Re: [Qemu-devel] [PATCH 8/8] docs: add mirroring to live block operations

[Eric Blake]

On 04/13/2012 02:41 PM, Paolo Bonzini wrote:
>> On 04/13/2012 11:32 AM, Paolo Bonzini wrote:
>>> Il 13/04/2012 19:09, Eric Blake ha scritto:
>>>>>> +It is also possible to use raw as the format of the destination
>>>>>> image.
>>>>>> +Converting an image to raw will properly support thin
>>>>>> provisioning.
>>>> Do you need to mention the use of block_job_cancel needed at the
>>>> point
>>>> where you are ready to stop qemu from writing into the copy?
>>>
>>> No, you need to do this with drive-reopen.  Using block_job_cancel
>>> will report completion, but will leave a race window that can cause to
>>> corruption in the guest.  Do your libvirt patches do that? O:)
>>
>> They don't in v4 [1], but I'll fix it to do that in v5.  Thanks for
>> the heads up.
> 
> I think we misunderstood each other.  I was asking if you were doing
> block-job-cancel + drive-reopen to switch to the target.  That would
> be wrong.
> 
> Doing block-job-cancel to revert to the source is perfectly fine.

Glad to hear it.  My current approach is:

request | phase > |    streaming     |    mirroring
== v =============+==================+===================
revert-source     | block-job-cancel |  block-job-cancel
pivot             | error            |  drive-reopen

with no combined commands (that is, I'm not doing
block-job-cancel+drive-reopen).  Although if I understand correctly, the
combination of mirroring/revert-source could also be implemented by
drive-reopen instead of block-job-cancel, with the same net results in
the end.

-- 
Eric Blake   address@hidden    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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