Re: [Qemu-devel] [PATCH 1/1] nbd: fix block-mirror NBD target

["Denis V. Lunev" <den@openvz.org>]

On 02/17/2016 06:21 PM, Max Reitz wrote:
> On 17.02.2016 16:08, Denis V. Lunev wrote:
>> There is VM with 8 GB QCOW2 storage. Real size of the image file is 2 GB.
>> 'drive-mirror' redirected to NBD target creates 8 GB image at destination.
>> The situation is even worse as zeroes are sent through the channel.
>>
>> The patch simply adds .bdrv_co_write_zeroes callback to NBD block driver
>> which works though NBD_TRIM to avoid transfer of zeroes.
> The specification[1] says the following about TRIM:
>
>> After issuing this command, a client MUST NOT make any assumptions
>> about the contents of the export affected by this command, until
>> overwriting it again with NBD_CMD_WRITE.
> So I don't think this is correct.
>
> The correct solution would probably to introduce a specific command to
> write zeroes, or at least to use detect-zeroes on the NBD server side
> (but this will still lead to the zeroes being sent over the line).
>
> Max
>
> [1] https://github.com/yoe/nbd/blob/master/doc/proto.md
>
>> Signed-off-by: Denis V. Lunev <den@openvz.org>
>> CC: Paolo Bonzini <pbonzini@redhat.com>
>> ---
>>   block/nbd-client.c | 10 ++++++++++
>>   block/nbd-client.h |  2 ++
>>   block/nbd.c        |  9 +++++++++
>>   3 files changed, 21 insertions(+)
>
This sounds not that good. OK. At least there is correct
lengthy way for this. White-outs must be supported
by the protocol.

Den