Re: Bug? qemu-img convert to preallocated image makes it sparse

[David Edmondson]

On Thursday, 2020-01-16 at 15:37:22 +01, Max Reitz wrote:

> So I suppose the best idea I can come up with is indeed a
> --target-is-zero flag for qemu-img convert -n.  Would that work for you?

I was looking at adding this for a slightly different reason (converting
sparse images to newly provisioned LUNs).

Followup is a naive patch (I'm new to hacking on qemu and the block
layer seems complex due to maturity) that works in my tests. Feedback
much appreciated.

The patch specifically targets the initial blanking of the image rather
than any other attempt to write zeroes, as I couldn't convince myself
that there was no control flow where qemu-img convert might need to
overwrite (with zeroes) data that it itself had previously written.

dme.
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