| Message ID | 20240619174022.1298578-1-nsoffer@redhat.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [v2] Consider discard option when writing zeros | expand |
Tested using:
$ cat test-unmap.sh
#!/bin/sh
qemu=${1:?Usage: $0 qemu-executable}
img=/tmp/test.raw
echo
echo "defaults - write zeroes"
fallocate -l 1m $img
echo -e 'qemu-io none0 "write -z 0 1m"\nquit' | $qemu -monitor stdio \
-drive if=none,file=$img,format=raw >/dev/null
du -sh $img
echo
echo "defaults - write zeroes unmap"
fallocate -l 1m $img
echo -e 'qemu-io none0 "write -zu 0 1m"\nquit' | $qemu -monitor stdio \
-drive if=none,file=$img,format=raw >/dev/null
du -sh $img
echo
echo "defaults - write actual zeros"
fallocate -l 1m $img
echo -e 'qemu-io none0 "write -P 0 0 1m"\nquit' | $qemu -monitor stdio \
-drive if=none,file=$img,format=raw >/dev/null
du -sh $img
echo
echo "discard=off - write zeroes unmap"
fallocate -l 1m $img
echo -e 'qemu-io none0 "write -zu 0 1m"\nquit' | $qemu -monitor stdio \
-drive if=none,file=$img,format=raw,discard=off >/dev/null
du -sh $img
echo
echo "detect-zeros=on - write actual zeros"
fallocate -l 1m $img
echo -e 'qemu-io none0 "write -P 0 0 1m"\nquit' | $qemu -monitor stdio \
-drive if=none,file=$img,format=raw,detect-zeroes=on >/dev/null
du -sh $img
echo
echo "detect-zeros=unmap,discard=unmap - write actual zeros"
fallocate -l 1m $img
echo -e 'qemu-io none0 "write -P 0 0 1m"\nquit' | $qemu -monitor stdio \
-drive if=none,file=$img,format=raw,detect-zeroes=unmap,discard=unmap
>/dev/null
du -sh $img
echo
echo "discard=unmap - write zeroes"
fallocate -l 1m $img
echo -e 'qemu-io none0 "write -z 0 1m"\nquit' | $qemu -monitor stdio \
-drive if=none,file=$img,format=raw,discard=unmap >/dev/null
du -sh $img
echo
echo "discard=unmap - write zeroes unmap"
fallocate -l 1m $img
echo -e 'qemu-io none0 "write -zu 0 1m"\nquit' | $qemu -monitor stdio \
-drive if=none,file=$img,format=raw,discard=unmap >/dev/null
du -sh $img
rm $img
Before this change:
$ cat before.out
defaults - write zeroes
1.0M /tmp/test.raw
defaults - write zeroes unmap
0 /tmp/test.raw
defaults - write actual zeros
1.0M /tmp/test.raw
discard=off - write zeroes unmap
0 /tmp/test.raw
detect-zeros=on - write actual zeros
1.0M /tmp/test.raw
detect-zeros=unmap,discard=unmap - write actual zeros
0 /tmp/test.raw
discard=unmap - write zeroes
1.0M /tmp/test.raw
discard=unmap - write zeroes unmap
0 /tmp/test.raw
[nsoffer build (consider-discard-option)]$
After this change:
$ cat after.out
defaults - write zeroes
1.0M /tmp/test.raw
defaults - write zeroes unmap
1.0M /tmp/test.raw
defaults - write actual zeros
1.0M /tmp/test.raw
discard=off - write zeroes unmap
1.0M /tmp/test.raw
detect-zeros=on - write actual zeros
1.0M /tmp/test.raw
detect-zeros=unmap,discard=unmap - write actual zeros
0 /tmp/test.raw
discard=unmap - write zeroes
1.0M /tmp/test.raw
discard=unmap - write zeroes unmap
0 /tmp/test.raw
Differences:
$ diff -u before.out after.out
--- before.out 2024-06-19 20:24:09.234083713 +0300
+++ after.out 2024-06-19 20:24:20.526165573 +0300
@@ -3,13 +3,13 @@
1.0M /tmp/test.raw
defaults - write zeroes unmap
-0 /tmp/test.raw
+1.0M /tmp/test.raw
defaults - write actual zeros
1.0M /tmp/test.raw
discard=off - write zeroes unmap
-0 /tmp/test.raw
+1.0M /tmp/test.raw
On Wed, Jun 19, 2024 at 8:40 PM Nir Soffer <nsoffer@redhat.com> wrote:
> When opening an image with discard=off, we punch hole in the image when
> writing zeroes, making the image sparse. This breaks users that want to
> ensure that writes cannot fail with ENOSPACE by using fully allocated
> images.
>
> bdrv_co_pwrite_zeroes() correctly disable BDRV_REQ_MAY_UNMAP if we
> opened the child without discard=unmap or discard=on. But we don't go
> through this function when accessing the top node. Move the check down
> to bdrv_co_do_pwrite_zeroes() which seems to be used in all code paths.
>
> Issues:
> - We don't punch hole by default, so images are kept allocated. Before
> this change we punched holes by default. I'm not sure this is a good
> change in behavior.
> - Need to run all block tests
> - Not sure that we have tests covering unmapping, we may need new tests
> - We may need new tests to cover this change
>
> Signed-off-by: Nir Soffer <nsoffer@redhat.com>
> ---
>
> Changes since v1:
> - Replace the incorrect has_discard change with the right fix
>
> v1 was here:
> https://lists.nongnu.org/archive/html/qemu-block/2024-06/msg00198.html
>
> block/io.c | 9 +++++----
> 1 file changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/block/io.c b/block/io.c
> index 7217cf811b..301514c880 100644
> --- a/block/io.c
> +++ b/block/io.c
> @@ -1860,10 +1860,15 @@ bdrv_co_do_pwrite_zeroes(BlockDriverState *bs,
> int64_t offset, int64_t bytes,
> /* By definition there is no user buffer so this flag doesn't make
> sense */
> if (flags & BDRV_REQ_REGISTERED_BUF) {
> return -EINVAL;
> }
>
> + /* If opened with discard=off we should never unmap. */
> + if (!(bs->open_flags & BDRV_O_UNMAP)) {
> + flags &= ~BDRV_REQ_MAY_UNMAP;
> + }
> +
> /* Invalidate the cached block-status data range if this write
> overlaps */
> bdrv_bsc_invalidate_range(bs, offset, bytes);
>
> assert(alignment % bs->bl.request_alignment == 0);
> head = offset % alignment;
> @@ -2313,14 +2318,10 @@ int coroutine_fn bdrv_co_pwrite_zeroes(BdrvChild
> *child, int64_t offset,
> {
> IO_CODE();
> trace_bdrv_co_pwrite_zeroes(child->bs, offset, bytes, flags);
> assert_bdrv_graph_readable();
>
> - if (!(child->bs->open_flags & BDRV_O_UNMAP)) {
> - flags &= ~BDRV_REQ_MAY_UNMAP;
> - }
> -
> return bdrv_co_pwritev(child, offset, bytes, NULL,
> BDRV_REQ_ZERO_WRITE | flags);
> }
>
> /*
> --
> 2.45.1
>
>
On Wed, Jun 19, 2024 at 8:40 PM Nir Soffer <nsoffer@redhat.com> wrote: > - Need to run all block tests > Stale note, make check pass
On Wed, Jun 19, 2024 at 08:43:25PM +0300, Nir Soffer wrote: > Tested using: Hi Nir, This looks like a good candidate for the qemu-iotests test suite. Adding it to the automated tests will protect against future regressions. Please add the script and the expected output to tests/qemu-iotests/test/write-zeroes-unmap and run it using `(cd build && tests/qemu-iotests/check write-zeroes-unmap)`. See the existing test cases in tests/qemu-iotests/ and tests/qemu-iotests/tests/ for examples. Some are shell scripts and others are Python. I think shell makes sense for this test case. You can copy the test framework boilerplate from an existing test case. Thanks, Stefan > > $ cat test-unmap.sh > #!/bin/sh > > qemu=${1:?Usage: $0 qemu-executable} > img=/tmp/test.raw > > echo > echo "defaults - write zeroes" > fallocate -l 1m $img > echo -e 'qemu-io none0 "write -z 0 1m"\nquit' | $qemu -monitor stdio \ > -drive if=none,file=$img,format=raw >/dev/null > du -sh $img > > echo > echo "defaults - write zeroes unmap" > fallocate -l 1m $img > echo -e 'qemu-io none0 "write -zu 0 1m"\nquit' | $qemu -monitor stdio \ > -drive if=none,file=$img,format=raw >/dev/null > du -sh $img > > echo > echo "defaults - write actual zeros" > fallocate -l 1m $img > echo -e 'qemu-io none0 "write -P 0 0 1m"\nquit' | $qemu -monitor stdio \ > -drive if=none,file=$img,format=raw >/dev/null > du -sh $img > > echo > echo "discard=off - write zeroes unmap" > fallocate -l 1m $img > echo -e 'qemu-io none0 "write -zu 0 1m"\nquit' | $qemu -monitor stdio \ > -drive if=none,file=$img,format=raw,discard=off >/dev/null > du -sh $img > > echo > echo "detect-zeros=on - write actual zeros" > fallocate -l 1m $img > echo -e 'qemu-io none0 "write -P 0 0 1m"\nquit' | $qemu -monitor stdio \ > -drive if=none,file=$img,format=raw,detect-zeroes=on >/dev/null > du -sh $img > > echo > echo "detect-zeros=unmap,discard=unmap - write actual zeros" > fallocate -l 1m $img > echo -e 'qemu-io none0 "write -P 0 0 1m"\nquit' | $qemu -monitor stdio \ > -drive if=none,file=$img,format=raw,detect-zeroes=unmap,discard=unmap > >/dev/null > du -sh $img > > echo > echo "discard=unmap - write zeroes" > fallocate -l 1m $img > echo -e 'qemu-io none0 "write -z 0 1m"\nquit' | $qemu -monitor stdio \ > -drive if=none,file=$img,format=raw,discard=unmap >/dev/null > du -sh $img > > echo > echo "discard=unmap - write zeroes unmap" > fallocate -l 1m $img > echo -e 'qemu-io none0 "write -zu 0 1m"\nquit' | $qemu -monitor stdio \ > -drive if=none,file=$img,format=raw,discard=unmap >/dev/null > du -sh $img > > rm $img > > > Before this change: > > $ cat before.out > > defaults - write zeroes > 1.0M /tmp/test.raw > > defaults - write zeroes unmap > 0 /tmp/test.raw > > defaults - write actual zeros > 1.0M /tmp/test.raw > > discard=off - write zeroes unmap > 0 /tmp/test.raw > > detect-zeros=on - write actual zeros > 1.0M /tmp/test.raw > > detect-zeros=unmap,discard=unmap - write actual zeros > 0 /tmp/test.raw > > discard=unmap - write zeroes > 1.0M /tmp/test.raw > > discard=unmap - write zeroes unmap > 0 /tmp/test.raw > [nsoffer build (consider-discard-option)]$ > > > After this change: > > $ cat after.out > > defaults - write zeroes > 1.0M /tmp/test.raw > > defaults - write zeroes unmap > 1.0M /tmp/test.raw > > defaults - write actual zeros > 1.0M /tmp/test.raw > > discard=off - write zeroes unmap > 1.0M /tmp/test.raw > > detect-zeros=on - write actual zeros > 1.0M /tmp/test.raw > > detect-zeros=unmap,discard=unmap - write actual zeros > 0 /tmp/test.raw > > discard=unmap - write zeroes > 1.0M /tmp/test.raw > > discard=unmap - write zeroes unmap > 0 /tmp/test.raw > > > Differences: > > $ diff -u before.out after.out > --- before.out 2024-06-19 20:24:09.234083713 +0300 > +++ after.out 2024-06-19 20:24:20.526165573 +0300 > @@ -3,13 +3,13 @@ > 1.0M /tmp/test.raw > > defaults - write zeroes unmap > -0 /tmp/test.raw > +1.0M /tmp/test.raw > > defaults - write actual zeros > 1.0M /tmp/test.raw > > discard=off - write zeroes unmap > -0 /tmp/test.raw > +1.0M /tmp/test.raw > > On Wed, Jun 19, 2024 at 8:40 PM Nir Soffer <nsoffer@redhat.com> wrote: > > > When opening an image with discard=off, we punch hole in the image when > > writing zeroes, making the image sparse. This breaks users that want to > > ensure that writes cannot fail with ENOSPACE by using fully allocated > > images. > > > > bdrv_co_pwrite_zeroes() correctly disable BDRV_REQ_MAY_UNMAP if we > > opened the child without discard=unmap or discard=on. But we don't go > > through this function when accessing the top node. Move the check down > > to bdrv_co_do_pwrite_zeroes() which seems to be used in all code paths. > > > > Issues: > > - We don't punch hole by default, so images are kept allocated. Before > > this change we punched holes by default. I'm not sure this is a good > > change in behavior. > > - Need to run all block tests > > - Not sure that we have tests covering unmapping, we may need new tests > > - We may need new tests to cover this change > > > > Signed-off-by: Nir Soffer <nsoffer@redhat.com> > > --- > > > > Changes since v1: > > - Replace the incorrect has_discard change with the right fix > > > > v1 was here: > > https://lists.nongnu.org/archive/html/qemu-block/2024-06/msg00198.html > > > > block/io.c | 9 +++++---- > > 1 file changed, 5 insertions(+), 4 deletions(-) > > > > diff --git a/block/io.c b/block/io.c > > index 7217cf811b..301514c880 100644 > > --- a/block/io.c > > +++ b/block/io.c > > @@ -1860,10 +1860,15 @@ bdrv_co_do_pwrite_zeroes(BlockDriverState *bs, > > int64_t offset, int64_t bytes, > > /* By definition there is no user buffer so this flag doesn't make > > sense */ > > if (flags & BDRV_REQ_REGISTERED_BUF) { > > return -EINVAL; > > } > > > > + /* If opened with discard=off we should never unmap. */ > > + if (!