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Date: Wed, 08 Dec 2010 09:54:08 +0100
From: Kevin Wolf <kwolf@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 1/1] qemu-img.c: Clean up handling of image
	size in img_create()
References: <1291743586-3518-1-git-send-email-Jes.Sorensen@redhat.com>
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To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: Jes.Sorensen@redhat.com, qemu-devel@nongnu.org

Am 07.12.2010 21:36, schrieb Stefan Hajnoczi:
> On Tue, Dec 7, 2010 at 5:39 PM,  <Jes.Sorensen@redhat.com> wrote:
>>     // The size for the image must always be specified, with one exception:
>>     // If we are using a backing file, we can obtain the size from there
>> -    if (get_option_parameter(param, BLOCK_OPT_SIZE)->value.n == -1) {
>> -
>> +    if (get_option_parameter(param, BLOCK_OPT_SIZE)->value.n == 0) {
>>         QEMUOptionParameter *backing_file =
>>             get_option_parameter(param, BLOCK_OPT_BACKING_FILE);
>>         QEMUOptionParameter *backing_fmt =
> 
> Today it is possible to create 0 byte sized images.  Your patch will
> change that:
> If there is a backing file, then the size will be taken from the backing file.
> If there is no backing file, then an error about missing size will be
> printed, even though a size of 0 has been given.

I can think of one use case for it: You can store the VM state on a
zero-sized qcow2 image for internal snapshots.

Otherwise it's probably rather useless, but we have supported it for a
long time, so I wouldn't remove it. People have actually noticed in the
past when something was broken with it.

Kevin