[RFC PATCH 0/2] qemu-img convert: Fix sparseness detection

[Kevin Wolf]

Peter, three years ago you changed 'qemu-img convert' to sacrifice some
sparsification in order to get aligned requests on the target image. At
the time, I thought the impact would be small, but it turns out that
this can end up wasting gigabytes of storagee (like converting a fully
zeroed 10 GB image taking 2.8 GB instead of a few kilobytes).

https://bugzilla.redhat.com/show_bug.cgi?id=1882917

I'm not entirely sure how to attack this best since this is a tradeoff,
but maybe the approach in this series is still good enough for the case
that you wanted to fix back then?

Of course, it would be possible to have a more complete fix like looking
forward a few blocks more before writing data, but that would probably
not be entirely trivial because you would have to merge blocks with ZERO
block status with DATA blocks that contain only zeros. I'm not sure if
it's worth this complication of the code.

Kevin Wolf (2):
  iotests: Test qemu-img convert of zeroed data cluster
  qemu-img convert: Fix sparseness detection

 qemu-img.c                 | 18 ++++--------------
 tests/qemu-iotests/122     |  1 +
 tests/qemu-iotests/122.out |  6 ++++--
 3 files changed, 9 insertions(+), 16 deletions(-)

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2.30.2