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

["Daniel P. Berrangé" <berrange@redhat.com>]

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
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