(bs->open_flags & BDRV_O_UNMAP)) { > > + flags &= ~BDRV_REQ_MAY_UNMAP; > > + } > > + > > /* Invalidate the cached block-status data range if this write > > overlaps */ > > bdrv_bsc_invalidate_range(bs, offset, bytes); > > > > assert(alignment % bs->bl.request_alignment == 0); > > head = offset % alignment; > > @@ -2313,14 +2318,10 @@ int coroutine_fn bdrv_co_pwrite_zeroes(BdrvChild > > *child, int64_t offset, > > { > > IO_CODE(); > > trace_bdrv_co_pwrite_zeroes(child->bs, offset, bytes, flags); > > assert_bdrv_graph_readable(); > > > > - if (!(child->bs->open_flags & BDRV_O_UNMAP)) { > > - flags &= ~BDRV_REQ_MAY_UNMAP; > > - } > > - > > return bdrv_co_pwritev(child, offset, bytes, NULL, > > BDRV_REQ_ZERO_WRITE | flags); > > } > > > > /* > > -- > > 2.45.1 > > > >
Am 24.06.2024 um 17:23 hat Stefan Hajnoczi geschrieben: > On Wed, Jun 19, 2024 at 08:43:25PM +0300, Nir Soffer wrote: > > Tested using: > > Hi Nir, > This looks like a good candidate for the qemu-iotests test suite. Adding > it to the automated tests will protect against future regressions. > > Please add the script and the expected output to > tests/qemu-iotests/test/write-zeroes-unmap and run it using > `(cd build && tests/qemu-iotests/check write-zeroes-unmap)`. > > See the existing test cases in tests/qemu-iotests/ and > tests/qemu-iotests/tests/ for examples. Some are shell scripts and > others are Python. I think shell makes sense for this test case. You > can copy the test framework boilerplate from an existing test case. 'du' can't be used like this in qemu-iotests because it makes assumptions that depend on the filesystem. A test case replicating what Nir did manually would likely fail on XFS with its preallocation. Maybe we could operate on a file exposed by the FUSE export that is backed by qcow2, and then you can use 'qemu-img map' on that qcow2 image to verify the allocation status. Somewhat complicated, but I think it could work. Kevin
On Mon, Jun 24, 2024 at 7:08 PM Kevin Wolf <kwolf@redhat.com> wrote: > Am 24.06.2024 um 17:23 hat Stefan Hajnoczi geschrieben: > > On Wed, Jun 19, 2024 at 08:43:25PM +0300, Nir Soffer wrote: > > > Tested using: > > > > Hi Nir, > > This looks like a good candidate for the qemu-iotests test suite. Adding > > it to the automated tests will protect against future regressions. > > > > Please add the script and the expected output to > > tests/qemu-iotests/test/write-zeroes-unmap and run it using > > `(cd build && tests/qemu-iotests/check write-zeroes-unmap)`. > > > > See the existing test cases in tests/qemu-iotests/ and > > tests/qemu-iotests/tests/ for examples. Some are shell scripts and > > others are Python. I think shell makes sense for this test case. You > > can copy the test framework boilerplate from an existing test case. > > 'du' can't be used like this in qemu-iotests because it makes > assumptions that depend on the filesystem. A test case replicating what > Nir did manually would likely fail on XFS with its preallocation. > This is why I did not try to add a new qemu-iotest yet. > Maybe we could operate on a file exposed by the FUSE export that is > backed by qcow2, and then you can use 'qemu-img map' on that qcow2 image > to verify the allocation status. Somewhat complicated, but I think it > could work. > Do we have examples of using the FUSE export? It sounds complicated but being able to test on any file system is awesome. The complexity can be hidden behind simple test helpers. Another option is to use a specific file system created for the tests, for example on a loop device. We used userstorage[1] in ovirt to test on specific file systems with known sector size. But more important, are you ok with the change? I'm not sure about not creating sparse images by default - this is not consistent with qemu-img convert and qemu-nbd, which do sparsify by default. The old behavior seems better. [1] https://github.com/nirs/userstorage Nir
