Re: [Qemu-devel] [PATCH v2 1/2] block: allow blockdev-backup from any source

[John Snow <jsnow@redhat.com>]

On 07/02/2018 03:59 AM, Kashyap Chamarthy wrote:
> On Thu, Jun 28, 2018 at 05:19:37PM -0400, John Snow wrote:
>>
>>
>> On 06/28/2018 02:05 PM, Eric Blake wrote:
>>> On 06/28/2018 01:00 PM, John Snow wrote:
>>>> In the case of image fleecing, the node we choose as the source
>>>> for a blockdev-backup is going to be both a root node AND the
>>>> backing node for the exported image. It does not qualify as a root
>>>> image in this case.
>>>>
>>>> Loosen the restriction.
>>>>
>>>> Signed-off-by: John Snow <jsnow@redhat.com>
>>>> ---
>>>>   blockdev.c | 2 +-
>>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> In v1, you mentioned that this used to work but then regressed,
>>> pinpointing that detail in the commit message might be nice, but not
>>> essential (since we didn't test it until now).  So,
>>>
>>
>> Really not sure when it regressed; I consider it unimportant as nothing
>> uses it presently: no docs, no tests, nothing in libvirt.
>>
>> My guess, though, is that it worked prior to
>> cef34eebf3d0f252a3b3e9a2a459b6c3ecc56f68.
> 
> So the above commit adds the ability to assign node names for
> `blockdev-backup`, which wasn't possible during 2.8 timeframe.
> 
>> In 2.8, possibly?
>>
>> Kashyap might know, I think he's experimented with this sometime in that
>> timezone.
> 
> I don't recall doing the "allow blockdev-backup from any source" test
> with 2.8.  (But I recall gradually moving from figuring out the source
> block device via wading through the output of `query-named-block-nodes`
> to using the 'node-name'.)
> 
> Looking at my test notes with QEMU 2.8, this was my
> workflow (see how `blockdev-backup` still doesn't use 'node-name's):
> 
> (QEMU) blockdev-add driver=qcow2 node-name=node2 file={"driver":"file","filename":"/export/target.qcow2"} backing={"driver":"qcow2","file":{"driver":"file","filename":"/export/base.qcow2"}}
> (QEMU) query-named-block-nodes # To dig through the source block device "#blockYYY"
> (QEMU) blockdev-backup device=#block197 target=node1 sync=none
> (QEMU) nbd-server-start addr={"type":"unix","data":{"path":"./nbd-sock"}}
> (QEMU) nbd-server-add device=node1
> (QEMU) nbd-server-stop
> 
> Later on I moved (thanks to the Git commit you mentioned earlier) to the
> much cleaner approach of using 'node-name' (the below syntax is
> from my test notes with QEMU 2.12):
> 
> (QEMU) blockdev-add driver=qcow2 node-name=node-E file={"driver":"file","filename":"e.qcow2"}
> (QEMU) blockdev-backup device=node-B target=node-E sync=full job-id=job0
> [...]
> 

Without this patch, "allow blockdev-backup from any source", the
fleecing workflow prohibits the blockdev-backup command. We're wondering
when that regressed.

If you managed to get fleecing working without this patch here, then it
was working at that time.

--js