Support for -XX:AllocateHeapAt on ZFS?

[Harald Eilertsen]

Hi everyone,

I tried asking this on the freebsd-java list too, but did not get a
response, so trying here instead:

The code backing the `-XX:AllocateHeapAt=path` option to java calls the
function posix_fallocate on most systems to reserve the space for the
file in the file system. This will fail and return `EINVAL` if the
underlying file system does not support the operation.

On FreeBSD this is particularly relevant because the commonly used ZFS
file system does not support this operation.

For rationale see:
https://freebsd-current.freebsd.narkive.com/5pbDPeIT/heads-up-posix-fallocate-support-removed-from-zfs-lld-affected
https://lists.freebsd.org/pipermail/freebsd-current/2018-February/068447.html

On OpenBSD the ftruncate function is used instead, and MacOSX will
attempt to set the size via fcntl, but falls back to ftruncate if that
does not work.

To make things even more interesting, the glibc implementation of
posix_fallocate used on most Linux based systems deviates from the posix
standard by 'emulating' the feature if not supported by the underlying
file system.
https://www.man7.org/linux/man-pages/man3/posix_fallocate.3.html#CAVEATS

My question is: Do we want to keep the strict posix compliance on
FreeBSD, which in practice means this feature is not supported on ZFS,
or do we want to be less strict like the other platforms mentioned
above?

Also, if someone can comment on the rationale for OpenBSD
implementation, that would also be helpful.

Thanks in advance,
Harald Eilertsen