Am 24.06.2024 um 23:12 hat Nir Soffer geschrieben: > On Mon, Jun 24, 2024 at 7:08 PM Kevin Wolf <kwolf@redhat.com> wrote: > > > Am 24.06.2024 um 17:23 hat Stefan Hajnoczi geschrieben: > > > On Wed, Jun 19, 2024 at 08:43:25PM +0300, Nir Soffer wrote: > > > > Tested using: > > > > > > Hi Nir, > > > This looks like a good candidate for the qemu-iotests test suite. Adding > > > it to the automated tests will protect against future regressions. > > > > > > Please add the script and the expected output to > > > tests/qemu-iotests/test/write-zeroes-unmap and run it using > > > `(cd build && tests/qemu-iotests/check write-zeroes-unmap)`. > > > > > > See the existing test cases in tests/qemu-iotests/ and > > > tests/qemu-iotests/tests/ for examples. Some are shell scripts and > > > others are Python. I think shell makes sense for this test case. You > > > can copy the test framework boilerplate from an existing test case. > > > > 'du' can't be used like this in qemu-iotests because it makes > > assumptions that depend on the filesystem. A test case replicating what > > Nir did manually would likely fail on XFS with its preallocation. > > This is why I did not try to add a new qemu-iotest yet. > > > Maybe we could operate on a file exposed by the FUSE export that is > > backed by qcow2, and then you can use 'qemu-img map' on that qcow2 image > > to verify the allocation status. Somewhat complicated, but I think it > > could work. > > Do we have examples of using the FUSE export? It sounds complicated but > being able to test on any file system is awesome. The complexity can be > hidden behind simple test helpers. We seem to have a few tests that use it, and then the fuse protocol implementation, too. 308 and file-io-error look relevant. > Another option is to use a specific file system created for the tests, > for example on a loop device. We used userstorage[1] in ovirt to test > on specific file systems with known sector size. Creating loop devices requires root privileges. If I understand correctly, userstorage solved that by having a setup phase as root before running the tests as a normal user? We don't really have that in qemu-iotests. Some tests require passwordless sudo and are skipped otherwise, but this means that in practice they are almost always skipped. > But more important, are you ok with the change? > > I'm not sure about not creating sparse images by default - this is not > consistent with qemu-img convert and qemu-nbd, which do sparsify by > default. The old behavior seems better. Well, your patches make it do what we always claimed it would do, so that consistency is certainly a good thing. Unmapping on write_zeroes and ignoring truncate is a weird combination anyway that doesn't really make any sense to me, so I don't think it's worth preserving. The other way around could have been more defensible, but that's not how our bug works. Now, if ignoring all discard requests is a good default these days is a separate question and I'm not sure really. Maybe discard=unmap should be the default (and apply to both discard are write_zeroes, of course). Kevin
On Mon, Jun 24, 2024 at 06:08:26PM +0200, Kevin Wolf wrote: > Am 24.06.2024 um 17:23 hat Stefan Hajnoczi geschrieben: > > On Wed, Jun 19, 2024 at 08:43:25PM +0300, Nir Soffer wrote: > > > Tested using: > > > > Hi Nir, > > This looks like a good candidate for the qemu-iotests test suite. Adding > > it to the automated tests will protect against future regressions. > > > > Please add the script and the expected output to > > tests/qemu-iotests/test/write-zeroes-unmap and run it using > > `(cd build && tests/qemu-iotests/check write-zeroes-unmap)`. > > > > See the existing test cases in tests/qemu-iotests/ and > > tests/qemu-iotests/tests/ for examples. Some are shell scripts and > > others are Python. I think shell makes sense for this test case. You > > can copy the test framework boilerplate from an existing test case. > > 'du' can't be used like this in qemu-iotests because it makes > assumptions that depend on the filesystem. A test case replicating what > Nir did manually would likely fail on XFS with its preallocation. > > Maybe we could operate on a file exposed by the FUSE export that is > backed by qcow2, and then you can use 'qemu-img map' on that qcow2 image > to verify the allocation status. Somewhat complicated, but I think it > could work. A simpler option would be to use 'du' but with a fuzzy range test, rather than an exact equality test. For the tests which write 1 MB, check the 'du' usage is "at least 1MB", for the tests which expect to unmap blocks, check that the 'du' usage is "less than 256kb". This should be within bounds of xfs speculative allocation. With regards, Daniel
