Re: [Qemu-devel] qemu-img convert with block driver without .bdrv_create (like iscsi)

[Kevin Wolf]

Am 25.10.2012 15:41, schrieb ronnie sahlberg:
> On Thu, Oct 25, 2012 at 1:02 AM, Kevin Wolf <address@hidden> wrote:
>> Am 25.10.2012 09:52, schrieb Paolo Bonzini:
>>> Il 25/10/2012 09:46, Kevin Wolf ha scritto:
>>>>>> 1)add a .bdrv_create in block/iscsi.c ?
>>>>>>
>>>>>> (like host_device block driver, only open/close the device and check if 
>>>>>> size if big enough)
>>>> Yes, this is the right way.
>>>
>>> Could it be a default implementation of .bdrv_create (i.e. something
>>> you'd do in bdrv_create if the protocol doesn't have it)?
>>
>> No, there are block drivers that really can't create images. They should
>> keep failing.
> 
> Technically, you can not create new LUNs via the iscsi protocol
> itself, you can only access pre-existing luns
> that have been created by some out-of-band method.
> 
> So basically, with iscsi you can only ever access already preexisting luns.
> 
> In that case I think requiring a .bdrv_create feels wrong. We are not
> creating a new LUN here but just opening an already existing LUN.

This is the very same as with host_device, which has always worked like
this. Obviously it can't create new block devices, but it can check that
the preexisting block device is suitable for the requested size and options.

> What about changing bdrv_create() so that IF there is no .bdrv_create
> method, then assume it might be a pre-existing file in which case we
> fallback to use .bdrv_open instead.

I don't think this is the correct behaviour for all block drivers. It's
wrong for any driver that would actually require a specific content of
the image file. For example let's overwrite an existing image:

$ touch /tmp/test.dmg # Or this could be a real DMG image as well
$ ./qemu-img create -f dmg /tmp/test.dmg 64M
Formatting '/tmp/test.dmg', fmt=dmg size=67108864
qemu-img: Formatting or formatting option not supported for file format
'dmg'

After your change it would appear to have succeeded, whereas of course
it still wouldn't have created the image.

Kevin