Re: [Qemu-devel] [PATCH 3/3] block: mirror - zero unallocated target sectors when zero init not present

[Paolo Bonzini <pbonzini@redhat.com>]

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On 29/09/2015 10:39, Kevin Wolf wrote:
> bdrv_has_zero_init() takes care of that, in theory. The "problem"
> here is that the target is opened with BDRV_O_NO_BACKING, so the
> block layer doesn't consider this an image with a backing file.

I think bdrv_has_zero_init() is working right. If you read the qcow2
file as it was opened (i.e. with BDRV_O_NO_BACKING), unallocated areas
will indeed read as zeroes.

Of course if the file is opened with BDRV_O_NO_BACKING but does have a
backing file, you ought not to read unallocated areas at all.

So it's not the answer (of bdrv_has_zero_init) that is wrong, but the
question that was not well-specified.

> Is there anything better than bs->backing_hd that we could check?

It's simply sync == 'full', I think.  Then the problematic case never
even reaches bdrv_has_zero_init.

Paolo
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