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

[Paolo Bonzini <pbonzini@redhat.com>]

Il 25/10/2012 15:41, ronnie sahlberg ha scritto:
> On Thu, Oct 25, 2012 at 1:02 AM, Kevin Wolf <kwolf@redhat.com> 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.

The problem is that bdrv_create is overloaded to mean both "create the
backing storage" and "format the image".  Only the latter applies to
iSCSI and, in general, as far as protocols are concerned bdrv_create is
usually a no-op.

However, Kevin said he prefers to have an explicit override of
bdrv_create for iSCSI.  Can you implement that?  (I'll then do the same
for NBD).

Paolo

> 
> 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.
> 
> regards
> ronnie sahlberg
>