[rleigh]

Regarding ACLs, I'd say that there's little choice here: it has to be NFSv4.

The rationale for this is that POSIX ACLs are firstly too simple to model what we need. And they are also non-standard (POSIX .1e ACLs are a DRAFT specification which was never ratified).

NFSv4 ACLs are vastly more featureful, already implemented to support NFSv4 in kernel, though not available in userspace AFAICT. On FreeBSD and other platforms using ZFS, they are also used by ZFS and are directly exposed to userspace, making rich ACLs usable as the default permissions model system-wide when running on ZFS. Linux, unfortunately, doesn't yet do any of this, even when using ZFS.

[JdeBP]

The irony is that whilst the standards document was never ratified most people implemented it anyway. So actually, they are a standard. (-: