On 04.05.2017 05:07, Eric Blake wrote:
> We've already improved discards to operate efficiently on the tail
> of an unaligned qcow2 image; it's time to make a similar improvement
> to write zeroes. The special case is only valid at the tail
> cluster of a file, where we must recognize that any sectors beyond
> the image end would implicitly read as zero, and therefore should
> not penalize our logic for widening a partial cluster into writing
> the whole cluster as zero.
>
> However, note that for now, the special case of end-of-file is only
> recognized if there is no backing file, or if the backing file has
> the same length; that's because when the backing file is shorter
> than the active layer, we don't have code in place to recognize
> that reads of a sector unallocated at the top and beyond the backing
> end-of-file are implicitly zero. It's not much of a real loss,
> because most people don't use images that aren't cluster-aligned,
> or where the active layer is a different size than the backing
> layer (especially where the difference falls within a single cluster).
>
> Update test 154 to cover the new scenarios, using two images of
> intentionally differing length.
>
> While at it, fix the test to gracefully skip when run as
> ./check -qcow2 -o compat=0.10 154
> since the older format lacks zero clusters already required earlier
> in the test.
>
> Signed-off-by: Eric Blake
>
> ---
> v12: fix testsuite problems, document shortcoming of differing
> v11: reserved for blkdebug half of v10
> size of backing file
>
> v10: rebase to better reporting of preallocated zero clusters
> v9: new patch
> ---
> block/qcow2.c | 7 ++
> tests/qemu-iotests/154 | 160
> -
> tests/qemu-iotests/154.out | 129
> 3 files changed, 294 insertions(+), 2 deletions(-)
[...]
> diff --git a/tests/qemu-iotests/154 b/tests/qemu-iotests/154
> index 7ca7219..687b8f3 100755
> --- a/tests/qemu-iotests/154
> +++ b/tests/qemu-iotests/154
[...]
> @@ -299,6 +302,159 @@ $QEMU_IO -c "read -P 0 75k 1k" "$TEST_IMG" |
> _filter_qemu_io
>
> $QEMU_IMG map --output=json "$TEST_IMG" | _filter_qemu_img_map
>
> +echo
> +echo == unaligned image tail cluster, no allocation needed ==
[...]
> +# A preallocated cluster maintains its allocation, whether it stays as
> +# data due to a partial write:
> +# Convert 128m... | XX XX => ... | XX 00
> +_make_test_img $((size + 1024))
> +$QEMU_IO -c "write -P 1 $((size)) 1024" "$TEST_IMG" | _filter_qemu_io
> +$QEMU_IO -c "write -z $((size + 512)) 512" "$TEST_IMG.base" | _filter_qemu_io
s/\.base//, I suppose?
(You should read your reference output. "Pattern verification failed" is
never good. ;-))
With that (and the reference output) fixed:
Reviewed-by: Max Reitz
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