Re: [Qemu-devel] [PATCH 3/3] block: mirror - zero unallocated target sectors when zero init not present

[Paolo Bonzini]

On 29/09/2015 10:10, Kevin Wolf wrote:
>> When mode == 'existing' for a shallow mirror (sync != 'full'),
>> that is the caller stating that the guest-visible contents of the
>> destination match the guest-visible contents of the backing
>> image.  The only sectors to be copied are those that differ from
>> the backing file, and we should not be zeroing unrelated sectors
>> because the user has already promised they have the same
>> guest-visible content as the backing image would report.
> 
> Where is this promise documented? I wasn't aware of it and can't
> seem to find it in the QAPI documentation of drive-mirror.

I don't think it is really a promise, but it's the only sensible way
to use this combination.

Paolo

>> When mode == 'existing' for a full mirror (sync == 'full'), that
>> is the caller stating that they want every single sector of the
>> destination written to hold the current state of the source (of
>> course, allowing for optimizations such as skipping the write
>> where the contents will read back the same as if the write had
>> been performed).
>> 
>> I think Paolo is right: we care about zeroing unallocated sectors
>> for sync == 'full', regardless of whether mode == 'existing'.
> 
> I agree.
> 
>> I also think the reason Jeff confused it for mode == 'existing'
>> is that the other modes let qemu create the file, but qemu does
>> not create block devices (the only way to mirror to a block
>> device is via mode == 'existing'), and it is primarily block
>> devices where zero init is not guaranteed.
> 
> 'qemu-img create' works on block devices (even though for raw it
> doesn't do more than checking if it's large enough; but for qcow2,
> it's obvious that it's necessary), so I'm pretty sure that mode !=
> 'existing' works on them as well.
> 
> Kevin
>