[Libguestfs] Re: copy tests failing in libnbd with ZoL

On Thu, Oct 23, 2025 at 06:45:33PM +0200, Arsen Arsenović wrote: > "Richard W.M. Jones" <rjones(a)redhat.com&gt; writes: > > > I was definitely expecting this one to fail with ENOTSUP or similar. > > That's with the file copy-allocated-destination-zero-synch.out on ZFS? > > Yes - my home directory is on ZoL, and so is ~/tmp. It appears this is a limitation of OpenZFS: https://github.com/openzfs/zfs/blob/fc519b2c1108b52aaad63e356645f63c2c167...

<quote> * The original mode=0 (allocate space) behavior can be reasonably emulated * by checking if enough space exists and creating a sparse file, as real * persistent space reservation is not possible due to COW, snapshots, etc. </quote> Looking at the code when mode == 0, I believe we hit the second case (line 707), which seems to sparsify the file and (if that succeeeds) returns no error :-( We should probably add a 'requires' to the test. I think it could still be a bug in libnbd that we don't fail if fallocate fails, but it wouldn't have helped here.