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Re: [Qemu-discuss] Cannot block-commit to nbd backend

From: Eric Blake
Subject: Re: [Qemu-discuss] Cannot block-commit to nbd backend
Date: Wed, 28 Jun 2017 08:18:14 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.0

On 06/28/2017 08:07 AM, Eric Blake wrote:
> On 06/27/2017 04:04 AM, Han Han wrote:
>> Hi all, I get an error when block-commit to nbd backend:
>> First, prepare a back file for nbd storage:
>> # qemu-img create -f qcow2 -b 'json:{"file.driver":"nbd",
>> "file.host":"xx.xx.xx.xx", "file.port":"10803", "file.export":"nbd_raw"}'
>> -o backing_fmt=raw /tmp/n2.s1
>> Formatting '/tmp/n2.s1', fmt=qcow2 size=10737418240
>> backing_file=json:{"file.driver":"nbd",, "file.host":"xx.xx.xx.xx",,
>> "file.port":"10803",, "file.export":"nbd_raw"} backing_fmt=raw
> This setup uses NBD as a read-only backing file.

I'll add a note that
https://bugzilla.redhat.com/show_bug.cgi?id=1465320 appears to
demonstrate a way to open NBD as a read-write file (different than what
is in your original email question), by creating snapshots on top, and
then attempts to commit; and that it demonstrates an oddity between
intermediate commit succeeding vs. live commit failing.  So I'm still
investigating if upstream qemu has a bug in the difference between those
two code paths.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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