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 <address@hidden>
CC: Paolo Bonzini <address@hidden>
---
block/nbd-client.c | 10 ++++++++++
block/nbd-client.h | 2 ++
block/nbd.c | 9 +++++++++
3 files changed, 21 insertions(+)