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Re: [Qemu-block] [PATCH v4 1/5] Discard blocks while copy-on-read

From: Max Reitz
Subject: Re: [Qemu-block] [PATCH v4 1/5] Discard blocks while copy-on-read
Date: Mon, 10 Dec 2018 16:24:18 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.3.1

On 30.11.18 09:55, Andrey Shinkevich wrote:
> Discards the block duplicated in an intermediate backing file
> after the block have been copied into the active layer during
> QMP block-stream operation.
> It saves the disk space while merging external snapshots.
> Signed-off-by: Andrey Shinkevich <address@hidden>
> ---
>  block/stream.c | 428 
> +++++++++++++++++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 413 insertions(+), 15 deletions(-)

I wonder why you aren't adding this functionality to the copy-on-read
filter driver; not just because it makes sense to have feature parity
between block-stream and that filter, but also because I imagine you
could use it as the filter node you're adding here.

I thought that block-stream could be rewritten to:
(1) Create a block-stream filter node on top of @device, with the
necessary parameters,
(2) Read everything that is allocated in (@base, @device), so that it is
copied to the target.

I think that can be done today.  There is no need for the COR filter to
support a base node parameter, as block-stream simply needs to take care
just to read things that are allocated above @base.


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