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Date: Tue, 15 Oct 2013 17:17:54 +0200
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From: Stefan Hajnoczi <stefanha@gmail.com>
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Subject: Re: [Qemu-devel] [Nbd]  Hibernate and qemu-nbd
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To: Mark Trumpold <markt@netqa.com>
Cc: nbd-general@lists.sourceforge.net, Wouter Verhelst <w@uter.be>, Paul Clements <Paul.Clements@steeleye.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, Paolo Bonzini <pbonzini@redhat.com>

On Thu, Oct 10, 2013 at 11:15 AM, Stefan Hajnoczi <stefanha@gmail.com> wrote:
> On Fri, Oct 04, 2013 at 07:30:45AM -0700, Mark Trumpold wrote:
>>
>>
>> On 9/26/13 10:18 PM, "Stefan Hajnoczi" <stefanha@gmail.com> wrote:
>>
>> >
>> >Try the qemu-nbd --persistent option.  That should prevent it from
>> >shutting down when nbd-client is disconnected.
>> >
>> >Stefan
>> >
>>
>>
>> Hi Stefan,
>>
>> Sorry for the delay..
>> I tried the following per your suggestion:
>>
>>   920  qemu-nbd --persistent -p 2000 /root/qemu/q1.img &
>>   921  nbd-client -persist localhost 2000 /dev/nbd0
>>   922  fsck /dev/nbd0
>>   923  mount /dev/nbd0 /mnt
>>   924  ls /mnt
>>   925  umount /dev/nbd0
>>   ::
>>
>>   927  echo reboot >/sys/power/disk
>>   928  echo disk >/sys/power/state
>>   929  mount /dev/nbd0 /mnt
>>
>> This seems to work; that is both sides (client and server) persist
>> after the hibernate cycle.
>>
>> However, if I don't 'umount' '/dev/nbd0' before the hibernate
>> cycle, and try to 'ls /mnt' after, the 'ls' hangs indefinitely.
>>
>> For my real use case we have the root filesystem mounted,
>> so unmounting is not an option (at least I don't think so).
>>
>> I also tried remounting readonly, and also 'blockdev --flushbufs ..'
>> before the hibernate cycle -- either or both did not help.
>>
>> I had thought about trying a 'chroot' and then a 'umount', but
>> have not yet tried this.
>>
>> This one was so close..

Could you try drop the umount/mount and capture the following straces:
strace -p $(pgrep nbd-client) -o /var/tmp/nbd-client.strace
strace -p $(pgrep qemu-nbd) -o /var/tmp/qemu-nbd.strace

In particular, let's find out what happens when you run ls /mnt after
resuming the machine.  The nbd-client should reconnect to qemu-nbd and
I/O requests would flow to qemu-nbd again.  But something is stuck;
the problem might be clear from the strace output.

Stefan