Re: [PATCH v2 07/11] mirror: Skip pre-zeroing destination if it is already zero

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Thu, Apr 17, 2025 at 01:39:12PM -0500, Eric Blake wrote:
> When doing a sync=full mirroring, QMP drive-mirror requests full
> zeroing if it did not just create the destination, and blockdev-mirror
> requests full zeroing unconditionally.  This is because during a full
> sync, we must ensure that the portions of the disk that are not
> otherwise touched by the source still read as zero upon completion.
> 
> However, in mirror_dirty_init(), we were blindly assuming that if the
> destination allows punching holes, we should pre-zero the entire
> image; and if it does not allow punching holes, then treat the entire
> source as dirty rather than mirroring just the allocated portions of
> the source.  Without the ability to punch holes, this results in the
> destination file being fully allocated; and even when punching holes
> is supported, it causes duplicate I/O to the portions of the
> destination corresponding to chunks of the source that are allocated
> but read as zero.
> 
> Smarter is to avoid the pre-zeroing pass over the destination if it
> can be proved the destination already reads as zero.  Note that a
> later patch will then further improve things to skip writing to the
> destination for parts of the image where the source is zero; but even
> with just this patch, it is possible to see a difference for any BDS
> that can quickly report that it already reads as zero.
> 
> Signed-off-by: Eric Blake <eblake@redhat.com>
> ---
>  block/mirror.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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