On Wed, Jun 26, 2024 at 11:42 AM Kevin Wolf <kwolf@redhat.com> wrote: > Am 24.06.2024 um 23:12 hat Nir Soffer geschrieben: > > On Mon, Jun 24, 2024 at 7:08 PM Kevin Wolf <kwolf@redhat.com> wrote: > > > > > Am 24.06.2024 um 17:23 hat Stefan Hajnoczi geschrieben: > > > > On Wed, Jun 19, 2024 at 08:43:25PM +0300, Nir Soffer wrote: > > > > > Tested using: > > > > > > > > Hi Nir, > > > > This looks like a good candidate for the qemu-iotests test suite. > Adding > > > > it to the automated tests will protect against future regressions. > > > > > > > > Please add the script and the expected output to > > > > tests/qemu-iotests/test/write-zeroes-unmap and run it using > > > > `(cd build && tests/qemu-iotests/check write-zeroes-unmap)`. > > > > > > > > See the existing test cases in tests/qemu-iotests/ and > > > > tests/qemu-iotests/tests/ for examples. Some are shell scripts and > > > > others are Python. I think shell makes sense for this test case. You > > > > can copy the test framework boilerplate from an existing test case. > > > > > > 'du' can't be used like this in qemu-iotests because it makes > > > assumptions that depend on the filesystem. A test case replicating what > > > Nir did manually would likely fail on XFS with its preallocation. > > > > This is why I did not try to add a new qemu-iotest yet. > > > > > Maybe we could operate on a file exposed by the FUSE export that is > > > backed by qcow2, and then you can use 'qemu-img map' on that qcow2 > image > > > to verify the allocation status. Somewhat complicated, but I think it > > > could work. > > > > Do we have examples of using the FUSE export? It sounds complicated but > > being able to test on any file system is awesome. The complexity can be > > hidden behind simple test helpers. > > We seem to have a few tests that use it, and then the fuse protocol > implementation, too. 308 and file-io-error look relevant. > > > Another option is to use a specific file system created for the tests, > > for example on a loop device. We used userstorage[1] in ovirt to test > > on specific file systems with known sector size. > > Creating loop devices requires root privileges. If I understand > correctly, userstorage solved that by having a setup phase as root > before running the tests as a normal user? We don't really have that in > qemu-iotests. > > Some tests require passwordless sudo and are skipped otherwise, but this > means that in practice they are almost always skipped. > Yes, this is the assumption the storage is being created before running the tests, for example when setting up a development or CI environment, and the tests can run with unprivileged user. > But more important, are you ok with the change? > > > > I'm not sure about not creating sparse images by default - this is not > > consistent with qemu-img convert and qemu-nbd, which do sparsify by > > default. The old behavior seems better. > > Well, your patches make it do what we always claimed it would do, so > that consistency is certainly a good thing. Unmapping on write_zeroes > and ignoring truncate is a weird combination anyway that doesn't really > make any sense to me, so I don't think it's worth preserving. The other > way around could have been more defensible, but that's not how our bug > works. > > Now, if ignoring all discard requests is a good default these days is a > separate question and I'm not sure really. Maybe discard=unmap should > be the default (and apply to both discard are write_zeroes, of course). > OK, lets limit the scope to fix the code to match the current docs. We can tweak the defaults later.
