Re: [Qemu-devel] [PATCH] mirror: add target-zeroed flag

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

On 03.06.2016 18:45, Denis V. Lunev wrote:
> On 06/03/2016 06:06 PM, Eric Blake wrote:
>> On 06/03/2016 08:05 AM, Vladimir Sementsov-Ogievskiy wrote:
>>> Add target-zeroed flag to allow user specify that target is already
>>> zeroed. With this flag set zeroes which was in source before mirror
>>> start will not be copyed.
>> With this flag set, any runs of zeroes in the source before the mirror
>> starts will not be copied.
>>
>>> Without this libvirt migration of empty disk takes too long time.
>>>
>>> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
>>> ---
>>>
>>> I've tested it with
>>> time virsh migrate --live test qemu+ssh://other_node/system 
>>> --copy-storage-all
>> Presumably with a libvirt patch to turn on the optional flag.
>>
>> I'm not sure I like this patch.  Libvirt uses NBD to implement
>> --copy-storage-all, I think we're better off improving NBD to
>> automatically handle sparse writes, than we are to add a one-off hack
>> that requires libvirt to change.  That is, once NBD is smarter, the copy
>> will be faster without needing a tweak.  And we ARE working on making
>> NBD smarter (one of my goals for the 2.7 release is to get all the
>> sparse file additions to NBD implemented)
>>
>> That said, I'll still review it.
> this is not enough, definitely.
>
> There is a problem that mirror_iteration code sleeps even for
> not read zeroes (and this IS slow). Moreover, even sending sparcified
> zeroes takes a lot of time for round trips.
>
> We have started with that and spent a lot of time trying to improve
> the situation.
>
> Also, as a side note, the QCOW2 file on a source and a target will
> be different without the flag - original image has empty blocks,
> target image will have blocks explicitly marked with zeroes.
>
> Though this is a matter of taste... For us this approach is the simplest.
>
> Den

Hey, what do think about it? Don't we forget somebody to be cc'ed?

-- 
Best regards,
Vladimir