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Re: [Qemu-devel] [PATCH 1/1] nbd: fix block-mirror NBD target
From: |
Max Reitz |
Subject: |
Re: [Qemu-devel] [PATCH 1/1] nbd: fix block-mirror NBD target |
Date: |
Wed, 17 Feb 2016 16:21:16 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.5.1 |
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(+)
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