Re: [PATCH v4 10/13] mirror: Skip pre-zeroing destination if it is already zero

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Fri, May 09, 2025 at 03:40:27PM -0500, Eric Blake wrote:
> When doing a sync=full mirroring, we can skip pre-zeroing the
> destination if it already reads as zeroes and we are not also trying
> to punch holes due to detect-zeroes.  With this patch, there are fewer
> scenarios that have to pass in an explicit target-is-zero, while still
> resulting in a sparse destination remaining sparse.
> 
> 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
> source that does not report itself as fully allocated, coupled with a
> destination BDS that can quickly report that it already reads as zero.
> (For a source that reports as fully allocated, such as a file, the
> rest of mirror_dirty_init() still sets the entire dirty bitmap to
> true, so even though we avoided the pre-zeroing, we are not yet
> avoiding all redundant I/O).
> 
> Iotest 194 detects the difference made by this patch: for a file
> source (where block status reports the entire image as allocated, and
> therefore we end up writing zeroes everywhere in the destination
> anyways), the job length remains the same.  But for a qcow2 source and
> a destination that reads as all zeroes, the dirty bitmap changes to
> just tracking the allocated portions of the source, which results in
> faster completion and smaller job statistics.  For the test to pass
> with both ./check -file and -qcow2, a new python filter is needed to
> mask out the now-varying job amounts (this matches the shell filters
> _filter_block_job_{offset,len} in common.filter).  A later test will
> also be added which further validates expected sparseness, so it does
> not matter that 194 is no longer explicitly looking at how many bytes
> were copied.
> 
> Signed-off-by: Eric Blake <eblake@redhat.com>
> 
> ---
> 
> v3: add exemption for "detect-zeroes":"unmap" on destination
> v4: Rebase later in series, revise logic for pre-zeroing [Sunny], add
> in python filter
> ---
>  block/mirror.c                | 24 ++++++++++++++++--------
>  tests/qemu-iotests/194        |  6 ++++--
>  tests/qemu-iotests/194.out    |  4 ++--
>  tests/qemu-iotests/iotests.py | 12 +++++++++++-
>  4 files changed, 33 insertions(+), 13 deletions(-)

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

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