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Re: [Qemu-devel] [PATCH 3/3] qemu-iotests: Some more write_zeroes tests

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH 3/3] qemu-iotests: Some more write_zeroes tests
Date: Tue, 17 May 2016 14:35:17 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0

On 05/17/2016 10:51 AM, Kevin Wolf wrote:
> This covers some more write_zeroes cases which are relevant for the
> recent qcow2 optimisations that check the allocation status of the
> backing file for partial cluster write_zeroes requests.
> Signed-off-by: Kevin Wolf <address@hidden>
> ---

> +# qcow2 specific bdrv_write_zeroes tests with backing files (complements 034)

> +_supported_fmt qcow2

I guess test 034 covered cases of larger-than-sector write_zero, and
that the reason you needed a new test id is because this one is qcow2
specific (the general test covers multiple formats, but the behavior on
sub-cluster zeroing is specific to qcow2). Makes sense so far.

> +
> +size=128M
> +
> +echo
> +echo == backing file contains zeros ==
> +
> +# Make sure that the whole cluster is allocated even for partial write_zeroes
> +# when the backing file contains zeros
> +
> +CLUSTER_SIZE=512 TEST_IMG="$TEST_IMG.base" _make_test_img $size
> +_make_test_img -b "$TEST_IMG.base"
> +
> +$QEMU_IO -c "write -z 0k 2k" "$TEST_IMG" | _filter_qemu_io

head aligned, tail is unaligned

> +$QEMU_IO -c "write -z 10k 2k" "$TEST_IMG" | _filter_qemu_io

head is unaligned, tail is unaligned

> +$QEMU_IO -c "write -z 17k 2k" "$TEST_IMG" | _filter_qemu_io

both unaligned, and completely contained within a single cluster

No test of crossing a cluster boundary at this point?  I would probably do:

$QEMU_IO -c "write -z 27k 2k" "$TEST_IMG" | _filter_qemu_io

then check that the map shows an allocation starting at 24k for 8192 bytes

> +$QEMU_IMG map --output=json "$TEST_IMG" | _filter_qemu_img_map
> +
> +echo
> +echo == backing file contains non-zero data before write_zeros ==
> +
> +CLUSTER_SIZE=512 TEST_IMG="$TEST_IMG.base" _make_test_img $size
> +_make_test_img -b "$TEST_IMG.base"
> +
> +$QEMU_IO -c "write -P 0x11 32k 1k" "$TEST_IMG.base" | _filter_qemu_io
> +$QEMU_IO -c "write -z 34k 1k" "$TEST_IMG" | _filter_qemu_io

cluster contains partial non-zero data, which does not overlap with the
write zero request. Good - you're testing that we don't nuke the rest of
the cluster (that is, we cannot convert an entire cluster to 0 unless
all sectors read as 0 before the conversion), but instead do proper

> +$QEMU_IO -c "read -P 0x11 32k 1k" "$TEST_IMG" | _filter_qemu_io
> +$QEMU_IO -c "read -P 0 33k 3k" "$TEST_IMG" | _filter_qemu_io
> +$QEMU_IMG map --output=json "$TEST_IMG" | _filter_qemu_img_map
> +
> +echo
> +echo == backing file contains non-zero data after write_zeros ==
> +
> +CLUSTER_SIZE=512 TEST_IMG="$TEST_IMG.base" _make_test_img $size
> +_make_test_img -b "$TEST_IMG.base"
> +
> +$QEMU_IO -c "write -P 0x11 43k 1k" "$TEST_IMG.base" | _filter_qemu_io
> +$QEMU_IO -c "write -z 41k 1k" "$TEST_IMG" | _filter_qemu_io
> +$QEMU_IO -c "read -P 0x11 43k 1k" "$TEST_IMG" | _filter_qemu_io
> +$QEMU_IO -c "read -P 0 40k 3k" "$TEST_IMG" | _filter_qemu_io
> +$QEMU_IMG map --output=json "$TEST_IMG" | _filter_qemu_img_map
> +
> +echo
> +echo == spanning multiple clusters ==
> +
> +CLUSTER_SIZE=512 TEST_IMG="$TEST_IMG.base" _make_test_img $size
> +_make_test_img -b "$TEST_IMG.base"
> +
> +$QEMU_IO -c "write -P 0x11 74k 2k" "$TEST_IMG.base" | _filter_qemu_io
> +$QEMU_IO -c "write -z 66k 7k" "$TEST_IMG" | _filter_qemu_io

the tail from 72k to 73k overlaps the sector that has to be
copy-on-write to pick up the data at 74k; while the earlier portion
picks up the clusters at 64k and 68k.  Looks good.

What you have looks fine, but again, I'd consider also testing a
sub-cluster size 'write -z' that spans a cluster boundary, including the
case where it is overwriting data from the backing file.  Something like:

$QEMU_IO -c "write -P 0x11 2k 4k" "$TEST_IMG.base" | _filter_qemu_io
$QEMU_IO -c "write -z 3k 2k" "$TEST_IMG" | _filter_qemu_io
$QEMU_IO -c "read -P 0 0k 2k" "$TEST_IMG" | _filter_qemu_io
$QEMU_IO -c "read -P 0x11 2k 1k" "$TEST_IMG" | _filter_qemu_io
$QEMU_IO -c "read -P 0 3k 2k" "$TEST_IMG" | _filter_qemu_io
$QEMU_IO -c "read -P 0x11 5k 1k" "$TEST_IMG" | _filter_qemu_io
$QEMU_IO -c "read -P 0 6k 2k" "$TEST_IMG" | _filter_qemu_io

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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