Re: [PATCH v2] Consider discard option when writing zeros

[Kevin Wolf <kwolf@redhat.com>]

Am 26.06.2024 um 18:27 hat Nir Soffer geschrieben:
> 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.

If we're okay with accepting tests that depend on filesystem behaviour,
then 'qemu-img map -f raw --output=json' should be the less risky
approach than checking 'du'.

Kevin