nbd' explicit flush
Date: Mon, 27 May 2013 14:36:19 +0200 [thread overview]
Message-ID: <20130527123619.GD23204@stefanha-thinkpad.redhat.com> (raw)
In-Reply-To: <CDC63015.85FF%markt@netqa.com>
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
next prev parent reply other threads:[~2013-05-27 12:36 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-05-23 21:58 [Qemu-devel] 'qemu-nbd' explicit flush Mark Trumpold
2013-05-24 9:05 ` Stefan Hajnoczi
2013-05-25 17:42 ` Mark Trumpold
2013-05-27 12:36 ` Stefan Hajnoczi [this message]
2013-05-24 12:10 ` Paolo Bonzini
-- strict thread matches above, loose matches on Subject: below --
2013-05-28 18:00 Mark Trumpold
2013-05-29 7:42 ` Stefan Hajnoczi
2013-05-29 15:29 ` Mark Trumpold
2013-06-07 14:00 ` Mark Trumpold
2013-05-23 23:35 Mark Trumpold
2013-05-24 9:06 ` Stefan Hajnoczi
2013-05-22 16:10 Mark Trumpold
2013-05-21 20:01 Mark Trumpold
2013-05-22 9:47 ` Stefan Hajnoczi
2013-05-22 11:07 ` Paolo Bonzini
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