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 <d...@openvz.org> > CC: Paolo Bonzini <pbonz...@redhat.com> > --- > block/nbd-client.c | 10 ++++++++++ > block/nbd-client.h | 2 ++ > block/nbd.c | 9 +++++++++ > 3 files changed, 21 insertions(+)
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