openZFS 2.0 rc3 reports fallocate supported, but not works

[wheelcomplex]

Read this Go issuse for reference.

System information

Distribution Name | Ubuntu
Distribution Version | 20.04
Linux Kernel | 5.4.0-48-generic
Architecture | x86_64
ZFS Version | 2.0.0-rc3
SPL Version | 2.0.0-rc3

Describe the problem you're observing

openZFS 2.0 rc3 reports fallocate supported, but not works

Describe how to reproduce the problem

git clone https://go.googlesource.com/go && cd go/src && ./all.bash

Include any warning/errors/backtraces from the system logs

--- FAIL: TestFallocate (0.00s)
    fallocate_test.go:61: unexpected disk usage: got 1 blocks, want 2048
    fallocate_test.go:61: unexpected disk usage: got 1 blocks, want 4096
    fallocate_test.go:61: unexpected disk usage: got 1 blocks, want 6144

[behlendorf]

@wheelcomplex thanks for opening this issue. The ZFS behavior you're seeing is what was intended. Partial support for fallocate(2) was only recently added because its expected semantics really don't make sense for a copy-on-write filesystem. This is explained nicely in the commit message, f734301.

    Since ZFS does COW and snapshots, preallocating blocks for a file
    cannot guarantee that writes to the file will not run out of space.
    Even if the first overwrite was guaranteed, it would not handle any
    later overwrite of blocks due to COW, so strict compliance is futile.
    Instead, make a best-effort check that at least enough free space is
    currently available in the pool (with a bit of margin), then create
    a sparse file of the requested size and continue on with life.

This is functionally correct and achieves what callers are really after while avoiding the expense of preallocating blocks which won't be used. There's a good discussion in issue #326 covering the various tradeoffs and concerns.

If possible I'd suggest updating the test case to not worry about the internal filesystem details of exactly how many blocks were actually allocated.

[wheelcomplex]

Thanks for the explaining and suggestion.