On Wed, Jun 26, 2024 at 12:17 PM Daniel P. Berrangé <berrange@redhat.com> wrote: > On Mon, Jun 24, 2024 at 06:08:26PM +0200, Kevin Wolf wrote: > > Am 24.06.2024 um 17:23 hat Stefan Hajnoczi geschrieben: > > > On Wed, Jun 19, 2024 at 08:43:25PM +0300, Nir Soffer wrote: > > > > Tested using: > > > > > > Hi Nir, > > > This looks like a good candidate for the qemu-iotests test suite. > Adding > > > it to the automated tests will protect against future regressions. > > > > > > Please add the script and the expected output to > > > tests/qemu-iotests/test/write-zeroes-unmap and run it using > > > `(cd build && tests/qemu-iotests/check write-zeroes-unmap)`. > > > > > > See the existing test cases in tests/qemu-iotests/ and > > > tests/qemu-iotests/tests/ for examples. Some are shell scripts and > > > others are Python. I think shell makes sense for this test case. You > > > can copy the test framework boilerplate from an existing test case. > > > > 'du' can't be used like this in qemu-iotests because it makes > > assumptions that depend on the filesystem. A test case replicating what > > Nir did manually would likely fail on XFS with its preallocation. > > > > Maybe we could operate on a file exposed by the FUSE export that is > > backed by qcow2, and then you can use 'qemu-img map' on that qcow2 image > > to verify the allocation status. Somewhat complicated, but I think it > > could work. > > A simpler option would be to use 'du' but with a fuzzy range test, > rather than an exact equality test. > > For the tests which write 1 MB, check the 'du' usage is "at least 1MB", > for the tests which expect to unmap blocks, check that the 'du' usage > is "less than 256kb". This should be within bounds of xfs speculative > allocation. > This should work, I'll start with this approach.
diff --git a/block/io.c b/block/io.c index 7217cf811b..301514c880 100644 --- a/block/io.c +++ b/block/io.c @@ -1860,10 +1860,15 @@ bdrv_co_do_pwrite_zeroes(BlockDriverState *bs, int64_t offset, int64_t bytes, /* By definition there is no user buffer so this flag doesn't make sense */ if (flags & BDRV_REQ_REGISTERED_BUF) { return -EINVAL; } + /* If opened with discard=off we should never unmap. */ + if (!(bs->open_flags & BDRV_O_UNMAP)) { + flags &= ~BDRV_REQ_MAY_UNMAP; + } + /* Invalidate the cached block-status data range if this write overlaps */ bdrv_bsc_invalidate_range(bs, offset, bytes); assert(alignment % bs->bl.request_alignment == 0); head = offset % alignment; @@ -2313,14 +2318,10 @@ int coroutine_fn bdrv_co_pwrite_zeroes(BdrvChild *child, int64_t offset, { IO_CODE(); trace_bdrv_co_pwrite_zeroes(child->bs, offset, bytes, flags); assert_bdrv_graph_readable(); - if (!(child->bs->open_flags & BDRV_O_UNMAP)) { - flags &= ~BDRV_REQ_MAY_UNMAP; - } - return bdrv_co_pwritev(child, offset, bytes, NULL, BDRV_REQ_ZERO_WRITE | flags); } /*
When opening an image with discard=off, we punch hole in the image when writing zeroes, making the image sparse. This breaks users that want to ensure that writes cannot fail with ENOSPACE by using fully allocated images. bdrv_co_pwrite_zeroes() correctly disable BDRV_REQ_MAY_UNMAP if we opened the child without discard=unmap or discard=on. But we don't go through this function when accessing the top node. Move the check down to bdrv_co_do_pwrite_zeroes() which seems to be used in all code paths. Issues: - We don't punch hole by default, so images are kept allocated. Before this change we punched holes by default. I'm not sure this is a good change in behavior. - Need to run all block tests - Not sure that we have tests covering unmapping, we may need new tests - We may need new tests to cover this change Signed-off-by: Nir Soffer <nsoffer@redhat.com> --- Changes since v1: - Replace the incorrect has_discard change with the right fix v1 was here: https://lists.nongnu.org/archive/html/qemu-block/2024-06/msg00198.html block/io.c | 9 +++++---- 1 file changed, 5 insertions(+), 4 deletions(-)