Implement O_DIRECT for open to bypass metadata cache #614
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Signed-off-by: Daniel Carl Jones <djonesoa@amazon.com>
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| // Open the file w/ O_DIRECT, which should see the new file on S3 despite the old file being cached | ||
| let mut buf = [0u8; OBJECT_SIZE]; |
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If I use vec![0u8; OBJECT_SIZE] here, I sometimes find that the buffer hasn't been written to (even though read_exact_at has returned Ok). This makes me super uncomfortable (which is why this PR is still draft, not ready to merge).
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O_DIRECT is really hard to test under the Rust test runner because it has undefined behavior if run concurrently with a fork, and all of our FUSE tests involve a fork to spawn fusermount. We've seen it before in #114. Do you still see the issue if you filter to run only one of these tests?
You probably don't see it with this version because buf is on the stack, so less likely to be immediately scribbled over by malloc in the forked process.
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Ah, I saw those limitations with O_DIRECT and fork but wasn't expecting us to be using it in the tests. That sounds like a probable cause!
I don't see the issue when running the test on its own.
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… serially Signed-off-by: Alessandro Passaro <alexpax@amazon.co.uk>
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Nov 21, 2023
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Signed-off-by: Alessandro Passaro <alexpax@amazon.co.uk>
Signed-off-by: Alessandro Passaro <alexpax@amazon.co.uk>
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LGTM, but its my PR so also needs your approval @passaro 😄 |
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Description of change
This is part of metadata and data caching feature currently in implementation: #581.
Some applications may want to opt-out of caching, maybe aware or not aware of Mountpoint itself. Applications use the
O_DIRECTflag when opening a file to indicate that file content should be read directly, bypassing the Kernel caches.We want to also skip Mountpoint's metadata cache when this flag is provided, so that open calls using it will see the latest object in S3 at open - as it works today without opting into caching.
Relevant issues: #255
Does this change impact existing behavior?
No breaking changes, changes only to a feature has not been released yet.
By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and I agree to the terms of the Developer Certificate of Origin (DCO).