Re: [Qemu-devel] [PATCH v10 8/9] qcow2: skip writing zero buffers to empty COW areas

[Vladimir Sementsov-Ogievskiy]

03.12.2018 13:14, Anton Nefedov wrote:
> If COW areas of the newly allocated clusters are zeroes on the backing image,
> efficient bdrv_write_zeroes(flags=BDRV_REQ_ALLOCATE) can be used on the whole
> cluster instead of writing explicit zero buffers later in perform_cow().
> 
> iotest 060:
> write to the discarded cluster does not trigger COW anymore.
> Use a backing image instead.
> 

[..]

> --- a/tests/qemu-iotests/060
> +++ b/tests/qemu-iotests/060
> @@ -150,27 +150,33 @@ $QEMU_IO -c "$OPEN_RO" -c "read -P 1 0 512" | 
> _filter_qemu_io
>   echo
>   echo "=== Testing overlap while COW is in flight ==="
>   echo
> +BACKING_IMG=$TEST_IMG.base
> +TEST_IMG=$BACKING_IMG _make_test_img 1G
> +
> +$QEMU_IO -c 'write 64k 64k' "$BACKING_IMG" | _filter_qemu_io
> +
>   # compat=0.10 is required in order to make the following discard actually
> -# unallocate the sector rather than make it a zero sector - we want COW, 
> after
> -# all.
> -IMGOPTS='compat=0.10' _make_test_img 1G
> +# unallocate the sector rather than make it a zero sector as we would like
> +# to reuse it for another guest offset
> +IMGOPTS='compat=0.10' _make_test_img -b "$BACKING_IMG" 1G
>   # Write two clusters, the second one enforces creation of an L2 table after
>   # the first data cluster.
>   $QEMU_IO -c 'write 0k 64k' -c 'write 512M 64k' "$TEST_IMG" | _filter_qemu_io
> -# Discard the first cluster. This cluster will soon enough be reallocated and
> -# used for COW.
> +# Discard the first cluster. This cluster will soon enough be reallocated
>   $QEMU_IO -c 'discard 0k 64k' "$TEST_IMG" | _filter_qemu_io
>   # Now, corrupt the image by marking the second L2 table cluster as free.
>   poke_file "$TEST_IMG" '131084' "\x00\x00" # 0x2000c
> -# Start a write operation requiring COW on the image stopping it right before
> -# doing the read; then, trigger the corruption prevention by writing 
> anything to
> -# any unallocated cluster, leading to an attempt to overwrite the second L2
> +# Start a write operation requiring COW on the image;
> +# this write will reuse the host offset released by a previous discard.
> +# Stop it right before doing the read.
> +# Then, trigger the corruption prevention by writing anything to
> +# another unallocated cluster, leading to an attempt to overwrite the second 
> L2
>   # table. Finally, resume the COW write and see it fail (but not crash).
>   echo "open -o file.driver=blkdebug $TEST_IMG
>   break cow_read 0
> -aio_write 0k 1k
> +aio_write 64k 1k
>   wait_break 0
> -write 64k 64k
> +write 128k 64k

don't understand why you need these changes.

works for me, without them, if write to backing at 0 offset, of course.

As I understand, discard create unallocated holes in top qcow2 for old qcow2 
version.

>   resume 0" | $QEMU_IO | _filter_qemu_io
>   
>   echo
> diff --git a/tests/qemu-iotests/060.out b/tests/qemu-iotests/060.out
> index d67c6234a4..ec8f15d2f0 100644
> --- a/tests/qemu-iotests/060.out
> +++ b/tests/qemu-iotests/060.out
> @@ -97,7 +97,10 @@ read 512/512 bytes at offset 0
>   
>   === Testing overlap while COW is in flight ===
>   
> -Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1073741824
> +Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=1073741824
> +wrote 65536/65536 bytes at offset 65536
> +64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
> +Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1073741824 
> backing_file=TEST_DIR/t.IMGFMT.base
>   wrote 65536/65536 bytes at offset 0
>   64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
>   wrote 65536/65536 bytes at offset 536870912
> 


-- 
Best regards,
Vladimir