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Date: Wed, 06 May 2015 15:04:32 +0200
From: Max Reitz <mreitz@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v2 0/3] block: Warn about usage of growing
 formats over non-growable protocols
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To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Stefan Hajnoczi <stefanha@redhat.com>, qemu-devel@nongnu.org, qemu-block@nongnu.org

On 05.05.2015 11:46, Stefan Hajnoczi wrote:
> On Thu, Mar 19, 2015 at 03:03:18PM -0400, Max Reitz wrote:
>> Some image formats (e.g. qcow2) require the underlying file to grow on
>> write accesses, but this is in fact not supported by all protocols (e.g.
>> nbd does not). If such a format requiring file growth is used
>> non-read-only over a protocol which does not support this, a warning
>> should be issued.
>>
>> This warning is issued for example whenever one tries to export a qcow2
>> image over nbd-server and use the export from qemu.
> The warning implies that the user should switch to read-only or a
> different protocol, but this configuration is perfectly normal.  For
> example, oVirt uses qcow2 on LVM volumes.
>
> Introducing a warning for a normal QEMU invocation is a bit weird.
>
> What is the point of this series?  Were users confused that they hit
> ENOSPC?

Users were confused when exporting a qcow2 image using nbd-server 
instead of qemu-img, and then accessing that NBD export with qemu 
(subsequently getting I/O errors on guest writes, if the image is not 
yet fully allocated): http://bugzilla.redhat.com/show_bug.cgi?id=1090713

Max