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

["Denis V. Lunev" <den@virtuozzo.com>]

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